1 /*
   2  * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.util;
  27 import java.io.Serializable;
  28 import java.io.ObjectOutputStream;
  29 import java.io.IOException;
  30 import java.lang.reflect.Array;
  31 import java.util.function.BiConsumer;
  32 import java.util.function.BiFunction;
  33 import java.util.function.Consumer;
  34 import java.util.function.Function;
  35 import java.util.function.Predicate;
  36 import java.util.function.UnaryOperator;
  37 import java.util.stream.IntStream;
  38 import java.util.stream.Stream;
  39 import java.util.stream.StreamSupport;
  40 
  41 /**
  42  * This class consists exclusively of static methods that operate on or return
  43  * collections.  It contains polymorphic algorithms that operate on
  44  * collections, "wrappers", which return a new collection backed by a
  45  * specified collection, and a few other odds and ends.
  46  *
  47  * <p>The methods of this class all throw a {@code NullPointerException}
  48  * if the collections or class objects provided to them are null.
  49  *
  50  * <p>The documentation for the polymorphic algorithms contained in this class
  51  * generally includes a brief description of the <i>implementation</i>.  Such
  52  * descriptions should be regarded as <i>implementation notes</i>, rather than
  53  * parts of the <i>specification</i>.  Implementors should feel free to
  54  * substitute other algorithms, so long as the specification itself is adhered
  55  * to.  (For example, the algorithm used by {@code sort} does not have to be
  56  * a mergesort, but it does have to be <i>stable</i>.)
  57  *
  58  * <p>The "destructive" algorithms contained in this class, that is, the
  59  * algorithms that modify the collection on which they operate, are specified
  60  * to throw {@code UnsupportedOperationException} if the collection does not
  61  * support the appropriate mutation primitive(s), such as the {@code set}
  62  * method.  These algorithms may, but are not required to, throw this
  63  * exception if an invocation would have no effect on the collection.  For
  64  * example, invoking the {@code sort} method on an unmodifiable list that is
  65  * already sorted may or may not throw {@code UnsupportedOperationException}.
  66  *
  67  * <p>This class is a member of the
  68  * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
  69  * Java Collections Framework</a>.
  70  *
  71  * @author  Josh Bloch
  72  * @author  Neal Gafter
  73  * @see     Collection
  74  * @see     Set
  75  * @see     List
  76  * @see     Map
  77  * @since   1.2
  78  */
  79 
  80 public class Collections {
  81     // Suppresses default constructor, ensuring non-instantiability.
  82     private Collections() {
  83     }
  84 
  85     // Algorithms
  86 
  87     /*
  88      * Tuning parameters for algorithms - Many of the List algorithms have
  89      * two implementations, one of which is appropriate for RandomAccess
  90      * lists, the other for "sequential."  Often, the random access variant
  91      * yields better performance on small sequential access lists.  The
  92      * tuning parameters below determine the cutoff point for what constitutes
  93      * a "small" sequential access list for each algorithm.  The values below
  94      * were empirically determined to work well for LinkedList. Hopefully
  95      * they should be reasonable for other sequential access List
  96      * implementations.  Those doing performance work on this code would
  97      * do well to validate the values of these parameters from time to time.
  98      * (The first word of each tuning parameter name is the algorithm to which
  99      * it applies.)
 100      */
 101     private static final int BINARYSEARCH_THRESHOLD   = 5000;
 102     private static final int REVERSE_THRESHOLD        =   18;
 103     private static final int SHUFFLE_THRESHOLD        =    5;
 104     private static final int FILL_THRESHOLD           =   25;
 105     private static final int ROTATE_THRESHOLD         =  100;
 106     private static final int COPY_THRESHOLD           =   10;
 107     private static final int REPLACEALL_THRESHOLD     =   11;
 108     private static final int INDEXOFSUBLIST_THRESHOLD =   35;
 109 
 110     /**
 111      * Sorts the specified list into ascending order, according to the
 112      * {@linkplain Comparable natural ordering} of its elements.
 113      * All elements in the list must implement the {@link Comparable}
 114      * interface.  Furthermore, all elements in the list must be
 115      * <i>mutually comparable</i> (that is, {@code e1.compareTo(e2)}
 116      * must not throw a {@code ClassCastException} for any elements
 117      * {@code e1} and {@code e2} in the list).
 118      *
 119      * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
 120      * not be reordered as a result of the sort.
 121      *
 122      * <p>The specified list must be modifiable, but need not be resizable.
 123      *
 124      * @implNote
 125      * This implementation defers to the {@link List#sort(Comparator)}
 126      * method using the specified list and a {@code null} comparator.
 127      *
 128      * @param  <T> the class of the objects in the list
 129      * @param  list the list to be sorted.
 130      * @throws ClassCastException if the list contains elements that are not
 131      *         <i>mutually comparable</i> (for example, strings and integers).
 132      * @throws UnsupportedOperationException if the specified list's
 133      *         list-iterator does not support the {@code set} operation.
 134      * @throws IllegalArgumentException (optional) if the implementation
 135      *         detects that the natural ordering of the list elements is
 136      *         found to violate the {@link Comparable} contract
 137      * @see List#sort(Comparator)
 138      */
 139     @SuppressWarnings("unchecked")
 140     public static <T extends Comparable<? super T>> void sort(List<T> list) {
 141         list.sort(null);
 142     }
 143 
 144     /**
 145      * Sorts the specified list according to the order induced by the
 146      * specified comparator.  All elements in the list must be <i>mutually
 147      * comparable</i> using the specified comparator (that is,
 148      * {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}
 149      * for any elements {@code e1} and {@code e2} in the list).
 150      *
 151      * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
 152      * not be reordered as a result of the sort.
 153      *
 154      * <p>The specified list must be modifiable, but need not be resizable.
 155      *
 156      * @implNote
 157      * This implementation defers to the {@link List#sort(Comparator)}
 158      * method using the specified list and comparator.
 159      *
 160      * @param  <T> the class of the objects in the list
 161      * @param  list the list to be sorted.
 162      * @param  c the comparator to determine the order of the list.  A
 163      *        {@code null} value indicates that the elements' <i>natural
 164      *        ordering</i> should be used.
 165      * @throws ClassCastException if the list contains elements that are not
 166      *         <i>mutually comparable</i> using the specified comparator.
 167      * @throws UnsupportedOperationException if the specified list's
 168      *         list-iterator does not support the {@code set} operation.
 169      * @throws IllegalArgumentException (optional) if the comparator is
 170      *         found to violate the {@link Comparator} contract
 171      * @see List#sort(Comparator)
 172      */
 173     @SuppressWarnings({"unchecked", "rawtypes"})
 174     public static <T> void sort(List<T> list, Comparator<? super T> c) {
 175         list.sort(c);
 176     }
 177 
 178 
 179     /**
 180      * Searches the specified list for the specified object using the binary
 181      * search algorithm.  The list must be sorted into ascending order
 182      * according to the {@linkplain Comparable natural ordering} of its
 183      * elements (as by the {@link #sort(List)} method) prior to making this
 184      * call.  If it is not sorted, the results are undefined.  If the list
 185      * contains multiple elements equal to the specified object, there is no
 186      * guarantee which one will be found.
 187      *
 188      * <p>This method runs in log(n) time for a "random access" list (which
 189      * provides near-constant-time positional access).  If the specified list
 190      * does not implement the {@link RandomAccess} interface and is large,
 191      * this method will do an iterator-based binary search that performs
 192      * O(n) link traversals and O(log n) element comparisons.
 193      *
 194      * @param  <T> the class of the objects in the list
 195      * @param  list the list to be searched.
 196      * @param  key the key to be searched for.
 197      * @return the index of the search key, if it is contained in the list;
 198      *         otherwise, <code>(-(<i>insertion point</i>) - 1)</code>.  The
 199      *         <i>insertion point</i> is defined as the point at which the
 200      *         key would be inserted into the list: the index of the first
 201      *         element greater than the key, or {@code list.size()} if all
 202      *         elements in the list are less than the specified key.  Note
 203      *         that this guarantees that the return value will be &gt;= 0 if
 204      *         and only if the key is found.
 205      * @throws ClassCastException if the list contains elements that are not
 206      *         <i>mutually comparable</i> (for example, strings and
 207      *         integers), or the search key is not mutually comparable
 208      *         with the elements of the list.
 209      */
 210     public static <T>
 211     int binarySearch(List<? extends Comparable<? super T>> list, T key) {
 212         if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
 213             return Collections.indexedBinarySearch(list, key);
 214         else
 215             return Collections.iteratorBinarySearch(list, key);
 216     }
 217 
 218     private static <T>
 219     int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {
 220         int low = 0;
 221         int high = list.size()-1;
 222 
 223         while (low <= high) {
 224             int mid = (low + high) >>> 1;
 225             Comparable<? super T> midVal = list.get(mid);
 226             int cmp = midVal.compareTo(key);
 227 
 228             if (cmp < 0)
 229                 low = mid + 1;
 230             else if (cmp > 0)
 231                 high = mid - 1;
 232             else
 233                 return mid; // key found
 234         }
 235         return -(low + 1);  // key not found
 236     }
 237 
 238     private static <T>
 239     int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)
 240     {
 241         int low = 0;
 242         int high = list.size()-1;
 243         ListIterator<? extends Comparable<? super T>> i = list.listIterator();
 244 
 245         while (low <= high) {
 246             int mid = (low + high) >>> 1;
 247             Comparable<? super T> midVal = get(i, mid);
 248             int cmp = midVal.compareTo(key);
 249 
 250             if (cmp < 0)
 251                 low = mid + 1;
 252             else if (cmp > 0)
 253                 high = mid - 1;
 254             else
 255                 return mid; // key found
 256         }
 257         return -(low + 1);  // key not found
 258     }
 259 
 260     /**
 261      * Gets the ith element from the given list by repositioning the specified
 262      * list listIterator.
 263      */
 264     private static <T> T get(ListIterator<? extends T> i, int index) {
 265         T obj = null;
 266         int pos = i.nextIndex();
 267         if (pos <= index) {
 268             do {
 269                 obj = i.next();
 270             } while (pos++ < index);
 271         } else {
 272             do {
 273                 obj = i.previous();
 274             } while (--pos > index);
 275         }
 276         return obj;
 277     }
 278 
 279     /**
 280      * Searches the specified list for the specified object using the binary
 281      * search algorithm.  The list must be sorted into ascending order
 282      * according to the specified comparator (as by the
 283      * {@link #sort(List, Comparator) sort(List, Comparator)}
 284      * method), prior to making this call.  If it is
 285      * not sorted, the results are undefined.  If the list contains multiple
 286      * elements equal to the specified object, there is no guarantee which one
 287      * will be found.
 288      *
 289      * <p>This method runs in log(n) time for a "random access" list (which
 290      * provides near-constant-time positional access).  If the specified list
 291      * does not implement the {@link RandomAccess} interface and is large,
 292      * this method will do an iterator-based binary search that performs
 293      * O(n) link traversals and O(log n) element comparisons.
 294      *
 295      * @param  <T> the class of the objects in the list
 296      * @param  list the list to be searched.
 297      * @param  key the key to be searched for.
 298      * @param  c the comparator by which the list is ordered.
 299      *         A {@code null} value indicates that the elements'
 300      *         {@linkplain Comparable natural ordering} should be used.
 301      * @return the index of the search key, if it is contained in the list;
 302      *         otherwise, <code>(-(<i>insertion point</i>) - 1)</code>.  The
 303      *         <i>insertion point</i> is defined as the point at which the
 304      *         key would be inserted into the list: the index of the first
 305      *         element greater than the key, or {@code list.size()} if all
 306      *         elements in the list are less than the specified key.  Note
 307      *         that this guarantees that the return value will be &gt;= 0 if
 308      *         and only if the key is found.
 309      * @throws ClassCastException if the list contains elements that are not
 310      *         <i>mutually comparable</i> using the specified comparator,
 311      *         or the search key is not mutually comparable with the
 312      *         elements of the list using this comparator.
 313      */
 314     @SuppressWarnings("unchecked")
 315     public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
 316         if (c==null)
 317             return binarySearch((List<? extends Comparable<? super T>>) list, key);
 318 
 319         if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
 320             return Collections.indexedBinarySearch(list, key, c);
 321         else
 322             return Collections.iteratorBinarySearch(list, key, c);
 323     }
 324 
 325     private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
 326         int low = 0;
 327         int high = l.size()-1;
 328 
 329         while (low <= high) {
 330             int mid = (low + high) >>> 1;
 331             T midVal = l.get(mid);
 332             int cmp = c.compare(midVal, key);
 333 
 334             if (cmp < 0)
 335                 low = mid + 1;
 336             else if (cmp > 0)
 337                 high = mid - 1;
 338             else
 339                 return mid; // key found
 340         }
 341         return -(low + 1);  // key not found
 342     }
 343 
 344     private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
 345         int low = 0;
 346         int high = l.size()-1;
 347         ListIterator<? extends T> i = l.listIterator();
 348 
 349         while (low <= high) {
 350             int mid = (low + high) >>> 1;
 351             T midVal = get(i, mid);
 352             int cmp = c.compare(midVal, key);
 353 
 354             if (cmp < 0)
 355                 low = mid + 1;
 356             else if (cmp > 0)
 357                 high = mid - 1;
 358             else
 359                 return mid; // key found
 360         }
 361         return -(low + 1);  // key not found
 362     }
 363 
 364     /**
 365      * Reverses the order of the elements in the specified list.<p>
 366      *
 367      * This method runs in linear time.
 368      *
 369      * @param  list the list whose elements are to be reversed.
 370      * @throws UnsupportedOperationException if the specified list or
 371      *         its list-iterator does not support the {@code set} operation.
 372      */
 373     @SuppressWarnings({"rawtypes", "unchecked"})
 374     public static void reverse(List<?> list) {
 375         int size = list.size();
 376         if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {
 377             for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)
 378                 swap(list, i, j);
 379         } else {
 380             // instead of using a raw type here, it's possible to capture
 381             // the wildcard but it will require a call to a supplementary
 382             // private method
 383             ListIterator fwd = list.listIterator();
 384             ListIterator rev = list.listIterator(size);
 385             for (int i=0, mid=list.size()>>1; i<mid; i++) {
 386                 Object tmp = fwd.next();
 387                 fwd.set(rev.previous());
 388                 rev.set(tmp);
 389             }
 390         }
 391     }
 392 
 393     /**
 394      * Randomly permutes the specified list using a default source of
 395      * randomness.  All permutations occur with approximately equal
 396      * likelihood.
 397      *
 398      * <p>The hedge "approximately" is used in the foregoing description because
 399      * default source of randomness is only approximately an unbiased source
 400      * of independently chosen bits. If it were a perfect source of randomly
 401      * chosen bits, then the algorithm would choose permutations with perfect
 402      * uniformity.
 403      *
 404      * <p>This implementation traverses the list backwards, from the last
 405      * element up to the second, repeatedly swapping a randomly selected element
 406      * into the "current position".  Elements are randomly selected from the
 407      * portion of the list that runs from the first element to the current
 408      * position, inclusive.
 409      *
 410      * <p>This method runs in linear time.  If the specified list does not
 411      * implement the {@link RandomAccess} interface and is large, this
 412      * implementation dumps the specified list into an array before shuffling
 413      * it, and dumps the shuffled array back into the list.  This avoids the
 414      * quadratic behavior that would result from shuffling a "sequential
 415      * access" list in place.
 416      *
 417      * @param  list the list to be shuffled.
 418      * @throws UnsupportedOperationException if the specified list or
 419      *         its list-iterator does not support the {@code set} operation.
 420      */
 421     public static void shuffle(List<?> list) {
 422         Random rnd = r;
 423         if (rnd == null)
 424             r = rnd = new Random(); // harmless race.
 425         shuffle(list, rnd);
 426     }
 427 
 428     private static Random r;
 429 
 430     /**
 431      * Randomly permute the specified list using the specified source of
 432      * randomness.  All permutations occur with equal likelihood
 433      * assuming that the source of randomness is fair.<p>
 434      *
 435      * This implementation traverses the list backwards, from the last element
 436      * up to the second, repeatedly swapping a randomly selected element into
 437      * the "current position".  Elements are randomly selected from the
 438      * portion of the list that runs from the first element to the current
 439      * position, inclusive.<p>
 440      *
 441      * This method runs in linear time.  If the specified list does not
 442      * implement the {@link RandomAccess} interface and is large, this
 443      * implementation dumps the specified list into an array before shuffling
 444      * it, and dumps the shuffled array back into the list.  This avoids the
 445      * quadratic behavior that would result from shuffling a "sequential
 446      * access" list in place.
 447      *
 448      * @param  list the list to be shuffled.
 449      * @param  rnd the source of randomness to use to shuffle the list.
 450      * @throws UnsupportedOperationException if the specified list or its
 451      *         list-iterator does not support the {@code set} operation.
 452      */
 453     @SuppressWarnings({"rawtypes", "unchecked"})
 454     public static void shuffle(List<?> list, Random rnd) {
 455         int size = list.size();
 456         if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
 457             for (int i=size; i>1; i--)
 458                 swap(list, i-1, rnd.nextInt(i));
 459         } else {
 460             Object arr[] = list.toArray();
 461 
 462             // Shuffle array
 463             for (int i=size; i>1; i--)
 464                 swap(arr, i-1, rnd.nextInt(i));
 465 
 466             // Dump array back into list
 467             // instead of using a raw type here, it's possible to capture
 468             // the wildcard but it will require a call to a supplementary
 469             // private method
 470             ListIterator it = list.listIterator();
 471             for (Object e : arr) {
 472                 it.next();
 473                 it.set(e);
 474             }
 475         }
 476     }
 477 
 478     /**
 479      * Swaps the elements at the specified positions in the specified list.
 480      * (If the specified positions are equal, invoking this method leaves
 481      * the list unchanged.)
 482      *
 483      * @param list The list in which to swap elements.
 484      * @param i the index of one element to be swapped.
 485      * @param j the index of the other element to be swapped.
 486      * @throws IndexOutOfBoundsException if either {@code i} or {@code j}
 487      *         is out of range (i &lt; 0 || i &gt;= list.size()
 488      *         || j &lt; 0 || j &gt;= list.size()).
 489      * @since 1.4
 490      */
 491     @SuppressWarnings({"rawtypes", "unchecked"})
 492     public static void swap(List<?> list, int i, int j) {
 493         // instead of using a raw type here, it's possible to capture
 494         // the wildcard but it will require a call to a supplementary
 495         // private method
 496         final List l = list;
 497         l.set(i, l.set(j, l.get(i)));
 498     }
 499 
 500     /**
 501      * Swaps the two specified elements in the specified array.
 502      */
 503     private static void swap(Object[] arr, int i, int j) {
 504         Object tmp = arr[i];
 505         arr[i] = arr[j];
 506         arr[j] = tmp;
 507     }
 508 
 509     /**
 510      * Replaces all of the elements of the specified list with the specified
 511      * element. <p>
 512      *
 513      * This method runs in linear time.
 514      *
 515      * @param  <T> the class of the objects in the list
 516      * @param  list the list to be filled with the specified element.
 517      * @param  obj The element with which to fill the specified list.
 518      * @throws UnsupportedOperationException if the specified list or its
 519      *         list-iterator does not support the {@code set} operation.
 520      */
 521     public static <T> void fill(List<? super T> list, T obj) {
 522         int size = list.size();
 523 
 524         if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
 525             for (int i=0; i<size; i++)
 526                 list.set(i, obj);
 527         } else {
 528             ListIterator<? super T> itr = list.listIterator();
 529             for (int i=0; i<size; i++) {
 530                 itr.next();
 531                 itr.set(obj);
 532             }
 533         }
 534     }
 535 
 536     /**
 537      * Copies all of the elements from one list into another.  After the
 538      * operation, the index of each copied element in the destination list
 539      * will be identical to its index in the source list.  The destination
 540      * list's size must be greater than or equal to the source list's size.
 541      * If it is greater, the remaining elements in the destination list are
 542      * unaffected. <p>
 543      *
 544      * This method runs in linear time.
 545      *
 546      * @param  <T> the class of the objects in the lists
 547      * @param  dest The destination list.
 548      * @param  src The source list.
 549      * @throws IndexOutOfBoundsException if the destination list is too small
 550      *         to contain the entire source List.
 551      * @throws UnsupportedOperationException if the destination list's
 552      *         list-iterator does not support the {@code set} operation.
 553      */
 554     public static <T> void copy(List<? super T> dest, List<? extends T> src) {
 555         int srcSize = src.size();
 556         if (srcSize > dest.size())
 557             throw new IndexOutOfBoundsException("Source does not fit in dest");
 558 
 559         if (srcSize < COPY_THRESHOLD ||
 560             (src instanceof RandomAccess && dest instanceof RandomAccess)) {
 561             for (int i=0; i<srcSize; i++)
 562                 dest.set(i, src.get(i));
 563         } else {
 564             ListIterator<? super T> di=dest.listIterator();
 565             ListIterator<? extends T> si=src.listIterator();
 566             for (int i=0; i<srcSize; i++) {
 567                 di.next();
 568                 di.set(si.next());
 569             }
 570         }
 571     }
 572 
 573     /**
 574      * Returns the minimum element of the given collection, according to the
 575      * <i>natural ordering</i> of its elements.  All elements in the
 576      * collection must implement the {@code Comparable} interface.
 577      * Furthermore, all elements in the collection must be <i>mutually
 578      * comparable</i> (that is, {@code e1.compareTo(e2)} must not throw a
 579      * {@code ClassCastException} for any elements {@code e1} and
 580      * {@code e2} in the collection).<p>
 581      *
 582      * This method iterates over the entire collection, hence it requires
 583      * time proportional to the size of the collection.
 584      *
 585      * @param  <T> the class of the objects in the collection
 586      * @param  coll the collection whose minimum element is to be determined.
 587      * @return the minimum element of the given collection, according
 588      *         to the <i>natural ordering</i> of its elements.
 589      * @throws ClassCastException if the collection contains elements that are
 590      *         not <i>mutually comparable</i> (for example, strings and
 591      *         integers).
 592      * @throws NoSuchElementException if the collection is empty.
 593      * @see Comparable
 594      */
 595     public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {
 596         Iterator<? extends T> i = coll.iterator();
 597         T candidate = i.next();
 598 
 599         while (i.hasNext()) {
 600             T next = i.next();
 601             if (next.compareTo(candidate) < 0)
 602                 candidate = next;
 603         }
 604         return candidate;
 605     }
 606 
 607     /**
 608      * Returns the minimum element of the given collection, according to the
 609      * order induced by the specified comparator.  All elements in the
 610      * collection must be <i>mutually comparable</i> by the specified
 611      * comparator (that is, {@code comp.compare(e1, e2)} must not throw a
 612      * {@code ClassCastException} for any elements {@code e1} and
 613      * {@code e2} in the collection).<p>
 614      *
 615      * This method iterates over the entire collection, hence it requires
 616      * time proportional to the size of the collection.
 617      *
 618      * @param  <T> the class of the objects in the collection
 619      * @param  coll the collection whose minimum element is to be determined.
 620      * @param  comp the comparator with which to determine the minimum element.
 621      *         A {@code null} value indicates that the elements' <i>natural
 622      *         ordering</i> should be used.
 623      * @return the minimum element of the given collection, according
 624      *         to the specified comparator.
 625      * @throws ClassCastException if the collection contains elements that are
 626      *         not <i>mutually comparable</i> using the specified comparator.
 627      * @throws NoSuchElementException if the collection is empty.
 628      * @see Comparable
 629      */
 630     @SuppressWarnings({"unchecked", "rawtypes"})
 631     public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
 632         if (comp==null)
 633             return (T)min((Collection) coll);
 634 
 635         Iterator<? extends T> i = coll.iterator();
 636         T candidate = i.next();
 637 
 638         while (i.hasNext()) {
 639             T next = i.next();
 640             if (comp.compare(next, candidate) < 0)
 641                 candidate = next;
 642         }
 643         return candidate;
 644     }
 645 
 646     /**
 647      * Returns the maximum element of the given collection, according to the
 648      * <i>natural ordering</i> of its elements.  All elements in the
 649      * collection must implement the {@code Comparable} interface.
 650      * Furthermore, all elements in the collection must be <i>mutually
 651      * comparable</i> (that is, {@code e1.compareTo(e2)} must not throw a
 652      * {@code ClassCastException} for any elements {@code e1} and
 653      * {@code e2} in the collection).<p>
 654      *
 655      * This method iterates over the entire collection, hence it requires
 656      * time proportional to the size of the collection.
 657      *
 658      * @param  <T> the class of the objects in the collection
 659      * @param  coll the collection whose maximum element is to be determined.
 660      * @return the maximum element of the given collection, according
 661      *         to the <i>natural ordering</i> of its elements.
 662      * @throws ClassCastException if the collection contains elements that are
 663      *         not <i>mutually comparable</i> (for example, strings and
 664      *         integers).
 665      * @throws NoSuchElementException if the collection is empty.
 666      * @see Comparable
 667      */
 668     public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {
 669         Iterator<? extends T> i = coll.iterator();
 670         T candidate = i.next();
 671 
 672         while (i.hasNext()) {
 673             T next = i.next();
 674             if (next.compareTo(candidate) > 0)
 675                 candidate = next;
 676         }
 677         return candidate;
 678     }
 679 
 680     /**
 681      * Returns the maximum element of the given collection, according to the
 682      * order induced by the specified comparator.  All elements in the
 683      * collection must be <i>mutually comparable</i> by the specified
 684      * comparator (that is, {@code comp.compare(e1, e2)} must not throw a
 685      * {@code ClassCastException} for any elements {@code e1} and
 686      * {@code e2} in the collection).<p>
 687      *
 688      * This method iterates over the entire collection, hence it requires
 689      * time proportional to the size of the collection.
