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