 690      *
 691      * @param  <T> the class of the objects in the collection
 692      * @param  coll the collection whose maximum element is to be determined.
 693      * @param  comp the comparator with which to determine the maximum element.
 694      *         A {@code null} value indicates that the elements' <i>natural
 695      *        ordering</i> should be used.
 696      * @return the maximum element of the given collection, according
 697      *         to the specified comparator.
 698      * @throws ClassCastException if the collection contains elements that are
 699      *         not <i>mutually comparable</i> using the specified comparator.
 700      * @throws NoSuchElementException if the collection is empty.
 701      * @see Comparable
 702      */
 703     @SuppressWarnings({"unchecked", "rawtypes"})
 704     public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
 705         if (comp==null)
 706             return (T)max((Collection) coll);
 707 
 708         Iterator<? extends T> i = coll.iterator();
 709         T candidate = i.next();
 710 
 711         while (i.hasNext()) {
 712             T next = i.next();
 713             if (comp.compare(next, candidate) > 0)
 714                 candidate = next;
 715         }
 716         return candidate;
 717     }
 718 
 719     /**
 720      * Rotates the elements in the specified list by the specified distance.
 721      * After calling this method, the element at index {@code i} will be
 722      * the element previously at index {@code (i - distance)} mod
 723      * {@code list.size()}, for all values of {@code i} between {@code 0}
 724      * and {@code list.size()-1}, inclusive.  (This method has no effect on
 725      * the size of the list.)
 726      *
 727      * <p>For example, suppose {@code list} comprises{@code  [t, a, n, k, s]}.
 728      * After invoking {@code Collections.rotate(list, 1)} (or
 729      * {@code Collections.rotate(list, -4)}), {@code list} will comprise
 730      * {@code [s, t, a, n, k]}.
 731      *
 732      * <p>Note that this method can usefully be applied to sublists to
 733      * move one or more elements within a list while preserving the
 734      * order of the remaining elements.  For example, the following idiom
 735      * moves the element at index {@code j} forward to position
 736      * {@code k} (which must be greater than or equal to {@code j}):
 737      * <pre>
 738      *     Collections.rotate(list.subList(j, k+1), -1);
 739      * </pre>
 740      * To make this concrete, suppose {@code list} comprises
 741      * {@code [a, b, c, d, e]}.  To move the element at index {@code 1}
 742      * ({@code b}) forward two positions, perform the following invocation:
 743      * <pre>
 744      *     Collections.rotate(l.subList(1, 4), -1);
 745      * </pre>
 746      * The resulting list is {@code [a, c, d, b, e]}.
 747      *
 748      * <p>To move more than one element forward, increase the absolute value
 749      * of the rotation distance.  To move elements backward, use a positive
 750      * shift distance.
 751      *
 752      * <p>If the specified list is small or implements the {@link
 753      * RandomAccess} interface, this implementation exchanges the first
 754      * element into the location it should go, and then repeatedly exchanges
 755      * the displaced element into the location it should go until a displaced
 756      * element is swapped into the first element.  If necessary, the process
 757      * is repeated on the second and successive elements, until the rotation
 758      * is complete.  If the specified list is large and doesn't implement the
 759      * {@code RandomAccess} interface, this implementation breaks the
 760      * list into two sublist views around index {@code -distance mod size}.
 761      * Then the {@link #reverse(List)} method is invoked on each sublist view,
 762      * and finally it is invoked on the entire list.  For a more complete
 763      * description of both algorithms, see Section 2.3 of Jon Bentley's
 764      * <i>Programming Pearls</i> (Addison-Wesley, 1986).
 765      *
 766      * @param list the list to be rotated.
 767      * @param distance the distance to rotate the list.  There are no
 768      *        constraints on this value; it may be zero, negative, or
 769      *        greater than {@code list.size()}.
 770      * @throws UnsupportedOperationException if the specified list or
 771      *         its list-iterator does not support the {@code set} operation.
 772      * @since 1.4
 773      */
 774     public static void rotate(List<?> list, int distance) {
 775         if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD)
 776             rotate1(list, distance);
 777         else
 778             rotate2(list, distance);
 779     }
 780 
 781     private static <T> void rotate1(List<T> list, int distance) {
 782         int size = list.size();
 783         if (size == 0)
 784             return;
 785         distance = distance % size;
 786         if (distance < 0)
 787             distance += size;
 788         if (distance == 0)
 789             return;
 790 
 791         for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {
 792             T displaced = list.get(cycleStart);
 793             int i = cycleStart;
 794             do {
 795                 i += distance;
 796                 if (i >= size)
 797                     i -= size;
 798                 displaced = list.set(i, displaced);
 799                 nMoved ++;
 800             } while (i != cycleStart);
 801         }
 802     }
 803 
 804     private static void rotate2(List<?> list, int distance) {
 805         int size = list.size();
 806         if (size == 0)
 807             return;
 808         int mid =  -distance % size;
 809         if (mid < 0)
 810             mid += size;
 811         if (mid == 0)
 812             return;
 813 
 814         reverse(list.subList(0, mid));
 815         reverse(list.subList(mid, size));
 816         reverse(list);
 817     }
 818 
 819     /**
 820      * Replaces all occurrences of one specified value in a list with another.
 821      * More formally, replaces with {@code newVal} each element {@code e}
 822      * in {@code list} such that
 823      * {@code (oldVal==null ? e==null : oldVal.equals(e))}.
 824      * (This method has no effect on the size of the list.)
 825      *
 826      * @param  <T> the class of the objects in the list
 827      * @param list the list in which replacement is to occur.
 828      * @param oldVal the old value to be replaced.
 829      * @param newVal the new value with which {@code oldVal} is to be
 830      *        replaced.
 831      * @return {@code true} if {@code list} contained one or more elements
 832      *         {@code e} such that
 833      *         {@code (oldVal==null ?  e==null : oldVal.equals(e))}.
 834      * @throws UnsupportedOperationException if the specified list or
 835      *         its list-iterator does not support the {@code set} operation.
 836      * @since  1.4
 837      */
 838     public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
 839         boolean result = false;
 840         int size = list.size();
 841         if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
 842             if (oldVal==null) {
 843                 for (int i=0; i<size; i++) {
 844                     if (list.get(i)==null) {
 845                         list.set(i, newVal);
 846                         result = true;
 847                     }
 848                 }
 849             } else {
 850                 for (int i=0; i<size; i++) {
 851                     if (oldVal.equals(list.get(i))) {
 852                         list.set(i, newVal);
 853                         result = true;
 854                     }
 855                 }
 856             }
 857         } else {
 858             ListIterator<T> itr=list.listIterator();
 859             if (oldVal==null) {
 860                 for (int i=0; i<size; i++) {
 861                     if (itr.next()==null) {
 862                         itr.set(newVal);
 863                         result = true;
 864                     }
 865                 }
 866             } else {
 867                 for (int i=0; i<size; i++) {
 868                     if (oldVal.equals(itr.next())) {
 869                         itr.set(newVal);
 870                         result = true;
 871                     }
 872                 }
 873             }
 874         }
 875         return result;
 876     }
 877 
 878     /**
 879      * Returns the starting position of the first occurrence of the specified
 880      * target list within the specified source list, or -1 if there is no
 881      * such occurrence.  More formally, returns the lowest index {@code i}
 882      * such that {@code source.subList(i, i+target.size()).equals(target)},
 883      * or -1 if there is no such index.  (Returns -1 if
 884      * {@code target.size() > source.size()})
 885      *
 886      * <p>This implementation uses the "brute force" technique of scanning
 887      * over the source list, looking for a match with the target at each
 888      * location in turn.
 889      *
 890      * @param source the list in which to search for the first occurrence
 891      *        of {@code target}.
 892      * @param target the list to search for as a subList of {@code source}.
 893      * @return the starting position of the first occurrence of the specified
 894      *         target list within the specified source list, or -1 if there
 895      *         is no such occurrence.
 896      * @since  1.4
 897      */
 898     public static int indexOfSubList(List<?> source, List<?> target) {
 899         int sourceSize = source.size();
 900         int targetSize = target.size();
 901         int maxCandidate = sourceSize - targetSize;
 902 
 903         if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
 904             (source instanceof RandomAccess&&target instanceof RandomAccess)) {
 905         nextCand:
 906             for (int candidate = 0; candidate <= maxCandidate; candidate++) {
 907                 for (int i=0, j=candidate; i<targetSize; i++, j++)
 908                     if (!eq(target.get(i), source.get(j)))
 909                         continue nextCand;  // Element mismatch, try next cand
 910                 return candidate;  // All elements of candidate matched target
 911             }
 912         } else {  // Iterator version of above algorithm
 913             ListIterator<?> si = source.listIterator();
 914         nextCand:
 915             for (int candidate = 0; candidate <= maxCandidate; candidate++) {
 916                 ListIterator<?> ti = target.listIterator();
 917                 for (int i=0; i<targetSize; i++) {
 918                     if (!eq(ti.next(), si.next())) {
 919                         // Back up source iterator to next candidate
 920                         for (int j=0; j<i; j++)
 921                             si.previous();
 922                         continue nextCand;
 923                     }
 924                 }
 925                 return candidate;
 926             }
 927         }
 928         return -1;  // No candidate matched the target
 929     }
 930 
 931     /**
 932      * Returns the starting position of the last occurrence of the specified
 933      * target list within the specified source list, or -1 if there is no such
 934      * occurrence.  More formally, returns the highest index {@code i}
 935      * such that {@code source.subList(i, i+target.size()).equals(target)},
 936      * or -1 if there is no such index.  (Returns -1 if
 937      * {@code target.size() > source.size()})
 938      *
 939      * <p>This implementation uses the "brute force" technique of iterating
 940      * over the source list, looking for a match with the target at each
 941      * location in turn.
 942      *
 943      * @param source the list in which to search for the last occurrence
 944      *        of {@code target}.
 945      * @param target the list to search for as a subList of {@code source}.
 946      * @return the starting position of the last occurrence of the specified
 947      *         target list within the specified source list, or -1 if there
 948      *         is no such occurrence.
 949      * @since  1.4
 950      */
 951     public static int lastIndexOfSubList(List<?> source, List<?> target) {
 952         int sourceSize = source.size();
 953         int targetSize = target.size();
 954         int maxCandidate = sourceSize - targetSize;
 955 
 956         if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
 957             source instanceof RandomAccess) {   // Index access version
 958         nextCand:
 959             for (int candidate = maxCandidate; candidate >= 0; candidate--) {
 960                 for (int i=0, j=candidate; i<targetSize; i++, j++)
 961                     if (!eq(target.get(i), source.get(j)))
 962                         continue nextCand;  // Element mismatch, try next cand
 963                 return candidate;  // All elements of candidate matched target
 964             }
 965         } else {  // Iterator version of above algorithm
 966             if (maxCandidate < 0)
 967                 return -1;
 968             ListIterator<?> si = source.listIterator(maxCandidate);
 969         nextCand:
 970             for (int candidate = maxCandidate; candidate >= 0; candidate--) {
 971                 ListIterator<?> ti = target.listIterator();
 972                 for (int i=0; i<targetSize; i++) {
 973                     if (!eq(ti.next(), si.next())) {
 974                         if (candidate != 0) {
 975                             // Back up source iterator to next candidate
 976                             for (int j=0; j<=i+1; j++)
 977                                 si.previous();
 978                         }
 979                         continue nextCand;
 980                     }
 981                 }
 982                 return candidate;
 983             }
 984         }
 985         return -1;  // No candidate matched the target
 986     }
 987 
 988 
 989     // Unmodifiable Wrappers
 990 
 991     /**
 992      * Returns an unmodifiable view of the specified collection.  This method
 993      * allows modules to provide users with "read-only" access to internal
 994      * collections.  Query operations on the returned collection "read through"
 995      * to the specified collection, and attempts to modify the returned
 996      * collection, whether direct or via its iterator, result in an
 997      * {@code UnsupportedOperationException}.<p>
 998      *
 999      * The returned collection does <i>not</i> pass the hashCode and equals
1000      * operations through to the backing collection, but relies on
1001      * {@code Object}'s {@code equals} and {@code hashCode} methods.  This
1002      * is necessary to preserve the contracts of these operations in the case
1003      * that the backing collection is a set or a list.<p>
1004      *
1005      * The returned collection will be serializable if the specified collection
1006      * is serializable.
1007      *
1008      * @param  <T> the class of the objects in the collection
1009      * @param  c the collection for which an unmodifiable view is to be
1010      *         returned.
1011      * @return an unmodifiable view of the specified collection.
1012      */
1013     public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
1014         return new UnmodifiableCollection<>(c);
1015     }
1016 
1017     /**
1018      * @serial include
1019      */
1020     static class UnmodifiableCollection<E> implements Collection<E>, Serializable {
1021         private static final long serialVersionUID = 1820017752578914078L;
1022 
1023         final Collection<? extends E> c;
1024 
1025         UnmodifiableCollection(Collection<? extends E> c) {
1026             if (c==null)
1027                 throw new NullPointerException();
1028             this.c = c;
1029         }
1030 
1031         public int size()                   {return c.size();}
1032         public boolean isEmpty()            {return c.isEmpty();}
1033         public boolean contains(Object o)   {return c.contains(o);}
1034         public Object[] toArray()           {return c.toArray();}
1035         public <T> T[] toArray(T[] a)       {return c.toArray(a);}
1036         public String toString()            {return c.toString();}
1037 
1038         public Iterator<E> iterator() {
1039             return new Iterator<E>() {
1040                 private final Iterator<? extends E> i = c.iterator();
1041 
1042                 public boolean hasNext() {return i.hasNext();}
1043                 public E next()          {return i.next();}
1044                 public void remove() {
1045                     throw new UnsupportedOperationException();
1046                 }
1047                 @Override
1048                 public void forEachRemaining(Consumer<? super E> action) {
1049                     // Use backing collection version
1050                     i.forEachRemaining(action);
1051                 }
1052             };
1053         }
1054 
1055         public boolean add(E e) {
1056             throw new UnsupportedOperationException();
1057         }
1058         public boolean remove(Object o) {
1059             throw new UnsupportedOperationException();
1060         }
1061 
1062         public boolean containsAll(Collection<?> coll) {
1063             return c.containsAll(coll);
1064         }
1065         public boolean addAll(Collection<? extends E> coll) {
1066             throw new UnsupportedOperationException();
1067         }
1068         public boolean removeAll(Collection<?> coll) {
1069             throw new UnsupportedOperationException();
1070         }
1071         public boolean retainAll(Collection<?> coll) {
1072             throw new UnsupportedOperationException();
1073         }
1074         public void clear() {
1075             throw new UnsupportedOperationException();
1076         }
1077 
1078         // Override default methods in Collection
1079         @Override
1080         public void forEach(Consumer<? super E> action) {
1081             c.forEach(action);
1082         }
1083         @Override
1084         public boolean removeIf(Predicate<? super E> filter) {
1085             throw new UnsupportedOperationException();
1086         }
1087         @SuppressWarnings("unchecked")
1088         @Override
1089         public Spliterator<E> spliterator() {
1090             return (Spliterator<E>)c.spliterator();
1091         }
1092         @SuppressWarnings("unchecked")
1093         @Override
1094         public Stream<E> stream() {
1095             return (Stream<E>)c.stream();
1096         }
1097         @SuppressWarnings("unchecked")
1098         @Override
1099         public Stream<E> parallelStream() {
1100             return (Stream<E>)c.parallelStream();
1101         }
1102     }
1103 
1104     /**
1105      * Returns an unmodifiable view of the specified set.  This method allows
1106      * modules to provide users with "read-only" access to internal sets.
1107      * Query operations on the returned set "read through" to the specified
1108      * set, and attempts to modify the returned set, whether direct or via its
1109      * iterator, result in an {@code UnsupportedOperationException}.<p>
1110      *
1111      * The returned set will be serializable if the specified set
1112      * is serializable.
1113      *
1114      * @param  <T> the class of the objects in the set
1115      * @param  s the set for which an unmodifiable view is to be returned.
1116      * @return an unmodifiable view of the specified set.
1117      */
1118     public static <T> Set<T> unmodifiableSet(Set<? extends T> s) {
1119         return new UnmodifiableSet<>(s);
1120     }
1121 
1122     /**
1123      * @serial include
1124      */
1125     static class UnmodifiableSet<E> extends UnmodifiableCollection<E>
1126                                  implements Set<E>, Serializable {
1127         private static final long serialVersionUID = -9215047833775013803L;
1128 
1129         UnmodifiableSet(Set<? extends E> s)     {super(s);}
1130         public boolean equals(Object o) {return o == this || c.equals(o);}
1131         public int hashCode()           {return c.hashCode();}
1132     }
1133 
1134     /**
1135      * Returns an unmodifiable view of the specified sorted set.  This method
1136      * allows modules to provide users with "read-only" access to internal
1137      * sorted sets.  Query operations on the returned sorted set "read
1138      * through" to the specified sorted set.  Attempts to modify the returned
1139      * sorted set, whether direct, via its iterator, or via its
1140      * {@code subSet}, {@code headSet}, or {@code tailSet} views, result in
1141      * an {@code UnsupportedOperationException}.<p>
1142      *
1143      * The returned sorted set will be serializable if the specified sorted set
1144      * is serializable.
1145      *
1146      * @param  <T> the class of the objects in the set
1147      * @param s the sorted set for which an unmodifiable view is to be
1148      *        returned.
1149      * @return an unmodifiable view of the specified sorted set.
1150      */
1151     public static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s) {
1152         return new UnmodifiableSortedSet<>(s);
1153     }
1154 
1155     /**
1156      * @serial include
1157      */
1158     static class UnmodifiableSortedSet<E>
1159                              extends UnmodifiableSet<E>
1160                              implements SortedSet<E>, Serializable {
1161         private static final long serialVersionUID = -4929149591599911165L;
1162         private final SortedSet<E> ss;
1163 
1164         UnmodifiableSortedSet(SortedSet<E> s) {super(s); ss = s;}
1165 
1166         public Comparator<? super E> comparator() {return ss.comparator();}
1167 
1168         public SortedSet<E> subSet(E fromElement, E toElement) {
1169             return new UnmodifiableSortedSet<>(ss.subSet(fromElement,toElement));
1170         }
1171         public SortedSet<E> headSet(E toElement) {
1172             return new UnmodifiableSortedSet<>(ss.headSet(toElement));
1173         }
1174         public SortedSet<E> tailSet(E fromElement) {
1175             return new UnmodifiableSortedSet<>(ss.tailSet(fromElement));
1176         }
1177 
1178         public E first()                   {return ss.first();}
1179         public E last()                    {return ss.last();}
1180     }
1181 
1182     /**
1183      * Returns an unmodifiable view of the specified navigable set.  This method
1184      * allows modules to provide users with "read-only" access to internal
1185      * navigable sets.  Query operations on the returned navigable set "read
1186      * through" to the specified navigable set.  Attempts to modify the returned
1187      * navigable set, whether direct, via its iterator, or via its
1188      * {@code subSet}, {@code headSet}, or {@code tailSet} views, result in
1189      * an {@code UnsupportedOperationException}.<p>
1190      *
1191      * The returned navigable set will be serializable if the specified
1192      * navigable set is serializable.
1193      *
1194      * @param  <T> the class of the objects in the set
1195      * @param s the navigable set for which an unmodifiable view is to be
1196      *        returned
1197      * @return an unmodifiable view of the specified navigable set
1198      * @since 1.8
1199      */
1200     public static <T> NavigableSet<T> unmodifiableNavigableSet(NavigableSet<T> s) {
1201         return new UnmodifiableNavigableSet<>(s);
1202     }
1203 
1204     /**
1205      * Wraps a navigable set and disables all of the mutative operations.
1206      *
1207      * @param <E> type of elements
1208      * @serial include
1209      */
1210     static class UnmodifiableNavigableSet<E>
1211                              extends UnmodifiableSortedSet<E>
1212                              implements NavigableSet<E>, Serializable {
1213 
1214         private static final long serialVersionUID = -6027448201786391929L;
1215 
1216         /**
1217          * A singleton empty unmodifiable navigable set used for
1218          * {@link #emptyNavigableSet()}.
1219          *
1220          * @param <E> type of elements, if there were any, and bounds
1221          */
1222         private static class EmptyNavigableSet<E> extends UnmodifiableNavigableSet<E>
1223             implements Serializable {
1224             private static final long serialVersionUID = -6291252904449939134L;
1225 
1226             public EmptyNavigableSet() {
1227                 super(new TreeSet<>());
1228             }
1229 
1230             private Object readResolve()        { return EMPTY_NAVIGABLE_SET; }
1231         }
1232 
1233         @SuppressWarnings("rawtypes")
1234         private static final NavigableSet<?> EMPTY_NAVIGABLE_SET =
1235                 new EmptyNavigableSet<>();
1236 
1237         /**
1238          * The instance we are protecting.
1239          */
1240         private final NavigableSet<E> ns;
1241 
1242         UnmodifiableNavigableSet(NavigableSet<E> s)         {super(s); ns = s;}
1243 
1244         public E lower(E e)                             { return ns.lower(e); }
1245         public E floor(E e)                             { return ns.floor(e); }
1246         public E ceiling(E e)                         { return ns.ceiling(e); }
1247         public E higher(E e)                           { return ns.higher(e); }
1248         public E pollFirst()     { throw new UnsupportedOperationException(); }
1249         public E pollLast()      { throw new UnsupportedOperationException(); }
1250         public NavigableSet<E> descendingSet()
1251                  { return new UnmodifiableNavigableSet<>(ns.descendingSet()); }
1252         public Iterator<E> descendingIterator()
1253                                          { return descendingSet().iterator(); }
1254 
1255         public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
1256             return new UnmodifiableNavigableSet<>(
1257                 ns.subSet(fromElement, fromInclusive, toElement, toInclusive));
1258         }
1259 
1260         public NavigableSet<E> headSet(E toElement, boolean inclusive) {
1261             return new UnmodifiableNavigableSet<>(
1262                 ns.headSet(toElement, inclusive));
1263         }
1264 
1265         public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
1266             return new UnmodifiableNavigableSet<>(
1267                 ns.tailSet(fromElement, inclusive));
1268         }
1269     }
1270 
1271     /**
1272      * Returns an unmodifiable view of the specified list.  This method allows
1273      * modules to provide users with "read-only" access to internal
1274      * lists.  Query operations on the returned list "read through" to the
1275      * specified list, and attempts to modify the returned list, whether
1276      * direct or via its iterator, result in an
1277      * {@code UnsupportedOperationException}.<p>
1278      *
1279      * The returned list will be serializable if the specified list
1280      * is serializable. Similarly, the returned list will implement
1281      * {@link RandomAccess} if the specified list does.
1282      *
1283      * @param  <T> the class of the objects in the list
1284      * @param  list the list for which an unmodifiable view is to be returned.
1285      * @return an unmodifiable view of the specified list.
1286      */
1287     public static <T> List<T> unmodifiableList(List<? extends T> list) {
1288         return (list instanceof RandomAccess ?
1289                 new UnmodifiableRandomAccessList<>(list) :
1290                 new UnmodifiableList<>(list));
1291     }
1292 
1293     /**
1294      * @serial include
1295      */
1296     static class UnmodifiableList<E> extends UnmodifiableCollection<E>
1297                                   implements List<E> {
1298         private static final long serialVersionUID = -283967356065247728L;
1299 
1300         final List<? extends E> list;
1301 
1302         UnmodifiableList(List<? extends E> list) {
1303             super(list);
1304             this.list = list;
1305         }
1306 
1307         public boolean equals(Object o) {return o == this || list.equals(o);}
1308         public int hashCode()           {return list.hashCode();}
1309 
1310         public E get(int index) {return list.get(index);}
1311         public E set(int index, E element) {
1312             throw new UnsupportedOperationException();
1313         }
1314         public void add(int index, E element) {
1315             throw new UnsupportedOperationException();
1316         }
1317         public E remove(int index) {
1318             throw new UnsupportedOperationException();
1319         }
1320         public int indexOf(Object o)            {return list.indexOf(o);}
1321         public int lastIndexOf(Object o)        {return list.lastIndexOf(o);}
1322         public boolean addAll(int index, Collection<? extends E> c) {
1323             throw new UnsupportedOperationException();
1324         }
1325 
1326         @Override
1327         public void replaceAll(UnaryOperator<E> operator) {
1328             throw new UnsupportedOperationException();
1329         }
1330         @Override
1331         public void sort(Comparator<? super E> c) {
1332             throw new UnsupportedOperationException();
1333         }
1334 
1335         public ListIterator<E> listIterator()   {return listIterator(0);}
1336 
1337         public ListIterator<E> listIterator(final int index) {
1338             return new ListIterator<E>() {
1339                 private final ListIterator<? extends E> i
1340                     = list.listIterator(index);
1341 
1342                 public boolean hasNext()     {return i.hasNext();}
1343                 public E next()              {return i.next();}
1344                 public boolean hasPrevious() {return i.hasPrevious();}
1345                 public E previous()          {return i.previous();}
1346                 public int nextIndex()       {return i.nextIndex();}
1347                 public int previousIndex()   {return i.previousIndex();}
1348 
1349                 public void remove() {
1350                     throw new UnsupportedOperationException();
1351                 }
1352                 public void set(E e) {
1353                     throw new UnsupportedOperationException();
1354                 }
1355                 public void add(E e) {
1356                     throw new UnsupportedOperationException();
1357                 }
1358 
1359                 @Override
1360                 public void forEachRemaining(Consumer<? super E> action) {
1361                     i.forEachRemaining(action);
1362                 }
1363             };
1364         }
1365 
1366         public List<E> subList(int fromIndex, int toIndex) {
1367             return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
1368         }
1369 
1370         /**
1371          * UnmodifiableRandomAccessList instances are serialized as
1372          * UnmodifiableList instances to allow them to be deserialized
1373          * in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
1374          * This method inverts the transformation.  As a beneficial
1375          * side-effect, it also grafts the RandomAccess marker onto
1376          * UnmodifiableList instances that were serialized in pre-1.4 JREs.
1377          *
1378          * Note: Unfortunately, UnmodifiableRandomAccessList instances
1379          * serialized in 1.4.1 and deserialized in 1.4 will become
1380          * UnmodifiableList instances, as this method was missing in 1.4.
1381          */
1382         private Object readResolve() {
1383             return (list instanceof RandomAccess
1384                     ? new UnmodifiableRandomAccessList<>(list)
1385                     : this);
1386         }
1387     }
1388 
1389     /**
1390      * @serial include
1391      */
1392     static class UnmodifiableRandomAccessList<E> extends UnmodifiableList<E>
1393                                               implements RandomAccess
1394     {
1395         UnmodifiableRandomAccessList(List<? extends E> list) {
1396             super(list);
1397         }
1398 
1399         public List<E> subList(int fromIndex, int toIndex) {
1400             return new UnmodifiableRandomAccessList<>(
1401                 list.subList(fromIndex, toIndex));
1402         }
1403 
1404         private static final long serialVersionUID = -2542308836966382001L;
1405 
1406         /**
1407          * Allows instances to be deserialized in pre-1.4 JREs (which do
1408          * not have UnmodifiableRandomAccessList).  UnmodifiableList has
1409          * a readResolve method that inverts this transformation upon
1410          * deserialization.
1411          */
1412         private Object writeReplace() {
1413             return new UnmodifiableList<>(list);
1414         }
1415     }
1416 
1417     /**
1418      * Returns an unmodifiable view of the specified map.  This method
1419      * allows modules to provide users with "read-only" access to internal
1420      * maps.  Query operations on the returned map "read through"
1421      * to the specified map, and attempts to modify the returned
1422      * map, whether direct or via its collection views, result in an
1423      * {@code UnsupportedOperationException}.<p>
1424      *
1425      * The returned map will be serializable if the specified map
1426      * is serializable.
1427      *
1428      * @param <K> the class of the map keys
1429      * @param <V> the class of the map values
1430      * @param  m the map for which an unmodifiable view is to be returned.
1431      * @return an unmodifiable view of the specified map.
1432      */
1433     public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m) {
1434         return new UnmodifiableMap<>(m);
1435     }
1436 
1437     /**
1438      * @serial include
1439      */
1440     private static class UnmodifiableMap<K,V> implements Map<K,V>, Serializable {
1441         private static final long serialVersionUID = -1034234728574286014L;
1442 
1443         private final Map<? extends K, ? extends V> m;
1444 
1445         UnmodifiableMap(Map<? extends K, ? extends V> m) {
1446             if (m==null)
1447                 throw new NullPointerException();
1448             this.m = m;
1449         }
1450 
1451         public int size()                        {return m.size();}
1452         public boolean isEmpty()                 {return m.isEmpty();}
1453         public boolean containsKey(Object key)   {return m.containsKey(key);}
1454         public boolean containsValue(Object val) {return m.containsValue(val);}
1455         public V get(Object key)                 {return m.get(key);}
1456 
1457         public V put(K key, V value) {
1458             throw new UnsupportedOperationException();
1459         }
1460         public V remove(Object key) {
1461             throw new UnsupportedOperationException();
1462         }
1463         public void putAll(Map<? extends K, ? extends V> m) {
1464             throw new UnsupportedOperationException();
1465         }
1466         public void clear() {
1467             throw new UnsupportedOperationException();
1468         }
1469 
1470         private transient Set<K> keySet;
1471         private transient Set<Map.Entry<K,V>> entrySet;
1472         private transient Collection<V> values;
1473 
1474         public Set<K> keySet() {
1475             if (keySet==null)
1476                 keySet = unmodifiableSet(m.keySet());
1477             return keySet;
1478         }
1479 
1480         public Set<Map.Entry<K,V>> entrySet() {
1481             if (entrySet==null)
1482                 entrySet = new UnmodifiableEntrySet<>(m.entrySet());
1483             return entrySet;
1484         }
1485 
1486         public Collection<V> values() {
1487             if (values==null)
1488                 values = unmodifiableCollection(m.values());
1489             return values;
1490         }
1491 
1492         public boolean equals(Object o) {return o == this || m.equals(o);}
1493         public int hashCode()           {return m.hashCode();}
1494         public String toString()        {return m.toString();}
1495 
1496         // Override default methods in Map
1497         @Override
1498         @SuppressWarnings("unchecked")
1499         public V getOrDefault(Object k, V defaultValue) {
1500             // Safe cast as we don't change the value
1501             return ((Map<K, V>)m).getOrDefault(k, defaultValue);
1502         }
1503 
1504         @Override
1505         public void forEach(BiConsumer<? super K, ? super V> action) {
1506             m.forEach(action);
1507         }
1508 
1509         @Override
1510         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
1511             throw new UnsupportedOperationException();
1512         }
1513 
1514         @Override
1515         public V putIfAbsent(K key, V value) {
1516             throw new UnsupportedOperationException();
1517         }
1518 
1519         @Override
1520         public boolean remove(Object key, Object value) {
1521             throw new UnsupportedOperationException();
1522         }
1523 
1524         @Override
1525         public boolean replace(K key, V oldValue, V newValue) {
1526             throw new UnsupportedOperationException();
1527         }
1528 
1529         @Override
1530         public V replace(K key, V value) {
1531             throw new UnsupportedOperationException();
1532         }
1533 
1534         @Override
1535         public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
1536             throw new UnsupportedOperationException();
1537         }
1538 
1539         @Override
1540         public V computeIfPresent(K key,
1541                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1542             throw new UnsupportedOperationException();
1543         }
1544 
1545         @Override
1546         public V compute(K key,
1547                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1548             throw new UnsupportedOperationException();
1549         }
1550 
1551         @Override
1552         public V merge(K key, V value,
1553                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
1554             throw new UnsupportedOperationException();
1555         }
1556 
1557         /**
1558          * We need this class in addition to UnmodifiableSet as
1559          * Map.Entries themselves permit modification of the backing Map
1560          * via their setValue operation.  This class is subtle: there are
1561          * many possible attacks that must be thwarted.
1562          *
1563          * @serial include
1564          */
1565         static class UnmodifiableEntrySet<K,V>
1566             extends UnmodifiableSet<Map.Entry<K,V>> {
1567             private static final long serialVersionUID = 7854390611657943733L;
1568 
1569             @SuppressWarnings({"unchecked", "rawtypes"})
1570             UnmodifiableEntrySet(Set<? extends Map.Entry<? extends K, ? extends V>> s) {
1571                 // Need to cast to raw in order to work around a limitation in the type system
1572                 super((Set)s);
1573             }
1574 
1575             static <K, V> Consumer<Map.Entry<K, V>> entryConsumer(Consumer<? super Entry<K, V>> action) {
1576                 return e -> action.accept(new UnmodifiableEntry<>(e));
1577             }
1578 
1579             public void forEach(Consumer<? super Entry<K, V>> action) {
1580                 Objects.requireNonNull(action);
1581                 c.forEach(entryConsumer(action));
1582             }
1583 
1584             static final class UnmodifiableEntrySetSpliterator<K, V>
1585                     implements Spliterator<Entry<K,V>> {
1586                 final Spliterator<Map.Entry<K, V>> s;
1587 
1588                 UnmodifiableEntrySetSpliterator(Spliterator<Entry<K, V>> s) {
1589                     this.s = s;
1590                 }
1591 
1592                 @Override
1593                 public boolean tryAdvance(Consumer<? super Entry<K, V>> action) {
1594                     Objects.requireNonNull(action);
1595                     return s.tryAdvance(entryConsumer(action));
1596                 }
1597 
1598                 @Override
1599                 public void forEachRemaining(Consumer<? super Entry<K, V>> action) {
1600                     Objects.requireNonNull(action);
1601                     s.forEachRemaining(entryConsumer(action));
1602                 }
1603 
1604                 @Override
1605                 public Spliterator<Entry<K, V>> trySplit() {
1606                     Spliterator<Entry<K, V>> split = s.trySplit();
1607                     return split == null
1608                            ? null
1609                            : new UnmodifiableEntrySetSpliterator<>(split);
1610                 }
1611 
1612                 @Override
1613                 public long estimateSize() {
1614                     return s.estimateSize();
1615                 }
1616 
1617                 @Override
1618                 public long getExactSizeIfKnown() {
1619                     return s.getExactSizeIfKnown();
1620                 }
1621 
1622                 @Override
1623                 public int characteristics() {
1624                     return s.characteristics();
1625                 }
1626 
1627                 @Override
1628                 public boolean hasCharacteristics(int characteristics) {
1629                     return s.hasCharacteristics(characteristics);
1630                 }
1631 
1632                 @Override
1633                 public Comparator<? super Entry<K, V>> getComparator() {
1634                     return s.getComparator();
1635                 }
1636             }
1637 
1638             @SuppressWarnings("unchecked")
1639             public Spliterator<Entry<K,V>> spliterator() {
1640                 return new UnmodifiableEntrySetSpliterator<>(
1641                         (Spliterator<Map.Entry<K, V>>) c.spliterator());
1642             }
1643 
1644             @Override
1645             public Stream<Entry<K,V>> stream() {
1646                 return StreamSupport.stream(spliterator(), false);
1647             }
1648 
1649             @Override
1650             public Stream<Entry<K,V>> parallelStream() {
1651                 return StreamSupport.stream(spliterator(), true);
1652             }
1653 
1654             public Iterator<Map.Entry<K,V>> iterator() {
1655                 return new Iterator<Map.Entry<K,V>>() {
1656                     private final Iterator<? extends Map.Entry<? extends K, ? extends V>> i = c.iterator();
1657 
1658                     public boolean hasNext() {
1659                         return i.hasNext();
1660                     }
1661                     public Map.Entry<K,V> next() {
1662                         return new UnmodifiableEntry<>(i.next());
1663                     }
1664                     public void remove() {
1665                         throw new UnsupportedOperationException();
1666                     }
1667                 };
1668             }
1669 
1670             @SuppressWarnings("unchecked")
1671             public Object[] toArray() {
1672                 Object[] a = c.toArray();
1673                 for (int i=0; i<a.length; i++)
1674                     a[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>)a[i]);
1675                 return a;
1676             }
1677 
1678             @SuppressWarnings("unchecked")
1679             public <T> T[] toArray(T[] a) {
1680                 // We don't pass a to c.toArray, to avoid window of
1681                 // vulnerability wherein an unscrupulous multithreaded client
1682                 // could get his hands on raw (unwrapped) Entries from c.
1683                 Object[] arr = c.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));
1684 
1685                 for (int i=0; i<arr.length; i++)
1686                     arr[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>)arr[i]);
1687 
1688                 if (arr.length > a.length)
1689                     return (T[])arr;
1690 
1691                 System.arraycopy(arr, 0, a, 0, arr.length);
1692                 if (a.length > arr.length)
1693                     a[arr.length] = null;
1694                 return a;
1695             }
1696 
1697             /**
1698              * This method is overridden to protect the backing set against
1699              * an object with a nefarious equals function that senses
1700              * that the equality-candidate is Map.Entry and calls its
1701              * setValue method.
1702              */
1703             public boolean contains(Object o) {
1704                 if (!(o instanceof Map.Entry))
1705                     return false;
1706                 return c.contains(
1707                     new UnmodifiableEntry<>((Map.Entry<?,?>) o));
1708             }
1709 
1710             /**
1711              * The next two methods are overridden to protect against
1712              * an unscrupulous List whose contains(Object o) method senses
1713              * when o is a Map.Entry, and calls o.setValue.
1714              */
1715             public boolean containsAll(Collection<?> coll) {
1716                 for (Object e : coll) {
1717                     if (!contains(e)) // Invokes safe contains() above
1718                         return false;
1719                 }
1720                 return true;
1721             }
1722             public boolean equals(Object o) {
1723                 if (o == this)
1724                     return true;
1725 
1726                 if (!(o instanceof Set))
1727                     return false;
1728                 Set<?> s = (Set<?>) o;
1729                 if (s.size() != c.size())
1730                     return false;
1731                 return containsAll(s); // Invokes safe containsAll() above
1732             }
1733 
1734             /**
1735              * This "wrapper class" serves two purposes: it prevents
1736              * the client from modifying the backing Map, by short-circuiting
1737              * the setValue method, and it protects the backing Map against
1738              * an ill-behaved Map.Entry that attempts to modify another
1739              * Map Entry when asked to perform an equality check.
1740              */
1741             private static class UnmodifiableEntry<K,V> implements Map.Entry<K,V> {
1742                 private Map.Entry<? extends K, ? extends V> e;
1743 
1744                 UnmodifiableEntry(Map.Entry<? extends K, ? extends V> e)
1745                         {this.e = Objects.requireNonNull(e);}
1746 
1747                 public K getKey()        {return e.getKey();}
1748                 public V getValue()      {return e.getValue();}
1749                 public V setValue(V value) {
1750                     throw new UnsupportedOperationException();
1751                 }
1752                 public int hashCode()    {return e.hashCode();}
1753                 public boolean equals(Object o) {
1754                     if (this == o)
1755                         return true;
1756                     if (!(o instanceof Map.Entry))
1757                         return false;
1758                     Map.Entry<?,?> t = (Map.Entry<?,?>)o;
1759                     return eq(e.getKey(),   t.getKey()) &&
1760                            eq(e.getValue(), t.getValue());
1761                 }
1762                 public String toString() {return e.toString();}
1763             }
1764         }
1765     }
1766 
1767     /**
1768      * Returns an unmodifiable view of the specified sorted map.  This method
1769      * allows modules to provide users with "read-only" access to internal
1770      * sorted maps.  Query operations on the returned sorted map "read through"
1771      * to the specified sorted map.  Attempts to modify the returned
1772      * sorted map, whether direct, via its collection views, or via its
1773      * {@code subMap}, {@code headMap}, or {@code tailMap} views, result in
1774      * an {@code UnsupportedOperationException}.<p>
1775      *
1776      * The returned sorted map will be serializable if the specified sorted map
1777      * is serializable.
1778      *
1779      * @param <K> the class of the map keys
1780      * @param <V> the class of the map values
1781      * @param m the sorted map for which an unmodifiable view is to be
1782      *        returned.
1783      * @return an unmodifiable view of the specified sorted map.
1784      */
1785     public static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) {
1786         return new UnmodifiableSortedMap<>(m);
1787     }
1788 
1789     /**
1790      * @serial include
1791      */
1792     static class UnmodifiableSortedMap<K,V>
1793           extends UnmodifiableMap<K,V>
1794           implements SortedMap<K,V>, Serializable {
1795         private static final long serialVersionUID = -8806743815996713206L;
1796 
1797         private final SortedMap<K, ? extends V> sm;
1798 
1799         UnmodifiableSortedMap(SortedMap<K, ? extends V> m) {super(m); sm = m; }
1800         public Comparator<? super K> comparator()   { return sm.comparator(); }
1801         public SortedMap<K,V> subMap(K fromKey, K toKey)
1802              { return new UnmodifiableSortedMap<>(sm.subMap(fromKey, toKey)); }
1803         public SortedMap<K,V> headMap(K toKey)
1804                      { return new UnmodifiableSortedMap<>(sm.headMap(toKey)); }
1805         public SortedMap<K,V> tailMap(K fromKey)
1806                    { return new UnmodifiableSortedMap<>(sm.tailMap(fromKey)); }
1807         public K firstKey()                           { return sm.firstKey(); }
1808         public K lastKey()                             { return sm.lastKey(); }
1809     }
1810 
1811     /**
1812      * Returns an unmodifiable view of the specified navigable map.  This method
1813      * allows modules to provide users with "read-only" access to internal
1814      * navigable maps.  Query operations on the returned navigable map "read
1815      * through" to the specified navigable map.  Attempts to modify the returned
1816      * navigable map, whether direct, via its collection views, or via its
1817      * {@code subMap}, {@code headMap}, or {@code tailMap} views, result in
1818      * an {@code UnsupportedOperationException}.<p>
1819      *
1820      * The returned navigable map will be serializable if the specified
1821      * navigable map is serializable.
1822      *
1823      * @param <K> the class of the map keys
1824      * @param <V> the class of the map values
1825      * @param m the navigable map for which an unmodifiable view is to be
1826      *        returned
1827      * @return an unmodifiable view of the specified navigable map
1828      * @since 1.8
1829      */
1830     public static <K,V> NavigableMap<K,V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> m) {
1831         return new UnmodifiableNavigableMap<>(m);
1832     }
1833 
1834     /**
1835      * @serial include
1836      */
1837     static class UnmodifiableNavigableMap<K,V>
1838           extends UnmodifiableSortedMap<K,V>
1839           implements NavigableMap<K,V>, Serializable {
1840         private static final long serialVersionUID = -4858195264774772197L;
1841 
1842         /**
1843          * A class for the {@link EMPTY_NAVIGABLE_MAP} which needs readResolve
1844          * to preserve singleton property.
1845          *
1846          * @param <K> type of keys, if there were any, and of bounds
1847          * @param <V> type of values, if there were any
1848          */
1849         private static class EmptyNavigableMap<K,V> extends UnmodifiableNavigableMap<K,V>
1850             implements Serializable {
1851 
1852             private static final long serialVersionUID = -2239321462712562324L;
1853 
1854             EmptyNavigableMap()                       { super(new TreeMap<>()); }
1855 
1856             @Override
1857             public NavigableSet<K> navigableKeySet()
1858                                                 { return emptyNavigableSet(); }
1859 
1860             private Object readResolve()        { return EMPTY_NAVIGABLE_MAP; }
1861         }
1862 
1863         /**
1864          * Singleton for {@link emptyNavigableMap()} which is also immutable.
1865          */
1866         private static final EmptyNavigableMap<?,?> EMPTY_NAVIGABLE_MAP =
1867             new EmptyNavigableMap<>();
1868 
1869         /**
1870          * The instance we wrap and protect.
1871          */
1872         private final NavigableMap<K, ? extends V> nm;
1873 
1874         UnmodifiableNavigableMap(NavigableMap<K, ? extends V> m)
1875                                                             {super(m); nm = m;}
1876 
1877         public K lowerKey(K key)                   { return nm.lowerKey(key); }
1878         public K floorKey(K key)                   { return nm.floorKey(key); }
1879         public K ceilingKey(K key)               { return nm.ceilingKey(key); }
1880         public K higherKey(K key)                 { return nm.higherKey(key); }
1881 
1882         @SuppressWarnings("unchecked")
1883         public Entry<K, V> lowerEntry(K key) {
1884             Entry<K,V> lower = (Entry<K, V>) nm.lowerEntry(key);
1885             return (null != lower)
1886                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(lower)
1887                 : null;
1888         }
1889 
1890         @SuppressWarnings("unchecked")
1891         public Entry<K, V> floorEntry(K key) {
1892             Entry<K,V> floor = (Entry<K, V>) nm.floorEntry(key);
1893             return (null != floor)
1894                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(floor)
1895                 : null;
1896         }
1897 
1898         @SuppressWarnings("unchecked")
1899         public Entry<K, V> ceilingEntry(K key) {
1900             Entry<K,V> ceiling = (Entry<K, V>) nm.ceilingEntry(key);
1901             return (null != ceiling)
1902                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(ceiling)
1903                 : null;
1904         }
1905 
1906 
1907         @SuppressWarnings("unchecked")
1908         public Entry<K, V> higherEntry(K key) {
1909             Entry<K,V> higher = (Entry<K, V>) nm.higherEntry(key);
1910             return (null != higher)
1911                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(higher)
1912                 : null;
1913         }
1914 
1915         @SuppressWarnings("unchecked")
1916         public Entry<K, V> firstEntry() {
1917             Entry<K,V> first = (Entry<K, V>) nm.firstEntry();
1918             return (null != first)
1919                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(first)
1920                 : null;
1921         }
1922 
1923         @SuppressWarnings("unchecked")
1924         public Entry<K, V> lastEntry() {
1925             Entry<K,V> last = (Entry<K, V>) nm.lastEntry();
1926             return (null != last)
1927                 ? new UnmodifiableEntrySet.UnmodifiableEntry<>(last)
1928                 : null;
1929         }
1930 
1931         public Entry<K, V> pollFirstEntry()
1932                                  { throw new UnsupportedOperationException(); }
1933         public Entry<K, V> pollLastEntry()
1934                                  { throw new UnsupportedOperationException(); }
1935         public NavigableMap<K, V> descendingMap()
1936                        { return unmodifiableNavigableMap(nm.descendingMap()); }
1937         public NavigableSet<K> navigableKeySet()
1938                      { return unmodifiableNavigableSet(nm.navigableKeySet()); }
1939         public NavigableSet<K> descendingKeySet()
1940                     { return unmodifiableNavigableSet(nm.descendingKeySet()); }
1941 
1942         public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
1943             return unmodifiableNavigableMap(
1944                 nm.subMap(fromKey, fromInclusive, toKey, toInclusive));
1945         }
1946 
1947         public NavigableMap<K, V> headMap(K toKey, boolean inclusive)
1948              { return unmodifiableNavigableMap(nm.headMap(toKey, inclusive)); }
1949         public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive)
1950            { return unmodifiableNavigableMap(nm.tailMap(fromKey, inclusive)); }
1951     }
1952 
1953     // Synch Wrappers
1954 
1955     /**
1956      * Returns a synchronized (thread-safe) collection backed by the specified
1957      * collection.  In order to guarantee serial access, it is critical that
1958      * <strong>all</strong> access to the backing collection is accomplished
1959      * through the returned collection.<p>
1960      *
1961      * It is imperative that the user manually synchronize on the returned
1962      * collection when traversing it via {@link Iterator}, {@link Spliterator}
1963      * or {@link Stream}:
1964      * <pre>
1965      *  Collection c = Collections.synchronizedCollection(myCollection);
1966      *     ...
1967      *  synchronized (c) {
1968      *      Iterator i = c.iterator(); // Must be in the synchronized block
1969      *      while (i.hasNext())
1970      *         foo(i.next());
1971      *  }
1972      * </pre>
1973      * Failure to follow this advice may result in non-deterministic behavior.
1974      *
1975      * <p>The returned collection does <i>not</i> pass the {@code hashCode}
1976      * and {@code equals} operations through to the backing collection, but
1977      * relies on {@code Object}'s equals and hashCode methods.  This is
1978      * necessary to preserve the contracts of these operations in the case
1979      * that the backing collection is a set or a list.<p>
1980      *
1981      * The returned collection will be serializable if the specified collection
1982      * is serializable.
1983      *
1984      * @param  <T> the class of the objects in the collection
1985      * @param  c the collection to be "wrapped" in a synchronized collection.
1986      * @return a synchronized view of the specified collection.
1987      */
1988     public static <T> Collection<T> synchronizedCollection(Collection<T> c) {
1989         return new SynchronizedCollection<>(c);
1990     }
1991 
1992     static <T> Collection<T> synchronizedCollection(Collection<T> c, Object mutex) {
1993         return new SynchronizedCollection<>(c, mutex);
1994     }
1995 
1996     /**
1997      * @serial include
1998      */
1999     static class SynchronizedCollection<E> implements Collection<E>, Serializable {
2000         private static final long serialVersionUID = 3053995032091335093L;
2001 
2002         final Collection<E> c;  // Backing Collection
2003         final Object mutex;     // Object on which to synchronize
2004 
2005         SynchronizedCollection(Collection<E> c) {
2006             this.c = Objects.requireNonNull(c);
2007             mutex = this;
2008         }
2009 
2010         SynchronizedCollection(Collection<E> c, Object mutex) {
2011             this.c = Objects.requireNonNull(c);
2012             this.mutex = Objects.requireNonNull(mutex);
2013         }
2014 
2015         public int size() {
2016             synchronized (mutex) {return c.size();}
2017         }
2018         public boolean isEmpty() {
2019             synchronized (mutex) {return c.isEmpty();}
2020         }
2021         public boolean contains(Object o) {
2022             synchronized (mutex) {return c.contains(o);}
2023         }
2024         public Object[] toArray() {
2025             synchronized (mutex) {return c.toArray();}
2026         }
2027         public <T> T[] toArray(T[] a) {
2028             synchronized (mutex) {return c.toArray(a);}
2029         }
2030 
2031         public Iterator<E> iterator() {
2032             return c.iterator(); // Must be manually synched by user!
2033         }
2034 
2035         public boolean add(E e) {
2036             synchronized (mutex) {return c.add(e);}
2037         }
2038         public boolean remove(Object o) {
2039             synchronized (mutex) {return c.remove(o);}
2040         }
2041 
2042         public boolean containsAll(Collection<?> coll) {
2043             synchronized (mutex) {return c.containsAll(coll);}
2044         }
2045         public boolean addAll(Collection<? extends E> coll) {
2046             synchronized (mutex) {return c.addAll(coll);}
2047         }
2048         public boolean removeAll(Collection<?> coll) {
2049             synchronized (mutex) {return c.removeAll(coll);}
2050         }
2051         public boolean retainAll(Collection<?> coll) {
2052             synchronized (mutex) {return c.retainAll(coll);}
2053         }
2054         public void clear() {
2055             synchronized (mutex) {c.clear();}
2056         }
2057         public String toString() {
2058             synchronized (mutex) {return c.toString();}
2059         }
2060         // Override default methods in Collection
2061         @Override
2062         public void forEach(Consumer<? super E> consumer) {
2063             synchronized (mutex) {c.forEach(consumer);}
2064         }
2065         @Override
2066         public boolean removeIf(Predicate<? super E> filter) {
2067             synchronized (mutex) {return c.removeIf(filter);}
2068         }
2069         @Override
2070         public Spliterator<E> spliterator() {
2071             return c.spliterator(); // Must be manually synched by user!
2072         }
2073         @Override
2074         public Stream<E> stream() {
2075             return c.stream(); // Must be manually synched by user!
2076         }
2077         @Override
2078         public Stream<E> parallelStream() {
2079             return c.parallelStream(); // Must be manually synched by user!
2080         }
2081         private void writeObject(ObjectOutputStream s) throws IOException {
2082             synchronized (mutex) {s.defaultWriteObject();}
2083         }
2084     }
2085 
2086     /**
2087      * Returns a synchronized (thread-safe) set backed by the specified
2088      * set.  In order to guarantee serial access, it is critical that
2089      * <strong>all</strong> access to the backing set is accomplished
2090      * through the returned set.<p>
2091      *
2092      * It is imperative that the user manually synchronize on the returned
2093      * collection when traversing it via {@link Iterator}, {@link Spliterator}
2094      * or {@link Stream}:
2095      * <pre>
2096      *  Set s = Collections.synchronizedSet(new HashSet());
2097      *      ...
2098      *  synchronized (s) {
2099      *      Iterator i = s.iterator(); // Must be in the synchronized block
2100      *      while (i.hasNext())
2101      *          foo(i.next());
2102      *  }
2103      * </pre>
2104      * Failure to follow this advice may result in non-deterministic behavior.
2105      *
2106      * <p>The returned set will be serializable if the specified set is
2107      * serializable.
2108      *
2109      * @param  <T> the class of the objects in the set
2110      * @param  s the set to be "wrapped" in a synchronized set.
2111      * @return a synchronized view of the specified set.
2112      */
2113     public static <T> Set<T> synchronizedSet(Set<T> s) {
2114         return new SynchronizedSet<>(s);
2115     }
2116 
2117     static <T> Set<T> synchronizedSet(Set<T> s, Object mutex) {
2118         return new SynchronizedSet<>(s, mutex);
2119     }
2120 
2121     /**
2122      * @serial include
2123      */
2124     static class SynchronizedSet<E>
2125           extends SynchronizedCollection<E>
2126           implements Set<E> {
2127         private static final long serialVersionUID = 487447009682186044L;
2128 
2129         SynchronizedSet(Set<E> s) {
2130             super(s);
2131         }
2132         SynchronizedSet(Set<E> s, Object mutex) {
2133             super(s, mutex);
2134         }
2135 
2136         public boolean equals(Object o) {
2137             if (this == o)
2138                 return true;
2139             synchronized (mutex) {return c.equals(o);}
2140         }
2141         public int hashCode() {
2142             synchronized (mutex) {return c.hashCode();}
2143         }
2144     }
2145 
2146     /**
2147      * Returns a synchronized (thread-safe) sorted set backed by the specified
2148      * sorted set.  In order to guarantee serial access, it is critical that
2149      * <strong>all</strong> access to the backing sorted set is accomplished
2150      * through the returned sorted set (or its views).<p>
2151      *
2152      * It is imperative that the user manually synchronize on the returned
2153      * sorted set when traversing it or any of its {@code subSet},
2154      * {@code headSet}, or {@code tailSet} views via {@link Iterator},
2155      * {@link Spliterator} or {@link Stream}:
2156      * <pre>
2157      *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
2158      *      ...
2159      *  synchronized (s) {
2160      *      Iterator i = s.iterator(); // Must be in the synchronized block
2161      *      while (i.hasNext())
2162      *          foo(i.next());
2163      *  }
2164      * </pre>
2165      * or:
2166      * <pre>
2167      *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
2168      *  SortedSet s2 = s.headSet(foo);
2169      *      ...
2170      *  synchronized (s) {  // Note: s, not s2!!!
2171      *      Iterator i = s2.iterator(); // Must be in the synchronized block
2172      *      while (i.hasNext())
2173      *          foo(i.next());
2174      *  }
2175      * </pre>
2176      * Failure to follow this advice may result in non-deterministic behavior.
2177      *
2178      * <p>The returned sorted set will be serializable if the specified
2179      * sorted set is serializable.
2180      *
2181      * @param  <T> the class of the objects in the set
2182      * @param  s the sorted set to be "wrapped" in a synchronized sorted set.
2183      * @return a synchronized view of the specified sorted set.
2184      */
2185     public static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) {
2186         return new SynchronizedSortedSet<>(s);
2187     }
2188 
2189     /**
2190      * @serial include
2191      */
2192     static class SynchronizedSortedSet<E>
2193         extends SynchronizedSet<E>
2194         implements SortedSet<E>
2195     {
2196         private static final long serialVersionUID = 8695801310862127406L;
2197 
2198         private final SortedSet<E> ss;
2199 
2200         SynchronizedSortedSet(SortedSet<E> s) {
2201             super(s);
2202             ss = s;
2203         }
2204         SynchronizedSortedSet(SortedSet<E> s, Object mutex) {
2205             super(s, mutex);
2206             ss = s;
2207         }
2208 
2209         public Comparator<? super E> comparator() {
2210             synchronized (mutex) {return ss.comparator();}
2211         }
2212 
2213         public SortedSet<E> subSet(E fromElement, E toElement) {
2214             synchronized (mutex) {
2215                 return new SynchronizedSortedSet<>(
2216                     ss.subSet(fromElement, toElement), mutex);
2217             }
2218         }
2219         public SortedSet<E> headSet(E toElement) {
2220             synchronized (mutex) {
2221                 return new SynchronizedSortedSet<>(ss.headSet(toElement), mutex);
2222             }
2223         }
2224         public SortedSet<E> tailSet(E fromElement) {
2225             synchronized (mutex) {
2226                return new SynchronizedSortedSet<>(ss.tailSet(fromElement),mutex);
2227             }
2228         }
2229 
2230         public E first() {
2231             synchronized (mutex) {return ss.first();}
2232         }
2233         public E last() {
2234             synchronized (mutex) {return ss.last();}
2235         }
2236     }
2237 
2238     /**
2239      * Returns a synchronized (thread-safe) navigable set backed by the
2240      * specified navigable set.  In order to guarantee serial access, it is
2241      * critical that <strong>all</strong> access to the backing navigable set is
2242      * accomplished through the returned navigable set (or its views).<p>
2243      *
2244      * It is imperative that the user manually synchronize on the returned
2245      * navigable set when traversing it, or any of its {@code subSet},
2246      * {@code headSet}, or {@code tailSet} views, via {@link Iterator},
2247      * {@link Spliterator} or {@link Stream}:
2248      * <pre>
2249      *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
2250      *      ...
2251      *  synchronized (s) {
2252      *      Iterator i = s.iterator(); // Must be in the synchronized block
2253      *      while (i.hasNext())
2254      *          foo(i.next());
2255      *  }
2256      * </pre>
2257      * or:
2258      * <pre>
2259      *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
2260      *  NavigableSet s2 = s.headSet(foo, true);
2261      *      ...
2262      *  synchronized (s) {  // Note: s, not s2!!!
2263      *      Iterator i = s2.iterator(); // Must be in the synchronized block
2264      *      while (i.hasNext())
2265      *          foo(i.next());
2266      *  }
2267      * </pre>
2268      * Failure to follow this advice may result in non-deterministic behavior.
2269      *
2270      * <p>The returned navigable set will be serializable if the specified
2271      * navigable set is serializable.
2272      *
2273      * @param  <T> the class of the objects in the set
2274      * @param  s the navigable set to be "wrapped" in a synchronized navigable
2275      * set
2276      * @return a synchronized view of the specified navigable set
2277      * @since 1.8
2278      */
2279     public static <T> NavigableSet<T> synchronizedNavigableSet(NavigableSet<T> s) {
2280         return new SynchronizedNavigableSet<>(s);
2281     }
2282 
2283     /**
2284      * @serial include
2285      */
2286     static class SynchronizedNavigableSet<E>
2287         extends SynchronizedSortedSet<E>
2288         implements NavigableSet<E>
2289     {
2290         private static final long serialVersionUID = -5505529816273629798L;
2291 
2292         private final NavigableSet<E> ns;
2293 
2294         SynchronizedNavigableSet(NavigableSet<E> s) {
2295             super(s);
2296             ns = s;
2297         }
2298 
2299         SynchronizedNavigableSet(NavigableSet<E> s, Object mutex) {
2300             super(s, mutex);
2301             ns = s;
2302         }
2303         public E lower(E e)      { synchronized (mutex) {return ns.lower(e);} }
2304         public E floor(E e)      { synchronized (mutex) {return ns.floor(e);} }
2305         public E ceiling(E e)  { synchronized (mutex) {return ns.ceiling(e);} }
2306         public E higher(E e)    { synchronized (mutex) {return ns.higher(e);} }
2307         public E pollFirst()  { synchronized (mutex) {return ns.pollFirst();} }
2308         public E pollLast()    { synchronized (mutex) {return ns.pollLast();} }
2309 
2310         public NavigableSet<E> descendingSet() {
2311             synchronized (mutex) {
2312                 return new SynchronizedNavigableSet<>(ns.descendingSet(), mutex);
2313             }
2314         }
2315 
2316         public Iterator<E> descendingIterator()
2317                  { synchronized (mutex) { return descendingSet().iterator(); } }
2318 
2319         public NavigableSet<E> subSet(E fromElement, E toElement) {
2320             synchronized (mutex) {
2321                 return new SynchronizedNavigableSet<>(ns.subSet(fromElement, true, toElement, false), mutex);
2322             }
2323         }
2324         public NavigableSet<E> headSet(E toElement) {
2325             synchronized (mutex) {
2326                 return new SynchronizedNavigableSet<>(ns.headSet(toElement, false), mutex);
2327             }
2328         }
2329         public NavigableSet<E> tailSet(E fromElement) {
2330             synchronized (mutex) {
2331                 return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, true), mutex);
2332             }
2333         }
2334 
2335         public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
2336             synchronized (mutex) {
2337                 return new SynchronizedNavigableSet<>(ns.subSet(fromElement, fromInclusive, toElement, toInclusive), mutex);
2338             }
2339         }
2340 
2341         public NavigableSet<E> headSet(E toElement, boolean inclusive) {
2342             synchronized (mutex) {
2343                 return new SynchronizedNavigableSet<>(ns.headSet(toElement, inclusive), mutex);
2344             }
2345         }
2346 
2347         public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
2348             synchronized (mutex) {
2349                 return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, inclusive), mutex);
2350             }
2351         }
2352     }
2353 
2354     /**
2355      * Returns a synchronized (thread-safe) list backed by the specified
2356      * list.  In order to guarantee serial access, it is critical that
2357      * <strong>all</strong> access to the backing list is accomplished
2358      * through the returned list.<p>
2359      *
2360      * It is imperative that the user manually synchronize on the returned
2361      * list when traversing it via {@link Iterator}, {@link Spliterator}
2362      * or {@link Stream}:
2363      * <pre>
2364      *  List list = Collections.synchronizedList(new ArrayList());
2365      *      ...
2366      *  synchronized (list) {
2367      *      Iterator i = list.iterator(); // Must be in synchronized block
2368      *      while (i.hasNext())
2369      *          foo(i.next());
2370      *  }
2371      * </pre>
2372      * Failure to follow this advice may result in non-deterministic behavior.
2373      *
2374      * <p>The returned list will be serializable if the specified list is
2375      * serializable.
2376      *
2377      * @param  <T> the class of the objects in the list
2378      * @param  list the list to be "wrapped" in a synchronized list.
2379      * @return a synchronized view of the specified list.
2380      */
2381     public static <T> List<T> synchronizedList(List<T> list) {
2382         return (list instanceof RandomAccess ?
2383                 new SynchronizedRandomAccessList<>(list) :
2384                 new SynchronizedList<>(list));
2385     }
2386 
2387     static <T> List<T> synchronizedList(List<T> list, Object mutex) {
2388         return (list instanceof RandomAccess ?
2389                 new SynchronizedRandomAccessList<>(list, mutex) :
2390                 new SynchronizedList<>(list, mutex));
2391     }
2392 
2393     /**
2394      * @serial include
2395      */
2396     static class SynchronizedList<E>
2397         extends SynchronizedCollection<E>
2398         implements List<E> {
2399         private static final long serialVersionUID = -7754090372962971524L;
2400 
2401         final List<E> list;
2402 
2403         SynchronizedList(List<E> list) {
2404             super(list);
2405             this.list = list;
2406         }
2407         SynchronizedList(List<E> list, Object mutex) {
2408             super(list, mutex);
2409             this.list = list;
2410         }
2411 
2412         public boolean equals(Object o) {
2413             if (this == o)
2414                 return true;
2415             synchronized (mutex) {return list.equals(o);}
2416         }
2417         public int hashCode() {
2418             synchronized (mutex) {return list.hashCode();}
2419         }
2420 
2421         public E get(int index) {
2422             synchronized (mutex) {return list.get(index);}
2423         }
2424         public E set(int index, E element) {
2425             synchronized (mutex) {return list.set(index, element);}
2426         }
2427         public void add(int index, E element) {
2428             synchronized (mutex) {list.add(index, element);}
2429         }
2430         public E remove(int index) {
2431             synchronized (mutex) {return list.remove(index);}
2432         }
2433 
2434         public int indexOf(Object o) {
2435             synchronized (mutex) {return list.indexOf(o);}
2436         }
2437         public int lastIndexOf(Object o) {
2438             synchronized (mutex) {return list.lastIndexOf(o);}
2439         }
2440 
2441         public boolean addAll(int index, Collection<? extends E> c) {
2442             synchronized (mutex) {return list.addAll(index, c);}
2443         }
2444 
2445         public ListIterator<E> listIterator() {
2446             return list.listIterator(); // Must be manually synched by user
2447         }
2448 
2449         public ListIterator<E> listIterator(int index) {
2450             return list.listIterator(index); // Must be manually synched by user
2451         }
2452 
2453         public List<E> subList(int fromIndex, int toIndex) {
2454             synchronized (mutex) {
2455                 return new SynchronizedList<>(list.subList(fromIndex, toIndex),
2456                                             mutex);
2457             }
2458         }
2459 
2460         @Override
2461         public void replaceAll(UnaryOperator<E> operator) {
2462             synchronized (mutex) {list.replaceAll(operator);}
2463         }
2464         @Override
2465         public void sort(Comparator<? super E> c) {
2466             synchronized (mutex) {list.sort(c);}
2467         }
2468 
2469         /**
2470          * SynchronizedRandomAccessList instances are serialized as
2471          * SynchronizedList instances to allow them to be deserialized
2472          * in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).
2473          * This method inverts the transformation.  As a beneficial
2474          * side-effect, it also grafts the RandomAccess marker onto
2475          * SynchronizedList instances that were serialized in pre-1.4 JREs.
2476          *
2477          * Note: Unfortunately, SynchronizedRandomAccessList instances
2478          * serialized in 1.4.1 and deserialized in 1.4 will become
2479          * SynchronizedList instances, as this method was missing in 1.4.
2480          */
2481         private Object readResolve() {
2482             return (list instanceof RandomAccess
2483                     ? new SynchronizedRandomAccessList<>(list)
2484                     : this);
2485         }
2486     }
2487 
2488     /**
2489      * @serial include
2490      */
2491     static class SynchronizedRandomAccessList<E>
2492         extends SynchronizedList<E>
2493         implements RandomAccess {
2494 
2495         SynchronizedRandomAccessList(List<E> list) {
2496             super(list);
2497         }
2498 
2499         SynchronizedRandomAccessList(List<E> list, Object mutex) {
2500             super(list, mutex);
2501         }
2502 
2503         public List<E> subList(int fromIndex, int toIndex) {
2504             synchronized (mutex) {
2505                 return new SynchronizedRandomAccessList<>(
2506                     list.subList(fromIndex, toIndex), mutex);
2507             }
2508         }
2509 
2510         private static final long serialVersionUID = 1530674583602358482L;
2511 
2512         /**
2513          * Allows instances to be deserialized in pre-1.4 JREs (which do
2514          * not have SynchronizedRandomAccessList).  SynchronizedList has
2515          * a readResolve method that inverts this transformation upon
2516          * deserialization.
2517          */
2518         private Object writeReplace() {
2519             return new SynchronizedList<>(list);
2520         }
2521     }
2522 
2523     /**
2524      * Returns a synchronized (thread-safe) map backed by the specified
2525      * map.  In order to guarantee serial access, it is critical that
2526      * <strong>all</strong> access to the backing map is accomplished
2527      * through the returned map.<p>
2528      *
2529      * It is imperative that the user manually synchronize on the returned
2530      * map when traversing any of its collection views via {@link Iterator},
2531      * {@link Spliterator} or {@link Stream}:
2532      * <pre>
2533      *  Map m = Collections.synchronizedMap(new HashMap());
2534      *      ...
2535      *  Set s = m.keySet();  // Needn't be in synchronized block
2536      *      ...
2537      *  synchronized (m) {  // Synchronizing on m, not s!
2538      *      Iterator i = s.iterator(); // Must be in synchronized block
2539      *      while (i.hasNext())
2540      *          foo(i.next());
2541      *  }
2542      * </pre>
2543      * Failure to follow this advice may result in non-deterministic behavior.
2544      *
2545      * <p>The returned map will be serializable if the specified map is
2546      * serializable.
2547      *
2548      * @param <K> the class of the map keys
2549      * @param <V> the class of the map values
2550      * @param  m the map to be "wrapped" in a synchronized map.
2551      * @return a synchronized view of the specified map.
2552      */
2553     public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
2554         return new SynchronizedMap<>(m);
2555     }
2556 
2557     /**
2558      * @serial include
2559      */
2560     private static class SynchronizedMap<K,V>
2561         implements Map<K,V>, Serializable {
2562         private static final long serialVersionUID = 1978198479659022715L;
2563 
2564         private final Map<K,V> m;     // Backing Map
2565         final Object      mutex;        // Object on which to synchronize
2566 
2567         SynchronizedMap(Map<K,V> m) {
2568             this.m = Objects.requireNonNull(m);
2569             mutex = this;
2570         }
2571 
2572         SynchronizedMap(Map<K,V> m, Object mutex) {
2573             this.m = m;
2574             this.mutex = mutex;
2575         }
2576 
2577         public int size() {
2578             synchronized (mutex) {return m.size();}
2579         }
2580         public boolean isEmpty() {
2581             synchronized (mutex) {return m.isEmpty();}
2582         }
2583         public boolean containsKey(Object key) {
2584             synchronized (mutex) {return m.containsKey(key);}
2585         }
2586         public boolean containsValue(Object value) {
2587             synchronized (mutex) {return m.containsValue(value);}
2588         }
2589         public V get(Object key) {
2590             synchronized (mutex) {return m.get(key);}
2591         }
2592 
2593         public V put(K key, V value) {
2594             synchronized (mutex) {return m.put(key, value);}
2595         }
2596         public V remove(Object key) {
2597             synchronized (mutex) {return m.remove(key);}
2598         }
2599         public void putAll(Map<? extends K, ? extends V> map) {
2600             synchronized (mutex) {m.putAll(map);}
2601         }
2602         public void clear() {
2603             synchronized (mutex) {m.clear();}
2604         }
2605 
2606         private transient Set<K> keySet;
2607         private transient Set<Map.Entry<K,V>> entrySet;
2608         private transient Collection<V> values;
2609 
2610         public Set<K> keySet() {
2611             synchronized (mutex) {
2612                 if (keySet==null)
2613                     keySet = new SynchronizedSet<>(m.keySet(), mutex);
2614                 return keySet;
2615             }
2616         }
2617 
2618         public Set<Map.Entry<K,V>> entrySet() {
2619             synchronized (mutex) {
2620                 if (entrySet==null)
2621                     entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
2622                 return entrySet;
2623             }
2624         }
2625 
2626         public Collection<V> values() {
2627             synchronized (mutex) {
2628                 if (values==null)
2629                     values = new SynchronizedCollection<>(m.values(), mutex);
2630                 return values;
2631             }
2632         }
2633 
2634         public boolean equals(Object o) {
2635             if (this == o)
2636                 return true;
2637             synchronized (mutex) {return m.equals(o);}
2638         }
2639         public int hashCode() {
2640             synchronized (mutex) {return m.hashCode();}
2641         }
2642         public String toString() {
2643             synchronized (mutex) {return m.toString();}
2644         }
2645 
2646         // Override default methods in Map
2647         @Override
2648         public V getOrDefault(Object k, V defaultValue) {
2649             synchronized (mutex) {return m.getOrDefault(k, defaultValue);}
2650         }
2651         @Override
2652         public void forEach(BiConsumer<? super K, ? super V> action) {
2653             synchronized (mutex) {m.forEach(action);}
2654         }
2655         @Override
2656         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
2657             synchronized (mutex) {m.replaceAll(function);}
2658         }
2659         @Override
2660         public V putIfAbsent(K key, V value) {
2661             synchronized (mutex) {return m.putIfAbsent(key, value);}
2662         }
2663         @Override
2664         public boolean remove(Object key, Object value) {
2665             synchronized (mutex) {return m.remove(key, value);}
2666         }
2667         @Override
2668         public boolean replace(K key, V oldValue, V newValue) {
2669             synchronized (mutex) {return m.replace(key, oldValue, newValue);}
2670         }
2671         @Override
2672         public V replace(K key, V value) {
2673             synchronized (mutex) {return m.replace(key, value);}
2674         }
2675         @Override
2676         public V computeIfAbsent(K key,
2677                 Function<? super K, ? extends V> mappingFunction) {
2678             synchronized (mutex) {return m.computeIfAbsent(key, mappingFunction);}
2679         }
2680         @Override
2681         public V computeIfPresent(K key,
2682                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
2683             synchronized (mutex) {return m.computeIfPresent(key, remappingFunction);}
2684         }
2685         @Override
2686         public V compute(K key,
2687                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
2688             synchronized (mutex) {return m.compute(key, remappingFunction);}
2689         }
2690         @Override
2691         public V merge(K key, V value,
2692                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
2693             synchronized (mutex) {return m.merge(key, value, remappingFunction);}
2694         }
2695 
2696         private void writeObject(ObjectOutputStream s) throws IOException {
2697             synchronized (mutex) {s.defaultWriteObject();}
2698         }
2699     }
2700 
2701     /**
2702      * Returns a synchronized (thread-safe) sorted map backed by the specified
2703      * sorted map.  In order to guarantee serial access, it is critical that
2704      * <strong>all</strong> access to the backing sorted map is accomplished
2705      * through the returned sorted map (or its views).<p>
2706      *
2707      * It is imperative that the user manually synchronize on the returned
2708      * sorted map when traversing any of its collection views, or the
2709      * collections views of any of its {@code subMap}, {@code headMap} or
2710      * {@code tailMap} views, via {@link Iterator}, {@link Spliterator} or
2711      * {@link Stream}:
2712      * <pre>
2713      *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
2714      *      ...
2715      *  Set s = m.keySet();  // Needn't be in synchronized block
2716      *      ...
2717      *  synchronized (m) {  // Synchronizing on m, not s!
2718      *      Iterator i = s.iterator(); // Must be in synchronized block
2719      *      while (i.hasNext())
2720      *          foo(i.next());
2721      *  }
2722      * </pre>
2723      * or:
2724      * <pre>
2725      *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
2726      *  SortedMap m2 = m.subMap(foo, bar);
2727      *      ...
2728      *  Set s2 = m2.keySet();  // Needn't be in synchronized block
2729      *      ...
2730      *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
2731      *      Iterator i = s2.iterator(); // Must be in synchronized block
2732      *      while (i.hasNext())
2733      *          foo(i.next());
2734      *  }
2735      * </pre>
2736      * Failure to follow this advice may result in non-deterministic behavior.
2737      *
2738      * <p>The returned sorted map will be serializable if the specified
2739      * sorted map is serializable.
2740      *
2741      * @param <K> the class of the map keys
2742      * @param <V> the class of the map values
2743      * @param  m the sorted map to be "wrapped" in a synchronized sorted map.
2744      * @return a synchronized view of the specified sorted map.
2745      */
2746     public static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m) {
2747         return new SynchronizedSortedMap<>(m);
2748     }
2749 
2750     /**
2751      * @serial include
2752      */
2753     static class SynchronizedSortedMap<K,V>
2754         extends SynchronizedMap<K,V>
2755         implements SortedMap<K,V>
2756     {
2757         private static final long serialVersionUID = -8798146769416483793L;
2758 
2759         private final SortedMap<K,V> sm;
2760 
2761         SynchronizedSortedMap(SortedMap<K,V> m) {
2762             super(m);
2763             sm = m;
2764         }
2765         SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {
2766             super(m, mutex);
2767             sm = m;
2768         }
2769 
2770         public Comparator<? super K> comparator() {
2771             synchronized (mutex) {return sm.comparator();}
2772         }
2773 
2774         public SortedMap<K,V> subMap(K fromKey, K toKey) {
2775             synchronized (mutex) {
2776                 return new SynchronizedSortedMap<>(
2777                     sm.subMap(fromKey, toKey), mutex);
2778             }
2779         }
2780         public SortedMap<K,V> headMap(K toKey) {
2781             synchronized (mutex) {
2782                 return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);
2783             }
2784         }
2785         public SortedMap<K,V> tailMap(K fromKey) {
2786             synchronized (mutex) {
2787                return new SynchronizedSortedMap<>(sm.tailMap(fromKey),mutex);
2788             }
2789         }
2790 
2791         public K firstKey() {
2792             synchronized (mutex) {return sm.firstKey();}
2793         }
2794         public K lastKey() {
2795             synchronized (mutex) {return sm.lastKey();}
2796         }
2797     }
2798 
2799     /**
2800      * Returns a synchronized (thread-safe) navigable map backed by the
2801      * specified navigable map.  In order to guarantee serial access, it is
2802      * critical that <strong>all</strong> access to the backing navigable map is
2803      * accomplished through the returned navigable map (or its views).<p>
2804      *
2805      * It is imperative that the user manually synchronize on the returned
2806      * navigable map when traversing any of its collection views, or the
2807      * collections views of any of its {@code subMap}, {@code headMap} or
2808      * {@code tailMap} views, via {@link Iterator}, {@link Spliterator} or
2809      * {@link Stream}:
2810      * <pre>
2811      *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
2812      *      ...
2813      *  Set s = m.keySet();  // Needn't be in synchronized block
2814      *      ...
2815      *  synchronized (m) {  // Synchronizing on m, not s!
2816      *      Iterator i = s.iterator(); // Must be in synchronized block
2817      *      while (i.hasNext())
2818      *          foo(i.next());
2819      *  }
2820      * </pre>
2821      * or:
2822      * <pre>
2823      *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
2824      *  NavigableMap m2 = m.subMap(foo, true, bar, false);
2825      *      ...
2826      *  Set s2 = m2.keySet();  // Needn't be in synchronized block
2827      *      ...
2828      *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
2829      *      Iterator i = s.iterator(); // Must be in synchronized block
2830      *      while (i.hasNext())
2831      *          foo(i.next());
2832      *  }
2833      * </pre>
2834      * Failure to follow this advice may result in non-deterministic behavior.
2835      *
2836      * <p>The returned navigable map will be serializable if the specified
2837      * navigable map is serializable.
2838      *
2839      * @param <K> the class of the map keys
2840      * @param <V> the class of the map values
2841      * @param  m the navigable map to be "wrapped" in a synchronized navigable
2842      *              map
2843      * @return a synchronized view of the specified navigable map.
2844      * @since 1.8
2845      */
2846     public static <K,V> NavigableMap<K,V> synchronizedNavigableMap(NavigableMap<K,V> m) {
2847         return new SynchronizedNavigableMap<>(m);
2848     }
2849 
2850     /**
2851      * A synchronized NavigableMap.
2852      *
2853      * @serial include
2854      */
2855     static class SynchronizedNavigableMap<K,V>
2856         extends SynchronizedSortedMap<K,V>
2857         implements NavigableMap<K,V>
2858     {
2859         private static final long serialVersionUID = 699392247599746807L;
2860 
2861         private final NavigableMap<K,V> nm;
2862 
2863         SynchronizedNavigableMap(NavigableMap<K,V> m) {
2864             super(m);
2865             nm = m;
2866         }
2867         SynchronizedNavigableMap(NavigableMap<K,V> m, Object mutex) {
2868             super(m, mutex);
2869             nm = m;
2870         }
2871 
2872         public Entry<K, V> lowerEntry(K key)
2873                         { synchronized (mutex) { return nm.lowerEntry(key); } }
2874         public K lowerKey(K key)
2875                           { synchronized (mutex) { return nm.lowerKey(key); } }
2876         public Entry<K, V> floorEntry(K key)
2877                         { synchronized (mutex) { return nm.floorEntry(key); } }
2878         public K floorKey(K key)
2879                           { synchronized (mutex) { return nm.floorKey(key); } }
2880         public Entry<K, V> ceilingEntry(K key)
2881                       { synchronized (mutex) { return nm.ceilingEntry(key); } }
2882         public K ceilingKey(K key)
2883                         { synchronized (mutex) { return nm.ceilingKey(key); } }
2884         public Entry<K, V> higherEntry(K key)
2885                        { synchronized (mutex) { return nm.higherEntry(key); } }
2886         public K higherKey(K key)
2887                          { synchronized (mutex) { return nm.higherKey(key); } }
2888         public Entry<K, V> firstEntry()
2889                            { synchronized (mutex) { return nm.firstEntry(); } }
2890         public Entry<K, V> lastEntry()
2891                             { synchronized (mutex) { return nm.lastEntry(); } }
2892         public Entry<K, V> pollFirstEntry()
2893                        { synchronized (mutex) { return nm.pollFirstEntry(); } }
2894         public Entry<K, V> pollLastEntry()
2895                         { synchronized (mutex) { return nm.pollLastEntry(); } }
2896 
2897         public NavigableMap<K, V> descendingMap() {
2898             synchronized (mutex) {
2899                 return
2900                     new SynchronizedNavigableMap<>(nm.descendingMap(), mutex);
2901             }
2902         }
2903 
2904         public NavigableSet<K> keySet() {
2905             return navigableKeySet();
2906         }
2907 
2908         public NavigableSet<K> navigableKeySet() {
2909             synchronized (mutex) {
2910                 return new SynchronizedNavigableSet<>(nm.navigableKeySet(), mutex);
2911             }
2912         }
2913 
2914         public NavigableSet<K> descendingKeySet() {
2915             synchronized (mutex) {
2916                 return new SynchronizedNavigableSet<>(nm.descendingKeySet(), mutex);
2917             }
2918         }
2919 
2920 
2921         public SortedMap<K,V> subMap(K fromKey, K toKey) {
2922             synchronized (mutex) {
2923                 return new SynchronizedNavigableMap<>(
2924                     nm.subMap(fromKey, true, toKey, false), mutex);
2925             }
2926         }
2927         public SortedMap<K,V> headMap(K toKey) {
2928             synchronized (mutex) {
2929                 return new SynchronizedNavigableMap<>(nm.headMap(toKey, false), mutex);
2930             }
2931         }
2932         public SortedMap<K,V> tailMap(K fromKey) {
2933             synchronized (mutex) {
2934         return new SynchronizedNavigableMap<>(nm.tailMap(fromKey, true),mutex);
2935             }
2936         }
2937 
2938         public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
2939             synchronized (mutex) {
2940                 return new SynchronizedNavigableMap<>(
2941                     nm.subMap(fromKey, fromInclusive, toKey, toInclusive), mutex);
2942             }
2943         }
2944 
2945         public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
2946             synchronized (mutex) {
2947                 return new SynchronizedNavigableMap<>(
2948                         nm.headMap(toKey, inclusive), mutex);
2949             }
2950         }
2951 
2952         public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
2953             synchronized (mutex) {
2954                 return new SynchronizedNavigableMap<>(
2955                     nm.tailMap(fromKey, inclusive), mutex);
2956             }
2957         }
2958     }
2959 
2960     // Dynamically typesafe collection wrappers
2961 
2962     /**
2963      * Returns a dynamically typesafe view of the specified collection.
2964      * Any attempt to insert an element of the wrong type will result in an
2965      * immediate {@link ClassCastException}.  Assuming a collection
2966      * contains no incorrectly typed elements prior to the time a
2967      * dynamically typesafe view is generated, and that all subsequent
2968      * access to the collection takes place through the view, it is
2969      * <i>guaranteed</i> that the collection cannot contain an incorrectly
2970      * typed element.
2971      *
2972      * <p>The generics mechanism in the language provides compile-time
2973      * (static) type checking, but it is possible to defeat this mechanism
2974      * with unchecked casts.  Usually this is not a problem, as the compiler
2975      * issues warnings on all such unchecked operations.  There are, however,
2976      * times when static type checking alone is not sufficient.  For example,
2977      * suppose a collection is passed to a third-party library and it is
2978      * imperative that the library code not corrupt the collection by
2979      * inserting an element of the wrong type.
2980      *
2981      * <p>Another use of dynamically typesafe views is debugging.  Suppose a
2982      * program fails with a {@code ClassCastException}, indicating that an
2983      * incorrectly typed element was put into a parameterized collection.
2984      * Unfortunately, the exception can occur at any time after the erroneous
2985      * element is inserted, so it typically provides little or no information
2986      * as to the real source of the problem.  If the problem is reproducible,
2987      * one can quickly determine its source by temporarily modifying the
2988      * program to wrap the collection with a dynamically typesafe view.
2989      * For example, this declaration:
2990      *  <pre> {@code
2991      *     Collection<String> c = new HashSet<>();
2992      * }</pre>
2993      * may be replaced temporarily by this one:
2994      *  <pre> {@code
2995      *     Collection<String> c = Collections.checkedCollection(
2996      *         new HashSet<>(), String.class);
2997      * }</pre>
2998      * Running the program again will cause it to fail at the point where
2999      * an incorrectly typed element is inserted into the collection, clearly
3000      * identifying the source of the problem.  Once the problem is fixed, the
3001      * modified declaration may be reverted back to the original.
3002      *
3003      * <p>The returned collection does <i>not</i> pass the hashCode and equals
3004      * operations through to the backing collection, but relies on
3005      * {@code Object}'s {@code equals} and {@code hashCode} methods.  This
3006      * is necessary to preserve the contracts of these operations in the case
3007      * that the backing collection is a set or a list.
3008      *
3009      * <p>The returned collection will be serializable if the specified
3010      * collection is serializable.
3011      *
3012      * <p>Since {@code null} is considered to be a value of any reference
3013      * type, the returned collection permits insertion of null elements
3014      * whenever the backing collection does.
3015      *
3016      * @param <E> the class of the objects in the collection
3017      * @param c the collection for which a dynamically typesafe view is to be
3018      *          returned
3019      * @param type the type of element that {@code c} is permitted to hold
3020      * @return a dynamically typesafe view of the specified collection
3021      * @since 1.5
3022      */
3023     public static <E> Collection<E> checkedCollection(Collection<E> c,
3024                                                       Class<E> type) {
3025         return new CheckedCollection<>(c, type);
3026     }
3027 
3028     @SuppressWarnings("unchecked")
3029     static <T> T[] zeroLengthArray(Class<T> type) {
3030         return (T[]) Array.newInstance(type, 0);
3031     }
3032 
3033     /**
3034      * @serial include
3035      */
3036     static class CheckedCollection<E> implements Collection<E>, Serializable {
3037         private static final long serialVersionUID = 1578914078182001775L;
3038 
3039         final Collection<E> c;
3040         final Class<E> type;
3041 
3042         @SuppressWarnings("unchecked")
3043         E typeCheck(Object o) {
3044             if (o != null && !type.isInstance(o))
3045                 throw new ClassCastException(badElementMsg(o));
3046             return (E) o;
3047         }
3048 
3049         private String badElementMsg(Object o) {
3050             return "Attempt to insert " + o.getClass() +
3051                 " element into collection with element type " + type;
3052         }
3053 
3054         CheckedCollection(Collection<E> c, Class<E> type) {
3055             this.c = Objects.requireNonNull(c, "c");
3056             this.type = Objects.requireNonNull(type, "type");
3057         }
3058 
3059         public int size()                 { return c.size(); }
3060         public boolean isEmpty()          { return c.isEmpty(); }
3061         public boolean contains(Object o) { return c.contains(o); }
3062         public Object[] toArray()         { return c.toArray(); }
3063         public <T> T[] toArray(T[] a)     { return c.toArray(a); }
3064         public String toString()          { return c.toString(); }
3065         public boolean remove(Object o)   { return c.remove(o); }
3066         public void clear()               {        c.clear(); }
3067 
3068         public boolean containsAll(Collection<?> coll) {
3069             return c.containsAll(coll);
3070         }
3071         public boolean removeAll(Collection<?> coll) {
3072             return c.removeAll(coll);
3073         }
3074         public boolean retainAll(Collection<?> coll) {
3075             return c.retainAll(coll);
3076         }
3077 
3078         public Iterator<E> iterator() {
3079             // JDK-6363904 - unwrapped iterator could be typecast to
3080             // ListIterator with unsafe set()
3081             final Iterator<E> it = c.iterator();
3082             return new Iterator<E>() {
3083                 public boolean hasNext() { return it.hasNext(); }
3084                 public E next()          { return it.next(); }
3085                 public void remove()     {        it.remove(); }};
3086         }
3087 
3088         public boolean add(E e)          { return c.add(typeCheck(e)); }
3089 
3090         private E[] zeroLengthElementArray; // Lazily initialized
3091 
3092         private E[] zeroLengthElementArray() {
3093             return zeroLengthElementArray != null ? zeroLengthElementArray :
3094                 (zeroLengthElementArray = zeroLengthArray(type));
3095         }
3096 
3097         @SuppressWarnings("unchecked")
3098         Collection<E> checkedCopyOf(Collection<? extends E> coll) {
3099             Object[] a;
3100             try {
3101                 E[] z = zeroLengthElementArray();
3102                 a = coll.toArray(z);
3103                 // Defend against coll violating the toArray contract
3104                 if (a.getClass() != z.getClass())
3105                     a = Arrays.copyOf(a, a.length, z.getClass());
3106             } catch (ArrayStoreException ignore) {
3107                 // To get better and consistent diagnostics,
3108                 // we call typeCheck explicitly on each element.
3109                 // We call clone() to defend against coll retaining a
3110                 // reference to the returned array and storing a bad
3111                 // element into it after it has been type checked.
3112                 a = coll.toArray().clone();
3113                 for (Object o : a)
3114                     typeCheck(o);
3115             }
3116             // A slight abuse of the type system, but safe here.
3117             return (Collection<E>) Arrays.asList(a);
3118         }
3119 
3120         public boolean addAll(Collection<? extends E> coll) {
3121             // Doing things this way insulates us from concurrent changes
3122             // in the contents of coll and provides all-or-nothing
3123             // semantics (which we wouldn't get if we type-checked each
3124             // element as we added it)
3125             return c.addAll(checkedCopyOf(coll));
3126         }
3127 
3128         // Override default methods in Collection
3129         @Override
3130         public void forEach(Consumer<? super E> action) {c.forEach(action);}
3131         @Override
3132         public boolean removeIf(Predicate<? super E> filter) {
3133             return c.removeIf(filter);
3134         }
3135         @Override
3136         public Spliterator<E> spliterator() {return c.spliterator();}
3137         @Override
3138         public Stream<E> stream()           {return c.stream();}
3139         @Override
3140         public Stream<E> parallelStream()   {return c.parallelStream();}
3141     }
3142 
3143     /**
3144      * Returns a dynamically typesafe view of the specified queue.
3145      * Any attempt to insert an element of the wrong type will result in
3146      * an immediate {@link ClassCastException}.  Assuming a queue contains
3147      * no incorrectly typed elements prior to the time a dynamically typesafe
3148      * view is generated, and that all subsequent access to the queue
3149      * takes place through the view, it is <i>guaranteed</i> that the
3150      * queue cannot contain an incorrectly typed element.
3151      *
3152      * <p>A discussion of the use of dynamically typesafe views may be
3153      * found in the documentation for the {@link #checkedCollection
3154      * checkedCollection} method.
3155      *
3156      * <p>The returned queue will be serializable if the specified queue
3157      * is serializable.
3158      *
3159      * <p>Since {@code null} is considered to be a value of any reference
3160      * type, the returned queue permits insertion of {@code null} elements
3161      * whenever the backing queue does.
3162      *
3163      * @param <E> the class of the objects in the queue
3164      * @param queue the queue for which a dynamically typesafe view is to be
3165      *             returned
3166      * @param type the type of element that {@code queue} is permitted to hold
3167      * @return a dynamically typesafe view of the specified queue
3168      * @since 1.8
3169      */
3170     public static <E> Queue<E> checkedQueue(Queue<E> queue, Class<E> type) {
3171         return new CheckedQueue<>(queue, type);
3172     }
3173 
3174     /**
3175      * @serial include
3176      */
3177     static class CheckedQueue<E>
3178         extends CheckedCollection<E>
3179         implements Queue<E>, Serializable
3180     {
3181         private static final long serialVersionUID = 1433151992604707767L;
3182         final Queue<E> queue;
3183 
3184         CheckedQueue(Queue<E> queue, Class<E> elementType) {
3185             super(queue, elementType);
3186             this.queue = queue;
3187         }
3188 
3189         public E element()              {return queue.element();}
3190         public boolean equals(Object o) {return o == this || c.equals(o);}
3191         public int hashCode()           {return c.hashCode();}
3192         public E peek()                 {return queue.peek();}
3193         public E poll()                 {return queue.poll();}
3194         public E remove()               {return queue.remove();}
3195         public boolean offer(E e)       {return queue.offer(typeCheck(e));}
3196     }
3197 
3198     /**
3199      * Returns a dynamically typesafe view of the specified set.
3200      * Any attempt to insert an element of the wrong type will result in
3201      * an immediate {@link ClassCastException}.  Assuming a set contains
3202      * no incorrectly typed elements prior to the time a dynamically typesafe
3203      * view is generated, and that all subsequent access to the set
3204      * takes place through the view, it is <i>guaranteed</i> that the
3205      * set cannot contain an incorrectly typed element.
3206      *
3207      * <p>A discussion of the use of dynamically typesafe views may be
3208      * found in the documentation for the {@link #checkedCollection
3209      * checkedCollection} method.
3210      *
3211      * <p>The returned set will be serializable if the specified set is
3212      * serializable.
3213      *
3214      * <p>Since {@code null} is considered to be a value of any reference
3215      * type, the returned set permits insertion of null elements whenever
3216      * the backing set does.
3217      *
3218      * @param <E> the class of the objects in the set
3219      * @param s the set for which a dynamically typesafe view is to be
3220      *          returned
3221      * @param type the type of element that {@code s} is permitted to hold
3222      * @return a dynamically typesafe view of the specified set
3223      * @since 1.5
3224      */
3225     public static <E> Set<E> checkedSet(Set<E> s, Class<E> type) {
3226         return new CheckedSet<>(s, type);
3227     }
3228 
3229     /**
3230      * @serial include
3231      */
3232     static class CheckedSet<E> extends CheckedCollection<E>
3233                                  implements Set<E>, Serializable
3234     {
3235         private static final long serialVersionUID = 4694047833775013803L;
3236 
3237         CheckedSet(Set<E> s, Class<E> elementType) { super(s, elementType); }
3238 
3239         public boolean equals(Object o) { return o == this || c.equals(o); }
3240         public int hashCode()           { return c.hashCode(); }
3241     }
3242 
3243     /**
3244      * Returns a dynamically typesafe view of the specified sorted set.
3245      * Any attempt to insert an element of the wrong type will result in an
3246      * immediate {@link ClassCastException}.  Assuming a sorted set
3247      * contains no incorrectly typed elements prior to the time a
3248      * dynamically typesafe view is generated, and that all subsequent
3249      * access to the sorted set takes place through the view, it is
3250      * <i>guaranteed</i> that the sorted set cannot contain an incorrectly
3251      * typed element.
3252      *
3253      * <p>A discussion of the use of dynamically typesafe views may be
3254      * found in the documentation for the {@link #checkedCollection
3255      * checkedCollection} method.
3256      *
3257      * <p>The returned sorted set will be serializable if the specified sorted
3258      * set is serializable.
3259      *
3260      * <p>Since {@code null} is considered to be a value of any reference
3261      * type, the returned sorted set permits insertion of null elements
3262      * whenever the backing sorted set does.
3263      *
3264      * @param <E> the class of the objects in the set
3265      * @param s the sorted set for which a dynamically typesafe view is to be
3266      *          returned
3267      * @param type the type of element that {@code s} is permitted to hold
3268      * @return a dynamically typesafe view of the specified sorted set
3269      * @since 1.5
3270      */
3271     public static <E> SortedSet<E> checkedSortedSet(SortedSet<E> s,
3272                                                     Class<E> type) {
3273         return new CheckedSortedSet<>(s, type);
3274     }
3275 
3276     /**
3277      * @serial include
3278      */
3279     static class CheckedSortedSet<E> extends CheckedSet<E>
3280         implements SortedSet<E>, Serializable
3281     {
3282         private static final long serialVersionUID = 1599911165492914959L;
3283 
3284         private final SortedSet<E> ss;
3285 
3286         CheckedSortedSet(SortedSet<E> s, Class<E> type) {
3287             super(s, type);
3288             ss = s;
3289         }
3290 
3291         public Comparator<? super E> comparator() { return ss.comparator(); }
3292         public E first()                   { return ss.first(); }
3293         public E last()                    { return ss.last(); }
3294 
3295         public SortedSet<E> subSet(E fromElement, E toElement) {
3296             return checkedSortedSet(ss.subSet(fromElement,toElement), type);
3297         }
3298         public SortedSet<E> headSet(E toElement) {
3299             return checkedSortedSet(ss.headSet(toElement), type);
3300         }
3301         public SortedSet<E> tailSet(E fromElement) {
3302             return checkedSortedSet(ss.tailSet(fromElement), type);
3303         }
3304     }
3305 
3306 /**
3307      * Returns a dynamically typesafe view of the specified navigable set.
3308      * Any attempt to insert an element of the wrong type will result in an
3309      * immediate {@link ClassCastException}.  Assuming a navigable set
3310      * contains no incorrectly typed elements prior to the time a
3311      * dynamically typesafe view is generated, and that all subsequent
3312      * access to the navigable set takes place through the view, it is
3313      * <em>guaranteed</em> that the navigable set cannot contain an incorrectly
3314      * typed element.
3315      *
3316      * <p>A discussion of the use of dynamically typesafe views may be
3317      * found in the documentation for the {@link #checkedCollection
3318      * checkedCollection} method.
3319      *
3320      * <p>The returned navigable set will be serializable if the specified
3321      * navigable set is serializable.
3322      *
3323      * <p>Since {@code null} is considered to be a value of any reference
3324      * type, the returned navigable set permits insertion of null elements
3325      * whenever the backing sorted set does.
3326      *
3327      * @param <E> the class of the objects in the set
3328      * @param s the navigable set for which a dynamically typesafe view is to be
3329      *          returned
3330      * @param type the type of element that {@code s} is permitted to hold
3331      * @return a dynamically typesafe view of the specified navigable set
3332      * @since 1.8
3333      */
3334     public static <E> NavigableSet<E> checkedNavigableSet(NavigableSet<E> s,
3335                                                     Class<E> type) {
3336         return new CheckedNavigableSet<>(s, type);
3337     }
3338 
3339     /**
3340      * @serial include
3341      */
3342     static class CheckedNavigableSet<E> extends CheckedSortedSet<E>
3343         implements NavigableSet<E>, Serializable
3344     {
3345         private static final long serialVersionUID = -5429120189805438922L;
3346 
3347         private final NavigableSet<E> ns;
3348 
3349         CheckedNavigableSet(NavigableSet<E> s, Class<E> type) {
3350             super(s, type);
3351             ns = s;
3352         }
3353 
3354         public E lower(E e)                             { return ns.lower(e); }
3355         public E floor(E e)                             { return ns.floor(e); }
3356         public E ceiling(E e)                         { return ns.ceiling(e); }
3357         public E higher(E e)                           { return ns.higher(e); }
3358         public E pollFirst()                         { return ns.pollFirst(); }
3359         public E pollLast()                            {return ns.pollLast(); }
3360         public NavigableSet<E> descendingSet()
3361                       { return checkedNavigableSet(ns.descendingSet(), type); }
3362         public Iterator<E> descendingIterator()
3363             {return checkedNavigableSet(ns.descendingSet(), type).iterator(); }
3364 
3365         public NavigableSet<E> subSet(E fromElement, E toElement) {
3366             return checkedNavigableSet(ns.subSet(fromElement, true, toElement, false), type);
3367         }
3368         public NavigableSet<E> headSet(E toElement) {
3369             return checkedNavigableSet(ns.headSet(toElement, false), type);
3370         }
3371         public NavigableSet<E> tailSet(E fromElement) {
3372             return checkedNavigableSet(ns.tailSet(fromElement, true), type);
3373         }
3374 
3375         public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
3376             return checkedNavigableSet(ns.subSet(fromElement, fromInclusive, toElement, toInclusive), type);
3377         }
3378 
3379         public NavigableSet<E> headSet(E toElement, boolean inclusive) {
3380             return checkedNavigableSet(ns.headSet(toElement, inclusive), type);
3381         }
3382 
3383         public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
3384             return checkedNavigableSet(ns.tailSet(fromElement, inclusive), type);
3385         }
3386     }
3387 
3388     /**
3389      * Returns a dynamically typesafe view of the specified list.
3390      * Any attempt to insert an element of the wrong type will result in
3391      * an immediate {@link ClassCastException}.  Assuming a list contains
3392      * no incorrectly typed elements prior to the time a dynamically typesafe
3393      * view is generated, and that all subsequent access to the list
3394      * takes place through the view, it is <i>guaranteed</i> that the
3395      * list cannot contain an incorrectly typed element.
3396      *
3397      * <p>A discussion of the use of dynamically typesafe views may be
3398      * found in the documentation for the {@link #checkedCollection
3399      * checkedCollection} method.
3400      *
3401      * <p>The returned list will be serializable if the specified list
3402      * is serializable.
3403      *
3404      * <p>Since {@code null} is considered to be a value of any reference
3405      * type, the returned list permits insertion of null elements whenever
3406      * the backing list does.
3407      *
3408      * @param <E> the class of the objects in the list
3409      * @param list the list for which a dynamically typesafe view is to be
3410      *             returned
3411      * @param type the type of element that {@code list} is permitted to hold
3412      * @return a dynamically typesafe view of the specified list
3413      * @since 1.5
3414      */
3415     public static <E> List<E> checkedList(List<E> list, Class<E> type) {
3416         return (list instanceof RandomAccess ?
3417                 new CheckedRandomAccessList<>(list, type) :
3418                 new CheckedList<>(list, type));
3419     }
3420 
3421     /**
3422      * @serial include
3423      */
3424     static class CheckedList<E>
3425         extends CheckedCollection<E>
3426         implements List<E>
3427     {
3428         private static final long serialVersionUID = 65247728283967356L;
3429         final List<E> list;
3430 
3431         CheckedList(List<E> list, Class<E> type) {
3432             super(list, type);
3433             this.list = list;
3434         }
3435 
3436         public boolean equals(Object o)  { return o == this || list.equals(o); }
3437         public int hashCode()            { return list.hashCode(); }
3438         public E get(int index)          { return list.get(index); }
3439         public E remove(int index)       { return list.remove(index); }
3440         public int indexOf(Object o)     { return list.indexOf(o); }
3441         public int lastIndexOf(Object o) { return list.lastIndexOf(o); }
3442 
3443         public E set(int index, E element) {
3444             return list.set(index, typeCheck(element));
3445         }
3446 
3447         public void add(int index, E element) {
3448             list.add(index, typeCheck(element));
3449         }
3450 
3451         public boolean addAll(int index, Collection<? extends E> c) {
3452             return list.addAll(index, checkedCopyOf(c));
3453         }
3454         public ListIterator<E> listIterator()   { return listIterator(0); }
3455 
3456         public ListIterator<E> listIterator(final int index) {
3457             final ListIterator<E> i = list.listIterator(index);
3458 
3459             return new ListIterator<E>() {
3460                 public boolean hasNext()     { return i.hasNext(); }
3461                 public E next()              { return i.next(); }
3462                 public boolean hasPrevious() { return i.hasPrevious(); }
3463                 public E previous()          { return i.previous(); }
3464                 public int nextIndex()       { return i.nextIndex(); }
3465                 public int previousIndex()   { return i.previousIndex(); }
3466                 public void remove()         {        i.remove(); }
3467 
3468                 public void set(E e) {
3469                     i.set(typeCheck(e));
3470                 }
3471 
3472                 public void add(E e) {
3473                     i.add(typeCheck(e));
3474                 }
3475 
3476                 @Override
3477                 public void forEachRemaining(Consumer<? super E> action) {
3478                     i.forEachRemaining(action);
3479                 }
3480             };
3481         }
3482 
3483         public List<E> subList(int fromIndex, int toIndex) {
3484             return new CheckedList<>(list.subList(fromIndex, toIndex), type);
3485         }
3486 
3487         /**
3488          * {@inheritDoc}
3489          *
3490          * @throws ClassCastException if the class of an element returned by the
3491          *         operator prevents it from being added to this collection. The
3492          *         exception may be thrown after some elements of the list have
3493          *         already been replaced.
3494          */
3495         @Override
3496         public void replaceAll(UnaryOperator<E> operator) {
3497             Objects.requireNonNull(operator);
3498             list.replaceAll(e -> typeCheck(operator.apply(e)));
3499         }
3500 
3501         @Override
3502         public void sort(Comparator<? super E> c) {
3503             list.sort(c);
3504         }
3505     }
3506 
3507     /**
3508      * @serial include
3509      */
3510     static class CheckedRandomAccessList<E> extends CheckedList<E>
3511                                             implements RandomAccess
3512     {
3513         private static final long serialVersionUID = 1638200125423088369L;
3514 
3515         CheckedRandomAccessList(List<E> list, Class<E> type) {
3516             super(list, type);
3517         }
3518 
3519         public List<E> subList(int fromIndex, int toIndex) {
3520             return new CheckedRandomAccessList<>(
3521                     list.subList(fromIndex, toIndex), type);
3522         }
3523     }
3524 
3525     /**
3526      * Returns a dynamically typesafe view of the specified map.
3527      * Any attempt to insert a mapping whose key or value have the wrong
3528      * type will result in an immediate {@link ClassCastException}.
3529      * Similarly, any attempt to modify the value currently associated with
3530      * a key will result in an immediate {@link ClassCastException},
3531      * whether the modification is attempted directly through the map
3532      * itself, or through a {@link Map.Entry} instance obtained from the
3533      * map's {@link Map#entrySet() entry set} view.
3534      *
3535      * <p>Assuming a map contains no incorrectly typed keys or values
3536      * prior to the time a dynamically typesafe view is generated, and
3537      * that all subsequent access to the map takes place through the view
3538      * (or one of its collection views), it is <i>guaranteed</i> that the
3539      * map cannot contain an incorrectly typed key or value.
3540      *
3541      * <p>A discussion of the use of dynamically typesafe views may be
3542      * found in the documentation for the {@link #checkedCollection
3543      * checkedCollection} method.
3544      *
3545      * <p>The returned map will be serializable if the specified map is
3546      * serializable.
3547      *
3548      * <p>Since {@code null} is considered to be a value of any reference
3549      * type, the returned map permits insertion of null keys or values
3550      * whenever the backing map does.
3551      *
3552      * @param <K> the class of the map keys
3553      * @param <V> the class of the map values
3554      * @param m the map for which a dynamically typesafe view is to be
3555      *          returned
3556      * @param keyType the type of key that {@code m} is permitted to hold
3557      * @param valueType the type of value that {@code m} is permitted to hold
3558      * @return a dynamically typesafe view of the specified map
3559      * @since 1.5
3560      */
3561     public static <K, V> Map<K, V> checkedMap(Map<K, V> m,
3562                                               Class<K> keyType,
3563                                               Class<V> valueType) {
3564         return new CheckedMap<>(m, keyType, valueType);
3565     }
3566 
3567 
3568     /**
3569      * @serial include
3570      */
3571     private static class CheckedMap<K,V>
3572         implements Map<K,V>, Serializable
3573     {
3574         private static final long serialVersionUID = 5742860141034234728L;
3575 
3576         private final Map<K, V> m;
3577         final Class<K> keyType;
3578         final Class<V> valueType;
3579 
3580         private void typeCheck(Object key, Object value) {
3581             if (key != null && !keyType.isInstance(key))
3582                 throw new ClassCastException(badKeyMsg(key));
3583 
3584             if (value != null && !valueType.isInstance(value))
3585                 throw new ClassCastException(badValueMsg(value));
3586         }
3587 
3588         private BiFunction<? super K, ? super V, ? extends V> typeCheck(
3589                 BiFunction<? super K, ? super V, ? extends V> func) {
3590             Objects.requireNonNull(func);
3591             return (k, v) -> {
3592                 V newValue = func.apply(k, v);
3593                 typeCheck(k, newValue);
3594                 return newValue;
3595             };
3596         }
3597 
3598         private String badKeyMsg(Object key) {
3599             return "Attempt to insert " + key.getClass() +
3600                     " key into map with key type " + keyType;
3601         }
3602 
3603         private String badValueMsg(Object value) {
3604             return "Attempt to insert " + value.getClass() +
3605                     " value into map with value type " + valueType;
3606         }
3607 
3608         CheckedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) {
3609             this.m = Objects.requireNonNull(m);
3610             this.keyType = Objects.requireNonNull(keyType);
3611             this.valueType = Objects.requireNonNull(valueType);
3612         }
3613 
3614         public int size()                      { return m.size(); }
3615         public boolean isEmpty()               { return m.isEmpty(); }
3616         public boolean containsKey(Object key) { return m.containsKey(key); }
3617         public boolean containsValue(Object v) { return m.containsValue(v); }
3618         public V get(Object key)               { return m.get(key); }
3619         public V remove(Object key)            { return m.remove(key); }
3620         public void clear()                    { m.clear(); }
3621         public Set<K> keySet()                 { return m.keySet(); }
3622         public Collection<V> values()          { return m.values(); }
3623         public boolean equals(Object o)        { return o == this || m.equals(o); }
3624         public int hashCode()                  { return m.hashCode(); }
3625         public String toString()               { return m.toString(); }
3626 
3627         public V put(K key, V value) {
3628             typeCheck(key, value);
3629             return m.put(key, value);
3630         }
3631 
3632         @SuppressWarnings("unchecked")
3633         public void putAll(Map<? extends K, ? extends V> t) {
3634             // Satisfy the following goals:
3635             // - good diagnostics in case of type mismatch
3636             // - all-or-nothing semantics
3637             // - protection from malicious t
3638             // - correct behavior if t is a concurrent map
3639             Object[] entries = t.entrySet().toArray();
3640             List<Map.Entry<K,V>> checked = new ArrayList<>(entries.length);
3641             for (Object o : entries) {
3642                 Map.Entry<?,?> e = (Map.Entry<?,?>) o;
3643                 Object k = e.getKey();
3644                 Object v = e.getValue();
3645                 typeCheck(k, v);
3646                 checked.add(
3647                         new AbstractMap.SimpleImmutableEntry<>((K)k, (V)v));
3648             }
3649             for (Map.Entry<K,V> e : checked)
3650                 m.put(e.getKey(), e.getValue());
3651         }
3652 
3653         private transient Set<Map.Entry<K,V>> entrySet;
3654 
3655         public Set<Map.Entry<K,V>> entrySet() {
3656             if (entrySet==null)
3657                 entrySet = new CheckedEntrySet<>(m.entrySet(), valueType);
3658             return entrySet;
3659         }
3660 
3661         // Override default methods in Map
3662         @Override
3663         public void forEach(BiConsumer<? super K, ? super V> action) {
3664             m.forEach(action);
3665         }
3666 
3667         @Override
3668         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
3669             m.replaceAll(typeCheck(function));
3670         }
3671 
3672         @Override
3673         public V putIfAbsent(K key, V value) {
3674             typeCheck(key, value);
3675             return m.putIfAbsent(key, value);
3676         }
3677 
3678         @Override
3679         public boolean remove(Object key, Object value) {
3680             return m.remove(key, value);
3681         }
3682 
3683         @Override
3684         public boolean replace(K key, V oldValue, V newValue) {
3685             typeCheck(key, newValue);
3686             return m.replace(key, oldValue, newValue);
3687         }
3688 
3689         @Override
3690         public V replace(K key, V value) {
3691             typeCheck(key, value);
3692             return m.replace(key, value);
3693         }
3694 
3695         @Override
3696         public V computeIfAbsent(K key,
3697                 Function<? super K, ? extends V> mappingFunction) {
3698             Objects.requireNonNull(mappingFunction);
3699             return m.computeIfAbsent(key, k -> {
3700                 V value = mappingFunction.apply(k);
3701                 typeCheck(k, value);
3702                 return value;
3703             });
3704         }
3705 
3706         @Override
3707         public V computeIfPresent(K key,
3708                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
3709             return m.computeIfPresent(key, typeCheck(remappingFunction));
3710         }
3711 
3712         @Override
3713         public V compute(K key,
3714                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
3715             return m.compute(key, typeCheck(remappingFunction));
3716         }
3717 
3718         @Override
3719         public V merge(K key, V value,
3720                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
3721             Objects.requireNonNull(remappingFunction);
3722             return m.merge(key, value, (v1, v2) -> {
3723                 V newValue = remappingFunction.apply(v1, v2);
3724                 typeCheck(null, newValue);
3725                 return newValue;
3726             });
3727         }
3728 
3729         /**
3730          * We need this class in addition to CheckedSet as Map.Entry permits
3731          * modification of the backing Map via the setValue operation.  This
3732          * class is subtle: there are many possible attacks that must be
3733          * thwarted.
3734          *
3735          * @serial exclude
3736          */
3737         static class CheckedEntrySet<K,V> implements Set<Map.Entry<K,V>> {
3738             private final Set<Map.Entry<K,V>> s;
3739             private final Class<V> valueType;
3740 
3741             CheckedEntrySet(Set<Map.Entry<K, V>> s, Class<V> valueType) {
3742                 this.s = s;
3743                 this.valueType = valueType;
3744             }
3745 
3746             public int size()        { return s.size(); }
3747             public boolean isEmpty() { return s.isEmpty(); }
3748             public String toString() { return s.toString(); }
3749             public int hashCode()    { return s.hashCode(); }
3750             public void clear()      {        s.clear(); }
3751 
3752             public boolean add(Map.Entry<K, V> e) {
3753                 throw new UnsupportedOperationException();
3754             }
3755             public boolean addAll(Collection<? extends Map.Entry<K, V>> coll) {
3756                 throw new UnsupportedOperationException();
3757             }
3758 
3759             public Iterator<Map.Entry<K,V>> iterator() {
3760                 final Iterator<Map.Entry<K, V>> i = s.iterator();
3761                 final Class<V> valueType = this.valueType;
3762 
3763                 return new Iterator<Map.Entry<K,V>>() {
3764                     public boolean hasNext() { return i.hasNext(); }
3765                     public void remove()     { i.remove(); }
3766 
3767                     public Map.Entry<K,V> next() {
3768                         return checkedEntry(i.next(), valueType);
3769                     }
3770                 };
3771             }
3772 
3773             @SuppressWarnings("unchecked")
3774             public Object[] toArray() {
3775                 Object[] source = s.toArray();
3776 
3777                 /*
3778                  * Ensure that we don't get an ArrayStoreException even if
3779                  * s.toArray returns an array of something other than Object
3780                  */
3781                 Object[] dest = (CheckedEntry.class.isInstance(
3782                     source.getClass().getComponentType()) ? source :
3783                                  new Object[source.length]);
3784 
3785                 for (int i = 0; i < source.length; i++)
3786                     dest[i] = checkedEntry((Map.Entry<K,V>)source[i],
3787                                            valueType);
3788                 return dest;
3789             }
3790 
3791             @SuppressWarnings("unchecked")
3792             public <T> T[] toArray(T[] a) {
3793                 // We don't pass a to s.toArray, to avoid window of
3794                 // vulnerability wherein an unscrupulous multithreaded client
3795                 // could get his hands on raw (unwrapped) Entries from s.
3796                 T[] arr = s.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));
3797 
3798                 for (int i=0; i<arr.length; i++)
3799                     arr[i] = (T) checkedEntry((Map.Entry<K,V>)arr[i],
3800                                               valueType);
3801                 if (arr.length > a.length)
3802                     return arr;
3803 
3804                 System.arraycopy(arr, 0, a, 0, arr.length);
3805                 if (a.length > arr.length)
3806                     a[arr.length] = null;
3807                 return a;
3808             }
3809 
3810             /**
3811              * This method is overridden to protect the backing set against
3812              * an object with a nefarious equals function that senses
3813              * that the equality-candidate is Map.Entry and calls its
3814              * setValue method.
3815              */
3816             public boolean contains(Object o) {
3817                 if (!(o instanceof Map.Entry))
3818                     return false;
3819                 Map.Entry<?,?> e = (Map.Entry<?,?>) o;
3820                 return s.contains(
3821                     (e instanceof CheckedEntry) ? e : checkedEntry(e, valueType));
3822             }
3823 
3824             /**
3825              * The bulk collection methods are overridden to protect
3826              * against an unscrupulous collection whose contains(Object o)
3827              * method senses when o is a Map.Entry, and calls o.setValue.
3828              */
3829             public boolean containsAll(Collection<?> c) {
3830                 for (Object o : c)
3831                     if (!contains(o)) // Invokes safe contains() above
3832                         return false;
3833                 return true;
3834             }
3835 
3836             public boolean remove(Object o) {
3837                 if (!(o instanceof Map.Entry))
3838                     return false;
3839                 return s.remove(new AbstractMap.SimpleImmutableEntry
3840                                 <>((Map.Entry<?,?>)o));
3841             }
3842 
3843             public boolean removeAll(Collection<?> c) {
3844                 return batchRemove(c, false);
3845             }
3846             public boolean retainAll(Collection<?> c) {
3847                 return batchRemove(c, true);
3848             }
3849             private boolean batchRemove(Collection<?> c, boolean complement) {
3850                 Objects.requireNonNull(c);
3851                 boolean modified = false;
3852                 Iterator<Map.Entry<K,V>> it = iterator();
3853                 while (it.hasNext()) {
3854                     if (c.contains(it.next()) != complement) {
3855                         it.remove();
3856                         modified = true;
3857                     }
3858                 }
3859                 return modified;
3860             }
3861 
3862             public boolean equals(Object o) {
3863                 if (o == this)
3864                     return true;
3865                 if (!(o instanceof Set))
3866                     return false;
3867                 Set<?> that = (Set<?>) o;
3868                 return that.size() == s.size()
3869                     && containsAll(that); // Invokes safe containsAll() above
3870             }
3871 
3872             static <K,V,T> CheckedEntry<K,V,T> checkedEntry(Map.Entry<K,V> e,
3873                                                             Class<T> valueType) {
3874                 return new CheckedEntry<>(e, valueType);
3875             }
3876 
3877             /**
3878              * This "wrapper class" serves two purposes: it prevents
3879              * the client from modifying the backing Map, by short-circuiting
3880              * the setValue method, and it protects the backing Map against
3881              * an ill-behaved Map.Entry that attempts to modify another
3882              * Map.Entry when asked to perform an equality check.
3883              */
3884             private static class CheckedEntry<K,V,T> implements Map.Entry<K,V> {
3885                 private final Map.Entry<K, V> e;
3886                 private final Class<T> valueType;
3887 
3888                 CheckedEntry(Map.Entry<K, V> e, Class<T> valueType) {
3889                     this.e = Objects.requireNonNull(e);
3890                     this.valueType = Objects.requireNonNull(valueType);
3891                 }
3892 
3893                 public K getKey()        { return e.getKey(); }
3894                 public V getValue()      { return e.getValue(); }
3895                 public int hashCode()    { return e.hashCode(); }
3896                 public String toString() { return e.toString(); }
3897 
3898                 public V setValue(V value) {
3899                     if (value != null && !valueType.isInstance(value))
3900                         throw new ClassCastException(badValueMsg(value));
3901                     return e.setValue(value);
3902                 }
3903 
3904                 private String badValueMsg(Object value) {
3905                     return "Attempt to insert " + value.getClass() +
3906                         " value into map with value type " + valueType;
3907                 }
3908 
3909                 public boolean equals(Object o) {
3910                     if (o == this)
3911                         return true;
3912                     if (!(o instanceof Map.Entry))
3913                         return false;
3914                     return e.equals(new AbstractMap.SimpleImmutableEntry
3915                                     <>((Map.Entry<?,?>)o));
3916                 }
3917             }
3918         }
3919     }
3920 
3921     /**
3922      * Returns a dynamically typesafe view of the specified sorted map.
3923      * Any attempt to insert a mapping whose key or value have the wrong
3924      * type will result in an immediate {@link ClassCastException}.
3925      * Similarly, any attempt to modify the value currently associated with
3926      * a key will result in an immediate {@link ClassCastException},
3927      * whether the modification is attempted directly through the map
3928      * itself, or through a {@link Map.Entry} instance obtained from the
3929      * map's {@link Map#entrySet() entry set} view.
3930      *
3931      * <p>Assuming a map contains no incorrectly typed keys or values
3932      * prior to the time a dynamically typesafe view is generated, and
3933      * that all subsequent access to the map takes place through the view
3934      * (or one of its collection views), it is <i>guaranteed</i> that the
3935      * map cannot contain an incorrectly typed key or value.
3936      *
3937      * <p>A discussion of the use of dynamically typesafe views may be
3938      * found in the documentation for the {@link #checkedCollection
3939      * checkedCollection} method.
3940      *
3941      * <p>The returned map will be serializable if the specified map is
3942      * serializable.
3943      *
3944      * <p>Since {@code null} is considered to be a value of any reference
3945      * type, the returned map permits insertion of null keys or values
3946      * whenever the backing map does.
3947      *
3948      * @param <K> the class of the map keys
3949      * @param <V> the class of the map values
3950      * @param m the map for which a dynamically typesafe view is to be
3951      *          returned
3952      * @param keyType the type of key that {@code m} is permitted to hold
3953      * @param valueType the type of value that {@code m} is permitted to hold
3954      * @return a dynamically typesafe view of the specified map
3955      * @since 1.5
3956      */
3957     public static <K,V> SortedMap<K,V> checkedSortedMap(SortedMap<K, V> m,
3958                                                         Class<K> keyType,
3959                                                         Class<V> valueType) {
3960         return new CheckedSortedMap<>(m, keyType, valueType);
3961     }
3962 
3963     /**
3964      * @serial include
3965      */
3966     static class CheckedSortedMap<K,V> extends CheckedMap<K,V>
3967         implements SortedMap<K,V>, Serializable
3968     {
3969         private static final long serialVersionUID = 1599671320688067438L;
3970 
3971         private final SortedMap<K, V> sm;
3972 
3973         CheckedSortedMap(SortedMap<K, V> m,
3974                          Class<K> keyType, Class<V> valueType) {
3975             super(m, keyType, valueType);
3976             sm = m;
3977         }
3978 
3979         public Comparator<? super K> comparator() { return sm.comparator(); }
3980         public K firstKey()                       { return sm.firstKey(); }
3981         public K lastKey()                        { return sm.lastKey(); }
3982 
3983         public SortedMap<K,V> subMap(K fromKey, K toKey) {
3984             return checkedSortedMap(sm.subMap(fromKey, toKey),
3985                                     keyType, valueType);
3986         }
3987         public SortedMap<K,V> headMap(K toKey) {
3988             return checkedSortedMap(sm.headMap(toKey), keyType, valueType);
3989         }
3990         public SortedMap<K,V> tailMap(K fromKey) {
3991             return checkedSortedMap(sm.tailMap(fromKey), keyType, valueType);
3992         }
3993     }
3994 
3995     /**
3996      * Returns a dynamically typesafe view of the specified navigable map.
3997      * Any attempt to insert a mapping whose key or value have the wrong
3998      * type will result in an immediate {@link ClassCastException}.
3999      * Similarly, any attempt to modify the value currently associated with
4000      * a key will result in an immediate {@link ClassCastException},
4001      * whether the modification is attempted directly through the map
4002      * itself, or through a {@link Map.Entry} instance obtained from the
4003      * map's {@link Map#entrySet() entry set} view.
4004      *
4005      * <p>Assuming a map contains no incorrectly typed keys or values
4006      * prior to the time a dynamically typesafe view is generated, and
4007      * that all subsequent access to the map takes place through the view
4008      * (or one of its collection views), it is <em>guaranteed</em> that the
4009      * map cannot contain an incorrectly typed key or value.
4010      *
4011      * <p>A discussion of the use of dynamically typesafe views may be
4012      * found in the documentation for the {@link #checkedCollection
4013      * checkedCollection} method.
4014      *
4015      * <p>The returned map will be serializable if the specified map is
4016      * serializable.
4017      *
4018      * <p>Since {@code null} is considered to be a value of any reference
4019      * type, the returned map permits insertion of null keys or values
4020      * whenever the backing map does.
4021      *
4022      * @param <K> type of map keys
4023      * @param <V> type of map values
4024      * @param m the map for which a dynamically typesafe view is to be
4025      *          returned
4026      * @param keyType the type of key that {@code m} is permitted to hold
4027      * @param valueType the type of value that {@code m} is permitted to hold
4028      * @return a dynamically typesafe view of the specified map
4029      * @since 1.8
4030      */
4031     public static <K,V> NavigableMap<K,V> checkedNavigableMap(NavigableMap<K, V> m,
4032                                                         Class<K> keyType,
4033                                                         Class<V> valueType) {
4034         return new CheckedNavigableMap<>(m, keyType, valueType);
4035     }
4036 
4037     /**
4038      * @serial include
4039      */
4040     static class CheckedNavigableMap<K,V> extends CheckedSortedMap<K,V>
4041         implements NavigableMap<K,V>, Serializable
4042     {
4043         private static final long serialVersionUID = -4852462692372534096L;
4044 
4045         private final NavigableMap<K, V> nm;
4046 
4047         CheckedNavigableMap(NavigableMap<K, V> m,
4048                          Class<K> keyType, Class<V> valueType) {
4049             super(m, keyType, valueType);
4050             nm = m;
4051         }
4052 
4053         public Comparator<? super K> comparator()   { return nm.comparator(); }
4054         public K firstKey()                           { return nm.firstKey(); }
4055         public K lastKey()                             { return nm.lastKey(); }
4056 
4057         public Entry<K, V> lowerEntry(K key) {
4058             Entry<K,V> lower = nm.lowerEntry(key);
4059             return (null != lower)
4060                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(lower, valueType)
4061                 : null;
4062         }
4063 
4064         public K lowerKey(K key)                   { return nm.lowerKey(key); }
4065 
4066         public Entry<K, V> floorEntry(K key) {
4067             Entry<K,V> floor = nm.floorEntry(key);
4068             return (null != floor)
4069                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(floor, valueType)
4070                 : null;
4071         }
4072 
4073         public K floorKey(K key)                   { return nm.floorKey(key); }
4074 
4075         public Entry<K, V> ceilingEntry(K key) {
4076             Entry<K,V> ceiling = nm.ceilingEntry(key);
4077             return (null != ceiling)
4078                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(ceiling, valueType)
4079                 : null;
4080         }
4081 
4082         public K ceilingKey(K key)               { return nm.ceilingKey(key); }
4083 
4084         public Entry<K, V> higherEntry(K key) {
4085             Entry<K,V> higher = nm.higherEntry(key);
4086             return (null != higher)
4087                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(higher, valueType)
4088                 : null;
4089         }
4090 
4091         public K higherKey(K key)                 { return nm.higherKey(key); }
4092 
4093         public Entry<K, V> firstEntry() {
4094             Entry<K,V> first = nm.firstEntry();
4095             return (null != first)
4096                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(first, valueType)
4097                 : null;
4098         }
4099 
4100         public Entry<K, V> lastEntry() {
4101             Entry<K,V> last = nm.lastEntry();
4102             return (null != last)
4103                 ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(last, valueType)
4104                 : null;
4105         }
4106 
4107         public Entry<K, V> pollFirstEntry() {
4108             Entry<K,V> entry = nm.pollFirstEntry();
4109             return (null == entry)
4110                 ? null
4111                 : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
4112         }
4113 
4114         public Entry<K, V> pollLastEntry() {
4115             Entry<K,V> entry = nm.pollLastEntry();
4116             return (null == entry)
4117                 ? null
4118                 : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
4119         }
4120 
4121         public NavigableMap<K, V> descendingMap() {
4122             return checkedNavigableMap(nm.descendingMap(), keyType, valueType);
4123         }
4124 
4125         public NavigableSet<K> keySet() {
4126             return navigableKeySet();
4127         }
4128 
4129         public NavigableSet<K> navigableKeySet() {
4130             return checkedNavigableSet(nm.navigableKeySet(), keyType);
4131         }
4132 
4133         public NavigableSet<K> descendingKeySet() {
4134             return checkedNavigableSet(nm.descendingKeySet(), keyType);
4135         }
4136 
4137         @Override
4138         public NavigableMap<K,V> subMap(K fromKey, K toKey) {
4139             return checkedNavigableMap(nm.subMap(fromKey, true, toKey, false),
4140                                     keyType, valueType);
4141         }
4142 
4143         @Override
4144         public NavigableMap<K,V> headMap(K toKey) {
4145             return checkedNavigableMap(nm.headMap(toKey, false), keyType, valueType);
4146         }
4147 
4148         @Override
4149         public NavigableMap<K,V> tailMap(K fromKey) {
4150             return checkedNavigableMap(nm.tailMap(fromKey, true), keyType, valueType);
4151         }
4152 
4153         public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
4154             return checkedNavigableMap(nm.subMap(fromKey, fromInclusive, toKey, toInclusive), keyType, valueType);
4155         }
4156 
4157         public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
4158             return checkedNavigableMap(nm.headMap(toKey, inclusive), keyType, valueType);
4159         }
4160 
4161         public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
4162             return checkedNavigableMap(nm.tailMap(fromKey, inclusive), keyType, valueType);
4163         }
4164     }
4165 
4166     // Empty collections
4167 
4168     /**
4169      * Returns an iterator that has no elements.  More precisely,
4170      *
4171      * <ul>
4172      * <li>{@link Iterator#hasNext hasNext} always returns {@code
4173      * false}.</li>
4174      * <li>{@link Iterator#next next} always throws {@link
4175      * NoSuchElementException}.</li>
4176      * <li>{@link Iterator#remove remove} always throws {@link
4177      * IllegalStateException}.</li>
4178      * </ul>
4179      *
4180      * <p>Implementations of this method are permitted, but not
4181      * required, to return the same object from multiple invocations.
4182      *
4183      * @param <T> type of elements, if there were any, in the iterator
4184      * @return an empty iterator
4185      * @since 1.7
4186      */
4187     @SuppressWarnings("unchecked")
4188     public static <T> Iterator<T> emptyIterator() {
4189         return (Iterator<T>) EmptyIterator.EMPTY_ITERATOR;
4190     }
4191 
4192     private static class EmptyIterator<E> implements Iterator<E> {
4193         static final EmptyIterator<Object> EMPTY_ITERATOR
4194             = new EmptyIterator<>();
4195 
4196         public boolean hasNext() { return false; }
4197         public E next() { throw new NoSuchElementException(); }
4198         public void remove() { throw new IllegalStateException(); }
4199         @Override
4200         public void forEachRemaining(Consumer<? super E> action) {
4201             Objects.requireNonNull(action);
4202         }
4203     }
4204 
4205     /**
4206      * Returns a list iterator that has no elements.  More precisely,
4207      *
4208      * <ul>
4209      * <li>{@link Iterator#hasNext hasNext} and {@link
4210      * ListIterator#hasPrevious hasPrevious} always return {@code
4211      * false}.</li>
4212      * <li>{@link Iterator#next next} and {@link ListIterator#previous
4213      * previous} always throw {@link NoSuchElementException}.</li>
4214      * <li>{@link Iterator#remove remove} and {@link ListIterator#set
4215      * set} always throw {@link IllegalStateException}.</li>
4216      * <li>{@link ListIterator#add add} always throws {@link
4217      * UnsupportedOperationException}.</li>
4218      * <li>{@link ListIterator#nextIndex nextIndex} always returns
4219      * {@code 0}.</li>
4220      * <li>{@link ListIterator#previousIndex previousIndex} always
4221      * returns {@code -1}.</li>
4222      * </ul>
4223      *
4224      * <p>Implementations of this method are permitted, but not
4225      * required, to return the same object from multiple invocations.
4226      *
4227      * @param <T> type of elements, if there were any, in the iterator
4228      * @return an empty list iterator
4229      * @since 1.7
4230      */
4231     @SuppressWarnings("unchecked")
4232     public static <T> ListIterator<T> emptyListIterator() {
4233         return (ListIterator<T>) EmptyListIterator.EMPTY_ITERATOR;
4234     }
4235 
4236     private static class EmptyListIterator<E>
4237         extends EmptyIterator<E>
4238         implements ListIterator<E>
4239     {
4240         static final EmptyListIterator<Object> EMPTY_ITERATOR
4241             = new EmptyListIterator<>();
4242 
4243         public boolean hasPrevious() { return false; }
4244         public E previous() { throw new NoSuchElementException(); }
4245         public int nextIndex()     { return 0; }
4246         public int previousIndex() { return -1; }
4247         public void set(E e) { throw new IllegalStateException(); }
4248         public void add(E e) { throw new UnsupportedOperationException(); }
4249     }
4250 
4251     /**
4252      * Returns an enumeration that has no elements.  More precisely,
4253      *
4254      * <ul>
4255      * <li>{@link Enumeration#hasMoreElements hasMoreElements} always
4256      * returns {@code false}.</li>
4257      * <li> {@link Enumeration#nextElement nextElement} always throws
4258      * {@link NoSuchElementException}.</li>
4259      * </ul>
4260      *
4261      * <p>Implementations of this method are permitted, but not
4262      * required, to return the same object from multiple invocations.
4263      *
4264      * @param  <T> the class of the objects in the enumeration
4265      * @return an empty enumeration
4266      * @since 1.7
4267      */
4268     @SuppressWarnings("unchecked")
4269     public static <T> Enumeration<T> emptyEnumeration() {
4270         return (Enumeration<T>) EmptyEnumeration.EMPTY_ENUMERATION;
4271     }
4272 
4273     private static class EmptyEnumeration<E> implements Enumeration<E> {
4274         static final EmptyEnumeration<Object> EMPTY_ENUMERATION
4275             = new EmptyEnumeration<>();
4276 
4277         public boolean hasMoreElements() { return false; }
4278         public E nextElement() { throw new NoSuchElementException(); }
4279         public Iterator<E> asIterator() { return emptyIterator(); }
4280     }
4281 
4282     /**
4283      * The empty set (immutable).  This set is serializable.
4284      *
4285      * @see #emptySet()
4286      */
4287     @SuppressWarnings("rawtypes")
4288     public static final Set EMPTY_SET = new EmptySet<>();
4289 
4290     /**
4291      * Returns an empty set (immutable).  This set is serializable.
4292      * Unlike the like-named field, this method is parameterized.
4293      *
4294      * <p>This example illustrates the type-safe way to obtain an empty set:
4295      * <pre>
4296      *     Set&lt;String&gt; s = Collections.emptySet();
4297      * </pre>
4298      * @implNote Implementations of this method need not create a separate
4299      * {@code Set} object for each call.  Using this method is likely to have
4300      * comparable cost to using the like-named field.  (Unlike this method, the
4301      * field does not provide type safety.)
4302      *
4303      * @param  <T> the class of the objects in the set
4304      * @return the empty set
4305      *
4306      * @see #EMPTY_SET
4307      * @since 1.5
4308      */
4309     @SuppressWarnings("unchecked")
4310     public static final <T> Set<T> emptySet() {
4311         return (Set<T>) EMPTY_SET;
4312     }
4313 
4314     /**
4315      * @serial include
4316      */
4317     private static class EmptySet<E>
4318         extends AbstractSet<E>
4319         implements Serializable
4320     {
4321         private static final long serialVersionUID = 1582296315990362920L;
4322 
4323         public Iterator<E> iterator() { return emptyIterator(); }
4324 
4325         public int size() {return 0;}
4326         public boolean isEmpty() {return true;}
4327         public void clear() {}
4328 
4329         public boolean contains(Object obj) {return false;}
4330         public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
4331 
4332         public Object[] toArray() { return new Object[0]; }
4333 
4334         public <T> T[] toArray(T[] a) {
4335             if (a.length > 0)
4336                 a[0] = null;
4337             return a;
4338         }
4339 
4340         // Override default methods in Collection
4341         @Override
4342         public void forEach(Consumer<? super E> action) {
4343             Objects.requireNonNull(action);
4344         }
4345         @Override
4346         public boolean removeIf(Predicate<? super E> filter) {
4347             Objects.requireNonNull(filter);
4348             return false;
4349         }
4350         @Override
4351         public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
4352 
4353         // Preserves singleton property
4354         private Object readResolve() {
4355             return EMPTY_SET;
4356         }
4357 
4358         @Override
4359         public int hashCode() {
4360             return 0;
4361         }
4362     }
4363 
4364     /**
4365      * Returns an empty sorted set (immutable).  This set is serializable.
4366      *
4367      * <p>This example illustrates the type-safe way to obtain an empty
4368      * sorted set:
4369      * <pre> {@code
4370      *     SortedSet<String> s = Collections.emptySortedSet();
4371      * }</pre>
4372      *
4373      * @implNote Implementations of this method need not create a separate
4374      * {@code SortedSet} object for each call.
4375      *
4376      * @param <E> type of elements, if there were any, in the set
4377      * @return the empty sorted set
4378      * @since 1.8
4379      */
4380     @SuppressWarnings("unchecked")
4381     public static <E> SortedSet<E> emptySortedSet() {
4382         return (SortedSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
4383     }
4384 
4385     /**
4386      * Returns an empty navigable set (immutable).  This set is serializable.
4387      *
4388      * <p>This example illustrates the type-safe way to obtain an empty
4389      * navigable set:
4390      * <pre> {@code
4391      *     NavigableSet<String> s = Collections.emptyNavigableSet();
4392      * }</pre>
4393      *
4394      * @implNote Implementations of this method need not
4395      * create a separate {@code NavigableSet} object for each call.
4396      *
4397      * @param <E> type of elements, if there were any, in the set
4398      * @return the empty navigable set
4399      * @since 1.8
4400      */
4401     @SuppressWarnings("unchecked")
4402     public static <E> NavigableSet<E> emptyNavigableSet() {
4403         return (NavigableSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
4404     }
4405 
4406     /**
4407      * The empty list (immutable).  This list is serializable.
4408      *
4409      * @see #emptyList()
4410      */
4411     @SuppressWarnings("rawtypes")
4412     public static final List EMPTY_LIST = new EmptyList<>();
4413 
4414     /**
4415      * Returns an empty list (immutable).  This list is serializable.
4416      *
4417      * <p>This example illustrates the type-safe way to obtain an empty list:
4418      * <pre>
4419      *     List&lt;String&gt; s = Collections.emptyList();
4420      * </pre>
4421      *
4422      * @implNote
4423      * Implementations of this method need not create a separate {@code List}
4424      * object for each call.   Using this method is likely to have comparable
4425      * cost to using the like-named field.  (Unlike this method, the field does
4426      * not provide type safety.)
4427      *
4428      * @param <T> type of elements, if there were any, in the list
4429      * @return an empty immutable list
4430      *
4431      * @see #EMPTY_LIST
4432      * @since 1.5
4433      */
4434     @SuppressWarnings("unchecked")
4435     public static final <T> List<T> emptyList() {
4436         return (List<T>) EMPTY_LIST;
4437     }
4438 
4439     /**
4440      * @serial include
4441      */
4442     private static class EmptyList<E>
4443         extends AbstractList<E>
4444         implements RandomAccess, Serializable {
4445         private static final long serialVersionUID = 8842843931221139166L;
4446 
4447         public Iterator<E> iterator() {
4448             return emptyIterator();
4449         }
4450         public ListIterator<E> listIterator() {
4451             return emptyListIterator();
4452         }
4453 
4454         public int size() {return 0;}
4455         public boolean isEmpty() {return true;}
4456         public void clear() {}
4457 
4458         public boolean contains(Object obj) {return false;}
4459         public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
4460 
4461         public Object[] toArray() { return new Object[0]; }
4462 
4463         public <T> T[] toArray(T[] a) {
4464             if (a.length > 0)
4465                 a[0] = null;
4466             return a;
4467         }
4468 
4469         public E get(int index) {
4470             throw new IndexOutOfBoundsException("Index: "+index);
4471         }
4472 
4473         public boolean equals(Object o) {
4474             return (o instanceof List) && ((List<?>)o).isEmpty();
4475         }
4476 
4477         public int hashCode() { return 1; }
4478 
4479         @Override
4480         public boolean removeIf(Predicate<? super E> filter) {
4481             Objects.requireNonNull(filter);
4482             return false;
4483         }
4484         @Override
4485         public void replaceAll(UnaryOperator<E> operator) {
4486             Objects.requireNonNull(operator);
4487         }
4488         @Override
4489         public void sort(Comparator<? super E> c) {
4490         }
4491 
4492         // Override default methods in Collection
4493         @Override
4494         public void forEach(Consumer<? super E> action) {
4495             Objects.requireNonNull(action);
4496         }
4497 
4498         @Override
4499         public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
4500 
4501         // Preserves singleton property
4502         private Object readResolve() {
4503             return EMPTY_LIST;
4504         }
4505     }
4506 
4507     /**
4508      * The empty map (immutable).  This map is serializable.
4509      *
4510      * @see #emptyMap()
4511      * @since 1.3
4512      */
4513     @SuppressWarnings("rawtypes")
4514     public static final Map EMPTY_MAP = new EmptyMap<>();
4515 
4516     /**
4517      * Returns an empty map (immutable).  This map is serializable.
4518      *
4519      * <p>This example illustrates the type-safe way to obtain an empty map:
4520      * <pre>
4521      *     Map&lt;String, Date&gt; s = Collections.emptyMap();
4522      * </pre>
4523      * @implNote Implementations of this method need not create a separate
4524      * {@code Map} object for each call.  Using this method is likely to have
4525      * comparable cost to using the like-named field.  (Unlike this method, the
4526      * field does not provide type safety.)
4527      *
4528      * @param <K> the class of the map keys
4529      * @param <V> the class of the map values
4530      * @return an empty map
4531      * @see #EMPTY_MAP
4532      * @since 1.5
4533      */
4534     @SuppressWarnings("unchecked")
4535     public static final <K,V> Map<K,V> emptyMap() {
4536         return (Map<K,V>) EMPTY_MAP;
4537     }
4538 
4539     /**
4540      * Returns an empty sorted map (immutable).  This map is serializable.
4541      *
4542      * <p>This example illustrates the type-safe way to obtain an empty map:
4543      * <pre> {@code
4544      *     SortedMap<String, Date> s = Collections.emptySortedMap();
4545      * }</pre>
4546      *
4547      * @implNote Implementations of this method need not create a separate
4548      * {@code SortedMap} object for each call.
4549      *
4550      * @param <K> the class of the map keys
4551      * @param <V> the class of the map values
4552      * @return an empty sorted map
4553      * @since 1.8
4554      */
4555     @SuppressWarnings("unchecked")
4556     public static final <K,V> SortedMap<K,V> emptySortedMap() {
4557         return (SortedMap<K,V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
4558     }
4559 
4560     /**
4561      * Returns an empty navigable map (immutable).  This map is serializable.
4562      *
4563      * <p>This example illustrates the type-safe way to obtain an empty map:
4564      * <pre> {@code
4565      *     NavigableMap<String, Date> s = Collections.emptyNavigableMap();
4566      * }</pre>
4567      *
4568      * @implNote Implementations of this method need not create a separate
4569      * {@code NavigableMap} object for each call.
4570      *
4571      * @param <K> the class of the map keys
4572      * @param <V> the class of the map values
4573      * @return an empty navigable map
4574      * @since 1.8
4575      */
4576     @SuppressWarnings("unchecked")
4577     public static final <K,V> NavigableMap<K,V> emptyNavigableMap() {
4578         return (NavigableMap<K,V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
4579     }
4580 
4581     /**
4582      * @serial include
4583      */
4584     private static class EmptyMap<K,V>
4585         extends AbstractMap<K,V>
4586         implements Serializable
4587     {
4588         private static final long serialVersionUID = 6428348081105594320L;
4589 
4590         public int size()                          {return 0;}
4591         public boolean isEmpty()                   {return true;}
4592         public void clear()                        {}
4593         public boolean containsKey(Object key)     {return false;}
4594         public boolean containsValue(Object value) {return false;}
4595         public V get(Object key)                   {return null;}
4596         public Set<K> keySet()                     {return emptySet();}
4597         public Collection<V> values()              {return emptySet();}
4598         public Set<Map.Entry<K,V>> entrySet()      {return emptySet();}
4599 
4600         public boolean equals(Object o) {
4601             return (o instanceof Map) && ((Map<?,?>)o).isEmpty();
4602         }
4603 
4604         public int hashCode()                      {return 0;}
4605 
4606         // Override default methods in Map
4607         @Override
4608         @SuppressWarnings("unchecked")
4609         public V getOrDefault(Object k, V defaultValue) {
4610             return defaultValue;
4611         }
4612 
4613         @Override
4614         public void forEach(BiConsumer<? super K, ? super V> action) {
4615             Objects.requireNonNull(action);
4616         }
4617 
4618         @Override
4619         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
4620             Objects.requireNonNull(function);
4621         }
4622 
4623         @Override
4624         public V putIfAbsent(K key, V value) {
4625             throw new UnsupportedOperationException();
4626         }
4627 
4628         @Override
4629         public boolean remove(Object key, Object value) {
4630             throw new UnsupportedOperationException();
4631         }
4632 
4633         @Override
4634         public boolean replace(K key, V oldValue, V newValue) {
4635             throw new UnsupportedOperationException();
4636         }
4637 
4638         @Override
4639         public V replace(K key, V value) {
4640             throw new UnsupportedOperationException();
4641         }
4642 
4643         @Override
4644         public V computeIfAbsent(K key,
4645                 Function<? super K, ? extends V> mappingFunction) {
4646             throw new UnsupportedOperationException();
4647         }
4648 
4649         @Override
4650         public V computeIfPresent(K key,
4651                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
4652             throw new UnsupportedOperationException();
4653         }
4654 
4655         @Override
4656         public V compute(K key,
4657                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
4658             throw new UnsupportedOperationException();
4659         }
4660 
4661         @Override
4662         public V merge(K key, V value,
4663                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
4664             throw new UnsupportedOperationException();
4665         }
4666 
4667         // Preserves singleton property
4668         private Object readResolve() {
4669             return EMPTY_MAP;
4670         }
4671     }
4672 
4673     // Singleton collections
4674 
4675     /**
4676      * Returns an immutable set containing only the specified object.
4677      * The returned set is serializable.
4678      *
4679      * @param  <T> the class of the objects in the set
4680      * @param o the sole object to be stored in the returned set.
4681      * @return an immutable set containing only the specified object.
4682      */
4683     public static <T> Set<T> singleton(T o) {
4684         return new SingletonSet<>(o);
4685     }
4686 
4687     static <E> Iterator<E> singletonIterator(final E e) {
4688         return new Iterator<E>() {
4689             private boolean hasNext = true;
4690             public boolean hasNext() {
4691                 return hasNext;
4692             }
4693             public E next() {
4694                 if (hasNext) {
4695                     hasNext = false;
4696                     return e;
4697                 }
4698                 throw new NoSuchElementException();
4699             }
4700             public void remove() {
4701                 throw new UnsupportedOperationException();
4702             }
4703             @Override
4704             public void forEachRemaining(Consumer<? super E> action) {
4705                 Objects.requireNonNull(action);
4706                 if (hasNext) {
4707                     hasNext = false;
4708                     action.accept(e);
4709                 }
4710             }
4711         };
4712     }
4713 
4714     /**
4715      * Creates a {@code Spliterator} with only the specified element
4716      *
4717      * @param <T> Type of elements
4718      * @return A singleton {@code Spliterator}
4719      */
4720     static <T> Spliterator<T> singletonSpliterator(final T element) {
4721         return new Spliterator<T>() {
4722             long est = 1;
4723 
4724             @Override
4725             public Spliterator<T> trySplit() {
4726                 return null;
4727             }
4728 
4729             @Override
4730             public boolean tryAdvance(Consumer<? super T> consumer) {
4731                 Objects.requireNonNull(consumer);
4732                 if (est > 0) {
4733                     est--;
4734                     consumer.accept(element);
4735                     return true;
4736                 }
4737                 return false;
4738             }
4739 
4740             @Override
4741             public void forEachRemaining(Consumer<? super T> consumer) {
4742                 tryAdvance(consumer);
4743             }
4744 
4745             @Override
4746             public long estimateSize() {
4747                 return est;
4748             }
4749 
4750             @Override
4751             public int characteristics() {
4752                 int value = (element != null) ? Spliterator.NONNULL : 0;
4753 
4754                 return value | Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.IMMUTABLE |
4755                        Spliterator.DISTINCT | Spliterator.ORDERED;
4756             }
4757         };
4758     }
4759 
4760     /**
4761      * @serial include
4762      */
4763     private static class SingletonSet<E>
4764         extends AbstractSet<E>
4765         implements Serializable
4766     {
4767         private static final long serialVersionUID = 3193687207550431679L;
4768 
4769         private final E element;
4770 
4771         SingletonSet(E e) {element = e;}
4772 
4773         public Iterator<E> iterator() {
4774             return singletonIterator(element);
4775         }
4776 
4777         public int size() {return 1;}
4778 
4779         public boolean contains(Object o) {return eq(o, element);}
4780 
4781         // Override default methods for Collection
4782         @Override
4783         public void forEach(Consumer<? super E> action) {
4784             action.accept(element);
4785         }
4786         @Override
4787         public Spliterator<E> spliterator() {
4788             return singletonSpliterator(element);
4789         }
4790         @Override
4791         public boolean removeIf(Predicate<? super E> filter) {
4792             throw new UnsupportedOperationException();
4793         }
4794         @Override
4795         public int hashCode() {
4796             return Objects.hashCode(element);
4797         }
4798     }
4799 
4800     /**
4801      * Returns an immutable list containing only the specified object.
4802      * The returned list is serializable.
4803      *
4804      * @param  <T> the class of the objects in the list
4805      * @param o the sole object to be stored in the returned list.
4806      * @return an immutable list containing only the specified object.
4807      * @since 1.3
4808      */
4809     public static <T> List<T> singletonList(T o) {
4810         return new SingletonList<>(o);
4811     }
4812 
4813     /**
4814      * @serial include
4815      */
4816     private static class SingletonList<E>
4817         extends AbstractList<E>
4818         implements RandomAccess, Serializable {
4819 
4820         private static final long serialVersionUID = 3093736618740652951L;
4821 
4822         private final E element;
4823 
4824         SingletonList(E obj)                {element = obj;}
4825 
4826         public Iterator<E> iterator() {
4827             return singletonIterator(element);
4828         }
4829 
4830         public int size()                   {return 1;}
4831 
4832         public boolean contains(Object obj) {return eq(obj, element);}
4833 
4834         public E get(int index) {
4835             if (index != 0)
4836               throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
4837             return element;
4838         }
4839 
4840         // Override default methods for Collection
4841         @Override
4842         public void forEach(Consumer<? super E> action) {
4843             action.accept(element);
4844         }
4845         @Override
4846         public boolean removeIf(Predicate<? super E> filter) {
4847             throw new UnsupportedOperationException();
4848         }
4849         @Override
4850         public void replaceAll(UnaryOperator<E> operator) {
4851             throw new UnsupportedOperationException();
4852         }
4853         @Override
4854         public void sort(Comparator<? super E> c) {
4855         }
4856         @Override
4857         public Spliterator<E> spliterator() {
4858             return singletonSpliterator(element);
4859         }
4860         @Override
4861         public int hashCode() {
4862             return 31 + Objects.hashCode(element);
4863         }
4864     }
4865 
4866     /**
4867      * Returns an immutable map, mapping only the specified key to the
4868      * specified value.  The returned map is serializable.
4869      *
4870      * @param <K> the class of the map keys
4871      * @param <V> the class of the map values
4872      * @param key the sole key to be stored in the returned map.
4873      * @param value the value to which the returned map maps {@code key}.
4874      * @return an immutable map containing only the specified key-value
4875      *         mapping.
4876      * @since 1.3
4877      */
4878     public static <K,V> Map<K,V> singletonMap(K key, V value) {
4879         return new SingletonMap<>(key, value);
4880     }
4881 
4882     /**
4883      * @serial include
4884      */
4885     private static class SingletonMap<K,V>
4886           extends AbstractMap<K,V>
4887           implements Serializable {
4888         private static final long serialVersionUID = -6979724477215052911L;
4889 
4890         private final K k;
4891         private final V v;
4892 
4893         SingletonMap(K key, V value) {
4894             k = key;
4895             v = value;
4896         }
4897 
4898         public int size()                                           {return 1;}
4899         public boolean isEmpty()                                {return false;}
4900         public boolean containsKey(Object key)             {return eq(key, k);}
4901         public boolean containsValue(Object value)       {return eq(value, v);}
4902         public V get(Object key)              {return (eq(key, k) ? v : null);}
4903 
4904         private transient Set<K> keySet;
4905         private transient Set<Map.Entry<K,V>> entrySet;
4906         private transient Collection<V> values;
4907 
4908         public Set<K> keySet() {
4909             if (keySet==null)
4910                 keySet = singleton(k);
4911             return keySet;
4912         }
4913 
4914         public Set<Map.Entry<K,V>> entrySet() {
4915             if (entrySet==null)
4916                 entrySet = Collections.<Map.Entry<K,V>>singleton(
4917                     new SimpleImmutableEntry<>(k, v));
4918             return entrySet;
4919         }
4920 
4921         public Collection<V> values() {
4922             if (values==null)
4923                 values = singleton(v);
4924             return values;
4925         }
4926 
4927         // Override default methods in Map
4928         @Override
4929         public V getOrDefault(Object key, V defaultValue) {
4930             return eq(key, k) ? v : defaultValue;
4931         }
4932 
4933         @Override
4934         public void forEach(BiConsumer<? super K, ? super V> action) {
4935             action.accept(k, v);
4936         }
4937 
4938         @Override
4939         public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
4940             throw new UnsupportedOperationException();
4941         }
4942 
4943         @Override
4944         public V putIfAbsent(K key, V value) {
4945             throw new UnsupportedOperationException();
4946         }
4947 
4948         @Override
4949         public boolean remove(Object key, Object value) {
4950             throw new UnsupportedOperationException();
4951         }
4952 
4953         @Override
4954         public boolean replace(K key, V oldValue, V newValue) {
4955             throw new UnsupportedOperationException();
4956         }
4957 
4958         @Override
4959         public V replace(K key, V value) {
4960             throw new UnsupportedOperationException();
4961         }
4962 
4963         @Override
4964         public V computeIfAbsent(K key,
4965                 Function<? super K, ? extends V> mappingFunction) {
4966             throw new UnsupportedOperationException();
4967         }
4968 
4969         @Override
4970         public V computeIfPresent(K key,
4971                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
4972             throw new UnsupportedOperationException();
4973         }
4974 
4975         @Override
4976         public V compute(K key,
4977                 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
4978             throw new UnsupportedOperationException();
4979         }
4980 
4981         @Override
4982         public V merge(K key, V value,
4983                 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
4984             throw new UnsupportedOperationException();
4985         }
4986 
4987         @Override
4988         public int hashCode() {
4989             return Objects.hashCode(k) ^ Objects.hashCode(v);
4990         }
4991     }
4992 
4993     // Miscellaneous
4994 
4995     /**
4996      * Returns an immutable list consisting of {@code n} copies of the
4997      * specified object.  The newly allocated data object is tiny (it contains
4998      * a single reference to the data object).  This method is useful in
4999      * combination with the {@code List.addAll} method to grow lists.
5000      * The returned list is serializable.
5001      *
5002      * @param  <T> the class of the object to copy and of the objects
5003      *         in the returned list.
5004      * @param  n the number of elements in the returned list.
5005      * @param  o the element to appear repeatedly in the returned list.
5006      * @return an immutable list consisting of {@code n} copies of the
5007      *         specified object.
5008      * @throws IllegalArgumentException if {@code n < 0}
5009      * @see    List#addAll(Collection)
5010      * @see    List#addAll(int, Collection)
5011      */
5012     public static <T> List<T> nCopies(int n, T o) {
5013         if (n < 0)
5014             throw new IllegalArgumentException("List length = " + n);
5015         return new CopiesList<>(n, o);
5016     }
5017 
5018     /**
5019      * @serial include
5020      */
5021     private static class CopiesList<E>
5022         extends AbstractList<E>
5023         implements RandomAccess, Serializable
5024     {
5025         private static final long serialVersionUID = 2739099268398711800L;
5026 
5027         final int n;
5028         final E element;
5029 
5030         CopiesList(int n, E e) {
5031             assert n >= 0;
5032             this.n = n;
5033             element = e;
5034         }
5035 
5036         public int size() {
5037             return n;
5038         }
5039 
5040         public boolean contains(Object obj) {
5041             return n != 0 && eq(obj, element);
5042         }
5043 
5044         public int indexOf(Object o) {
5045             return contains(o) ? 0 : -1;
5046         }
5047 
5048         public int lastIndexOf(Object o) {
5049             return contains(o) ? n - 1 : -1;
5050         }
5051 
5052         public E get(int index) {
5053             if (index < 0 || index >= n)
5054                 throw new IndexOutOfBoundsException("Index: "+index+
5055                                                     ", Size: "+n);
5056             return element;
5057         }
5058 
5059         public Object[] toArray() {
5060             final Object[] a = new Object[n];
5061             if (element != null)
5062                 Arrays.fill(a, 0, n, element);
5063             return a;
5064         }
5065 
5066         @SuppressWarnings("unchecked")
5067         public <T> T[] toArray(T[] a) {
5068             final int n = this.n;
5069             if (a.length < n) {
5070                 a = (T[])java.lang.reflect.Array
5071                     .newInstance(a.getClass().getComponentType(), n);
5072                 if (element != null)
5073                     Arrays.fill(a, 0, n, element);
5074             } else {
5075                 Arrays.fill(a, 0, n, element);
5076                 if (a.length > n)
5077                     a[n] = null;
5078             }
5079             return a;
5080         }
5081 
5082         public List<E> subList(int fromIndex, int toIndex) {
5083             if (fromIndex < 0)
5084                 throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
5085             if (toIndex > n)
5086                 throw new IndexOutOfBoundsException("toIndex = " + toIndex);
5087             if (fromIndex > toIndex)
5088                 throw new IllegalArgumentException("fromIndex(" + fromIndex +
5089                                                    ") > toIndex(" + toIndex + ")");
5090             return new CopiesList<>(toIndex - fromIndex, element);
5091         }
5092 
5093         // Override default methods in Collection
5094         @Override
5095         public Stream<E> stream() {
5096             return IntStream.range(0, n).mapToObj(i -> element);
5097         }
5098 
5099         @Override
5100         public Stream<E> parallelStream() {
5101             return IntStream.range(0, n).parallel().mapToObj(i -> element);
5102         }
5103 
5104         @Override
5105         public Spliterator<E> spliterator() {
5106             return stream().spliterator();
5107         }
5108     }
5109 
5110     /**
5111      * Returns a comparator that imposes the reverse of the <em>natural
5112      * ordering</em> on a collection of objects that implement the
5113      * {@code Comparable} interface.  (The natural ordering is the ordering
5114      * imposed by the objects' own {@code compareTo} method.)  This enables a
5115      * simple idiom for sorting (or maintaining) collections (or arrays) of
5116      * objects that implement the {@code Comparable} interface in
5117      * reverse-natural-order.  For example, suppose {@code a} is an array of
5118      * strings. Then: <pre>
5119      *          Arrays.sort(a, Collections.reverseOrder());
5120      * </pre> sorts the array in reverse-lexicographic (alphabetical) order.<p>
5121      *
5122      * The returned comparator is serializable.
5123      *
5124      * @param  <T> the class of the objects compared by the comparator
5125      * @return A comparator that imposes the reverse of the <i>natural
5126      *         ordering</i> on a collection of objects that implement
5127      *         the {@code Comparable} interface.
5128      * @see Comparable
5129      */
5130     @SuppressWarnings("unchecked")
5131     public static <T> Comparator<T> reverseOrder() {
5132         return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
5133     }
5134 
5135     /**
5136      * @serial include
5137      */
5138     private static class ReverseComparator
5139         implements Comparator<Comparable<Object>>, Serializable {
5140 
5141         private static final long serialVersionUID = 7207038068494060240L;
5142 
5143         static final ReverseComparator REVERSE_ORDER
5144             = new ReverseComparator();
5145 
5146         public int compare(Comparable<Object> c1, Comparable<Object> c2) {
5147             return c2.compareTo(c1);
5148         }
5149 
5150         private Object readResolve() { return Collections.reverseOrder(); }
5151 
5152         @Override
5153         public Comparator<Comparable<Object>> reversed() {
5154             return Comparator.naturalOrder();
5155         }
5156     }
5157 
5158     /**
5159      * Returns a comparator that imposes the reverse ordering of the specified
5160      * comparator.  If the specified comparator is {@code null}, this method is
5161      * equivalent to {@link #reverseOrder()} (in other words, it returns a
5162      * comparator that imposes the reverse of the <em>natural ordering</em> on
5163      * a collection of objects that implement the Comparable interface).
5164      *
5165      * <p>The returned comparator is serializable (assuming the specified
5166      * comparator is also serializable or {@code null}).
5167      *
5168      * @param <T> the class of the objects compared by the comparator
5169      * @param cmp a comparator who's ordering is to be reversed by the returned
5170      * comparator or {@code null}
5171      * @return A comparator that imposes the reverse ordering of the
5172      *         specified comparator.
5173      * @since 1.5
5174      */
5175     public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
5176         if (cmp == null)
5177             return reverseOrder();
5178 
5179         if (cmp instanceof ReverseComparator2)
5180             return ((ReverseComparator2<T>)cmp).cmp;
5181 
5182         return new ReverseComparator2<>(cmp);
5183     }
5184 
5185     /**
5186      * @serial include
5187      */
5188     private static class ReverseComparator2<T> implements Comparator<T>,
5189         Serializable
5190     {
5191         private static final long serialVersionUID = 4374092139857L;
5192 
5193         /**
5194          * The comparator specified in the static factory.  This will never
5195          * be null, as the static factory returns a ReverseComparator
5196          * instance if its argument is null.
5197          *
5198          * @serial
5199          */
5200         final Comparator<T> cmp;
5201 
5202         ReverseComparator2(Comparator<T> cmp) {
5203             assert cmp != null;
5204             this.cmp = cmp;
5205         }
5206 
5207         public int compare(T t1, T t2) {
5208             return cmp.compare(t2, t1);
5209         }
5210 
5211         public boolean equals(Object o) {
5212             return (o == this) ||
5213                 (o instanceof ReverseComparator2 &&
5214                  cmp.equals(((ReverseComparator2)o).cmp));
5215         }
5216 
5217         public int hashCode() {
5218             return cmp.hashCode() ^ Integer.MIN_VALUE;
5219         }
5220 
5221         @Override
5222         public Comparator<T> reversed() {
5223             return cmp;
5224         }
5225     }
5226 
5227     /**
5228      * Returns an enumeration over the specified collection.  This provides
5229      * interoperability with legacy APIs that require an enumeration
5230      * as input.
5231      *
5232      * <p>The iterator returned from a call to {@link Enumeration#asIterator()}
5233      * does not support removal of elements from the specified collection.  This
5234      * is necessary to avoid unintentionally increasing the capabilities of the
5235      * returned enumeration.
5236      *
5237      * @param  <T> the class of the objects in the collection
5238      * @param c the collection for which an enumeration is to be returned.
5239      * @return an enumeration over the specified collection.
5240      * @see Enumeration
5241      */
5242     public static <T> Enumeration<T> enumeration(final Collection<T> c) {
5243         return new Enumeration<T>() {
5244             private final Iterator<T> i = c.iterator();
5245 
5246             public boolean hasMoreElements() {
5247                 return i.hasNext();
5248             }
5249 
5250             public T nextElement() {
5251                 return i.next();
5252             }
5253         };
5254     }
5255 
5256     /**
5257      * Returns an array list containing the elements returned by the
5258      * specified enumeration in the order they are returned by the
5259      * enumeration.  This method provides interoperability between
5260      * legacy APIs that return enumerations and new APIs that require
5261      * collections.
5262      *
5263      * @param <T> the class of the objects returned by the enumeration
5264      * @param e enumeration providing elements for the returned
5265      *          array list
5266      * @return an array list containing the elements returned
5267      *         by the specified enumeration.
5268      * @since 1.4
5269      * @see Enumeration
5270      * @see ArrayList
5271      */
5272     public static <T> ArrayList<T> list(Enumeration<T> e) {
5273         ArrayList<T> l = new ArrayList<>();
5274         while (e.hasMoreElements())
5275             l.add(e.nextElement());
5276         return l;
5277     }
5278 
5279     /**
5280      * Returns true if the specified arguments are equal, or both null.
5281      *
5282      * NB: Do not replace with Object.equals until JDK-8015417 is resolved.
5283      */
5284     static boolean eq(Object o1, Object o2) {
5285         return o1==null ? o2==null : o1.equals(o2);
5286     }
5287 
5288     /**
5289      * Returns the number of elements in the specified collection equal to the
5290      * specified object.  More formally, returns the number of elements
5291      * {@code e} in the collection such that
5292      * {@code Objects.equals(o, e)}.
5293      *
5294      * @param c the collection in which to determine the frequency
5295      *     of {@code o}
5296      * @param o the object whose frequency is to be determined
5297      * @return the number of elements in {@code c} equal to {@code o}
5298      * @throws NullPointerException if {@code c} is null
5299      * @since 1.5
5300      */
5301     public static int frequency(Collection<?> c, Object o) {
5302         int result = 0;
5303         if (o == null) {
5304             for (Object e : c)
5305                 if (e == null)
5306                     result++;
5307         } else {
5308             for (Object e : c)
5309                 if (o.equals(e))
5310                     result++;
5311         }
5312         return result;
5313     }
5314 
5315     /**
5316      * Returns {@code true} if the two specified collections have no
5317      * elements in common.
5318      *
5319      * <p>Care must be exercised if this method is used on collections that
5320      * do not comply with the general contract for {@code Collection}.
5321      * Implementations may elect to iterate over either collection and test
5322      * for containment in the other collection (or to perform any equivalent
5323      * computation).  If either collection uses a nonstandard equality test
5324      * (as does a {@link SortedSet} whose ordering is not <em>compatible with
5325      * equals</em>, or the key set of an {@link IdentityHashMap}), both
5326      * collections must use the same nonstandard equality test, or the
5327      * result of this method is undefined.
5328      *
5329      * <p>Care must also be exercised when using collections that have
5330      * restrictions on the elements that they may contain. Collection
5331      * implementations are allowed to throw exceptions for any operation
5332      * involving elements they deem ineligible. For absolute safety the
5333      * specified collections should contain only elements which are
5334      * eligible elements for both collections.
5335      *
5336      * <p>Note that it is permissible to pass the same collection in both
5337      * parameters, in which case the method will return {@code true} if and
5338      * only if the collection is empty.
5339      *
5340      * @param c1 a collection
5341      * @param c2 a collection
5342      * @return {@code true} if the two specified collections have no
5343      * elements in common.
5344      * @throws NullPointerException if either collection is {@code null}.
5345      * @throws NullPointerException if one collection contains a {@code null}
5346      * element and {@code null} is not an eligible element for the other collection.
5347      * (<a href="Collection.html#optional-restrictions">optional</a>)
5348      * @throws ClassCastException if one collection contains an element that is
5349      * of a type which is ineligible for the other collection.
5350      * (<a href="Collection.html#optional-restrictions">optional</a>)
5351      * @since 1.5
5352      */
5353     public static boolean disjoint(Collection<?> c1, Collection<?> c2) {
5354         // The collection to be used for contains(). Preference is given to
5355         // the collection who's contains() has lower O() complexity.
5356         Collection<?> contains = c2;
5357         // The collection to be iterated. If the collections' contains() impl
5358         // are of different O() complexity, the collection with slower
5359         // contains() will be used for iteration. For collections who's
5360         // contains() are of the same complexity then best performance is
5361         // achieved by iterating the smaller collection.
5362         Collection<?> iterate = c1;
5363 
5364         // Performance optimization cases. The heuristics:
5365         //   1. Generally iterate over c1.
5366         //   2. If c1 is a Set then iterate over c2.
5367         //   3. If either collection is empty then result is always true.
5368         //   4. Iterate over the smaller Collection.
5369         if (c1 instanceof Set) {
5370             // Use c1 for contains as a Set's contains() is expected to perform
5371             // better than O(N/2)
5372             iterate = c2;
5373             contains = c1;
5374         } else if (!(c2 instanceof Set)) {
5375             // Both are mere Collections. Iterate over smaller collection.
5376             // Example: If c1 contains 3 elements and c2 contains 50 elements and
5377             // assuming contains() requires ceiling(N/2) comparisons then
5378             // checking for all c1 elements in c2 would require 75 comparisons
5379             // (3 * ceiling(50/2)) vs. checking all c2 elements in c1 requiring
5380             // 100 comparisons (50 * ceiling(3/2)).
5381             int c1size = c1.size();
5382             int c2size = c2.size();
5383             if (c1size == 0 || c2size == 0) {
5384                 // At least one collection is empty. Nothing will match.
5385                 return true;
5386             }
5387 
5388             if (c1size > c2size) {
5389                 iterate = c2;
5390                 contains = c1;
5391             }
5392         }
5393 
5394         for (Object e : iterate) {
5395             if (contains.contains(e)) {
5396                // Found a common element. Collections are not disjoint.
5397                 return false;
5398             }
5399         }
5400 
5401         // No common elements were found.
5402         return true;
5403     }
5404 
5405     /**
5406      * Adds all of the specified elements to the specified collection.
5407      * Elements to be added may be specified individually or as an array.
5408      * The behavior of this convenience method is identical to that of
5409      * {@code c.addAll(Arrays.asList(elements))}, but this method is likely
5410      * to run significantly faster under most implementations.
5411      *
5412      * <p>When elements are specified individually, this method provides a
5413      * convenient way to add a few elements to an existing collection:
5414      * <pre>
5415      *     Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
5416      * </pre>
5417      *
5418      * @param  <T> the class of the elements to add and of the collection
5419      * @param c the collection into which {@code elements} are to be inserted
5420      * @param elements the elements to insert into {@code c}
5421      * @return {@code true} if the collection changed as a result of the call
5422      * @throws UnsupportedOperationException if {@code c} does not support
5423      *         the {@code add} operation
5424      * @throws NullPointerException if {@code elements} contains one or more
5425      *         null values and {@code c} does not permit null elements, or
5426      *         if {@code c} or {@code elements} are {@code null}
5427      * @throws IllegalArgumentException if some property of a value in
5428      *         {@code elements} prevents it from being added to {@code c}
5429      * @see Collection#addAll(Collection)
5430      * @since 1.5
5431      */
5432     @SafeVarargs
5433     public static <T> boolean addAll(Collection<? super T> c, T... elements) {
5434         boolean result = false;
5435         for (T element : elements)
5436             result |= c.add(element);
5437         return result;
5438     }
5439 
5440     /**
5441      * Returns a set backed by the specified map.  The resulting set displays
5442      * the same ordering, concurrency, and performance characteristics as the
5443      * backing map.  In essence, this factory method provides a {@link Set}
5444      * implementation corresponding to any {@link Map} implementation.  There
5445      * is no need to use this method on a {@link Map} implementation that
5446      * already has a corresponding {@link Set} implementation (such as {@link
5447      * HashMap} or {@link TreeMap}).
5448      *
5449      * <p>Each method invocation on the set returned by this method results in
5450      * exactly one method invocation on the backing map or its {@code keySet}
5451      * view, with one exception.  The {@code addAll} method is implemented
5452      * as a sequence of {@code put} invocations on the backing map.
5453      *
5454      * <p>The specified map must be empty at the time this method is invoked,
5455      * and should not be accessed directly after this method returns.  These
5456      * conditions are ensured if the map is created empty, passed directly
5457      * to this method, and no reference to the map is retained, as illustrated
5458      * in the following code fragment:
5459      * <pre>
5460      *    Set&lt;Object&gt; weakHashSet = Collections.newSetFromMap(
5461      *        new WeakHashMap&lt;Object, Boolean&gt;());
5462      * </pre>
5463      *
5464      * @param <E> the class of the map keys and of the objects in the
5465      *        returned set
5466      * @param map the backing map
5467      * @return the set backed by the map
5468      * @throws IllegalArgumentException if {@code map} is not empty
5469      * @since 1.6
5470      */
5471     public static <E> Set<E> newSetFromMap(Map<E, Boolean> map) {
5472         return new SetFromMap<>(map);
5473     }
5474 
5475     /**
5476      * @serial include
5477      */
5478     private static class SetFromMap<E> extends AbstractSet<E>
5479         implements Set<E>, Serializable
5480     {
5481         private final Map<E, Boolean> m;  // The backing map
5482         private transient Set<E> s;       // Its keySet
5483 
5484         SetFromMap(Map<E, Boolean> map) {
5485             if (!map.isEmpty())
5486                 throw new IllegalArgumentException("Map is non-empty");
5487             m = map;
5488             s = map.keySet();
5489         }
5490 
5491         public void clear()               {        m.clear(); }
5492         public int size()                 { return m.size(); }
5493         public boolean isEmpty()          { return m.isEmpty(); }
5494         public boolean contains(Object o) { return m.containsKey(o); }
5495         public boolean remove(Object o)   { return m.remove(o) != null; }
5496         public boolean add(E e) { return m.put(e, Boolean.TRUE) == null; }
5497         public Iterator<E> iterator()     { return s.iterator(); }
5498         public Object[] toArray()         { return s.toArray(); }
5499         public <T> T[] toArray(T[] a)     { return s.toArray(a); }
5500         public String toString()          { return s.toString(); }
5501         public int hashCode()             { return s.hashCode(); }
5502         public boolean equals(Object o)   { return o == this || s.equals(o); }
5503         public boolean containsAll(Collection<?> c) {return s.containsAll(c);}
5504         public boolean removeAll(Collection<?> c)   {return s.removeAll(c);}
5505         public boolean retainAll(Collection<?> c)   {return s.retainAll(c);}
5506         // addAll is the only inherited implementation
5507 
5508         // Override default methods in Collection
5509         @Override
5510         public void forEach(Consumer<? super E> action) {
5511             s.forEach(action);
5512         }
5513         @Override
5514         public boolean removeIf(Predicate<? super E> filter) {
5515             return s.removeIf(filter);
5516         }
5517 
5518         @Override
5519         public Spliterator<E> spliterator() {return s.spliterator();}
5520         @Override
5521         public Stream<E> stream()           {return s.stream();}
5522         @Override
5523         public Stream<E> parallelStream()   {return s.parallelStream();}
5524 
5525         private static final long serialVersionUID = 2454657854757543876L;
5526 
5527         private void readObject(java.io.ObjectInputStream stream)
5528             throws IOException, ClassNotFoundException
5529         {
5530             stream.defaultReadObject();
5531             s = m.keySet();
5532         }
5533     }
5534 
5535     /**
5536      * Returns a view of a {@link Deque} as a Last-in-first-out (Lifo)
5537      * {@link Queue}. Method {@code add} is mapped to {@code push},
5538      * {@code remove} is mapped to {@code pop} and so on. This
5539      * view can be useful when you would like to use a method
5540      * requiring a {@code Queue} but you need Lifo ordering.
5541      *
5542      * <p>Each method invocation on the queue returned by this method
5543      * results in exactly one method invocation on the backing deque, with
5544      * one exception.  The {@link Queue#addAll addAll} method is
5545      * implemented as a sequence of {@link Deque#addFirst addFirst}
5546      * invocations on the backing deque.
5547      *
5548      * @param  <T> the class of the objects in the deque
5549      * @param deque the deque
5550      * @return the queue
5551      * @since  1.6
5552      */
5553     public static <T> Queue<T> asLifoQueue(Deque<T> deque) {
5554         return new AsLIFOQueue<>(Objects.requireNonNull(deque));
5555     }
5556 
5557     /**
5558      * @serial include
5559      */
5560     static class AsLIFOQueue<E> extends AbstractQueue<E>
5561         implements Queue<E>, Serializable {
5562         private static final long serialVersionUID = 1802017725587941708L;
5563         private final Deque<E> q;
5564         AsLIFOQueue(Deque<E> q)           { this.q = q; }
5565         public boolean add(E e)           { q.addFirst(e); return true; }
5566         public boolean offer(E e)         { return q.offerFirst(e); }
5567         public E poll()                   { return q.pollFirst(); }
5568         public E remove()                 { return q.removeFirst(); }
5569         public E peek()                   { return q.peekFirst(); }
5570         public E element()                { return q.getFirst(); }
5571         public void clear()               {        q.clear(); }
5572         public int size()                 { return q.size(); }
5573         public boolean isEmpty()          { return q.isEmpty(); }
5574         public boolean contains(Object o) { return q.contains(o); }
5575         public boolean remove(Object o)   { return q.remove(o); }
5576         public Iterator<E> iterator()     { return q.iterator(); }
5577         public Object[] toArray()         { return q.toArray(); }
5578         public <T> T[] toArray(T[] a)     { return q.toArray(a); }
5579         public String toString()          { return q.toString(); }
5580         public boolean containsAll(Collection<?> c) {return q.containsAll(c);}
5581         public boolean removeAll(Collection<?> c)   {return q.removeAll(c);}
5582         public boolean retainAll(Collection<?> c)   {return q.retainAll(c);}
5583         // We use inherited addAll; forwarding addAll would be wrong
5584 
5585         // Override default methods in Collection
5586         @Override
5587         public void forEach(Consumer<? super E> action) {q.forEach(action);}
5588         @Override
5589         public boolean removeIf(Predicate<? super E> filter) {
5590             return q.removeIf(filter);
5591         }
5592         @Override
5593         public Spliterator<E> spliterator() {return q.spliterator();}
5594         @Override
5595         public Stream<E> stream()           {return q.stream();}
5596         @Override
5597         public Stream<E> parallelStream()   {return q.parallelStream();}
5598     }
5599 }