1 /*
   2  * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.util;
  27 import java.io.Serializable;
  28 import java.io.ObjectOutputStream;
  29 import java.io.IOException;
  30 import java.lang.reflect.Array;
  31 
  32 /**
  33  * This class consists exclusively of static methods that operate on or return
  34  * collections.  It contains polymorphic algorithms that operate on
  35  * collections, "wrappers", which return a new collection backed by a
  36  * specified collection, and a few other odds and ends.
  37  *
  38  * <p>The methods of this class all throw a <tt>NullPointerException</tt>
  39  * if the collections or class objects provided to them are null.
  40  *
  41  * <p>The documentation for the polymorphic algorithms contained in this class
  42  * generally includes a brief description of the <i>implementation</i>.  Such
  43  * descriptions should be regarded as <i>implementation notes</i>, rather than
  44  * parts of the <i>specification</i>.  Implementors should feel free to
  45  * substitute other algorithms, so long as the specification itself is adhered
  46  * to.  (For example, the algorithm used by <tt>sort</tt> does not have to be
  47  * a mergesort, but it does have to be <i>stable</i>.)
  48  *
  49  * <p>The "destructive" algorithms contained in this class, that is, the
  50  * algorithms that modify the collection on which they operate, are specified
  51  * to throw <tt>UnsupportedOperationException</tt> if the collection does not
  52  * support the appropriate mutation primitive(s), such as the <tt>set</tt>
  53  * method.  These algorithms may, but are not required to, throw this
  54  * exception if an invocation would have no effect on the collection.  For
  55  * example, invoking the <tt>sort</tt> method on an unmodifiable list that is
  56  * already sorted may or may not throw <tt>UnsupportedOperationException</tt>.
  57  *
  58  * <p>This class is a member of the
  59  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  60  * Java Collections Framework</a>.
  61  *
  62  * @author  Josh Bloch
  63  * @author  Neal Gafter
  64  * @see     Collection
  65  * @see     Set
  66  * @see     List
  67  * @see     Map
  68  * @since   1.2
  69  */
  70 
  71 public class Collections {
  72     // Suppresses default constructor, ensuring non-instantiability.
  73     private Collections() {
  74     }
  75 
  76     // Algorithms
  77 
  78     /*
  79      * Tuning parameters for algorithms - Many of the List algorithms have
  80      * two implementations, one of which is appropriate for RandomAccess
  81      * lists, the other for "sequential."  Often, the random access variant
  82      * yields better performance on small sequential access lists.  The
  83      * tuning parameters below determine the cutoff point for what constitutes
  84      * a "small" sequential access list for each algorithm.  The values below
  85      * were empirically determined to work well for LinkedList. Hopefully
  86      * they should be reasonable for other sequential access List
  87      * implementations.  Those doing performance work on this code would
  88      * do well to validate the values of these parameters from time to time.
  89      * (The first word of each tuning parameter name is the algorithm to which
  90      * it applies.)
  91      */
  92     private static final int BINARYSEARCH_THRESHOLD   = 5000;
  93     private static final int REVERSE_THRESHOLD        =   18;
  94     private static final int SHUFFLE_THRESHOLD        =    5;
  95     private static final int FILL_THRESHOLD           =   25;
  96     private static final int ROTATE_THRESHOLD         =  100;
  97     private static final int COPY_THRESHOLD           =   10;
  98     private static final int REPLACEALL_THRESHOLD     =   11;
  99     private static final int INDEXOFSUBLIST_THRESHOLD =   35;
 100 
 101     /**
 102      * Sorts the specified list into ascending order, according to the
 103      * {@linkplain Comparable natural ordering} of its elements.
 104      * All elements in the list must implement the {@link Comparable}
 105      * interface.  Furthermore, all elements in the list must be
 106      * <i>mutually comparable</i> (that is, {@code e1.compareTo(e2)}
 107      * must not throw a {@code ClassCastException} for any elements
 108      * {@code e1} and {@code e2} in the list).
 109      *
 110      * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
 111      * not be reordered as a result of the sort.
 112      *
 113      * <p>The specified list must be modifiable, but need not be resizable.
 114      *
 115      * <p>Implementation note: This implementation is a stable, adaptive,
 116      * iterative mergesort that requires far fewer than n lg(n) comparisons
 117      * when the input array is partially sorted, while offering the
 118      * performance of a traditional mergesort when the input array is
 119      * randomly ordered.  If the input array is nearly sorted, the
 120      * implementation requires approximately n comparisons.  Temporary
 121      * storage requirements vary from a small constant for nearly sorted
 122      * input arrays to n/2 object references for randomly ordered input
 123      * arrays.
 124      *
 125      * <p>The implementation takes equal advantage of ascending and
 126      * descending order in its input array, and can take advantage of
 127      * ascending and descending order in different parts of the same
 128      * input array.  It is well-suited to merging two or more sorted arrays:
 129      * simply concatenate the arrays and sort the resulting array.
 130      *
 131      * <p>The implementation was adapted from Tim Peters's list sort for Python
 132      * (<a href="http://svn.python.org/projects/python/trunk/Objects/listsort.txt">
 133      * TimSort</a>).  It uses techiques from Peter McIlroy's "Optimistic
 134      * Sorting and Information Theoretic Complexity", in Proceedings of the
 135      * Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474,
 136      * January 1993.
 137      *
 138      * <p>This implementation dumps the specified list into an array, sorts
 139      * the array, and iterates over the list resetting each element
 140      * from the corresponding position in the array.  This avoids the
 141      * n<sup>2</sup> log(n) performance that would result from attempting
 142      * to sort a linked list in place.
 143      *
 144      * @param  list the list to be sorted.
 145      * @throws ClassCastException if the list contains elements that are not
 146      *         <i>mutually comparable</i> (for example, strings and integers).
 147      * @throws UnsupportedOperationException if the specified list's
 148      *         list-iterator does not support the {@code set} operation.
 149      * @throws IllegalArgumentException (optional) if the implementation
 150      *         detects that the natural ordering of the list elements is
 151      *         found to violate the {@link Comparable} contract
 152      */
 153     public static <T extends Comparable<? super T>> void sort(List<T> list) {
 154         Object[] a = list.toArray();
 155         if(a.length <= 1) {
 156             return;
 157         }
 158         Arrays.sort(a);
 159         ListIterator<T> i = list.listIterator();
 160         for (int j=0; j<a.length; j++) {
 161             i.next();
 162             i.set((T)a[j]);
 163         }
 164     }
 165 
 166     /**
 167      * Sorts the specified list according to the order induced by the
 168      * specified comparator.  All elements in the list must be <i>mutually
 169      * comparable</i> using the specified comparator (that is,
 170      * {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}
 171      * for any elements {@code e1} and {@code e2} in the list).
 172      *
 173      * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
 174      * not be reordered as a result of the sort.
 175      *
 176      * <p>The specified list must be modifiable, but need not be resizable.
 177      *
 178      * <p>Implementation note: This implementation is a stable, adaptive,
 179      * iterative mergesort that requires far fewer than n lg(n) comparisons
 180      * when the input array is partially sorted, while offering the
 181      * performance of a traditional mergesort when the input array is
 182      * randomly ordered.  If the input array is nearly sorted, the
 183      * implementation requires approximately n comparisons.  Temporary
 184      * storage requirements vary from a small constant for nearly sorted
 185      * input arrays to n/2 object references for randomly ordered input
 186      * arrays.
 187      *
 188      * <p>The implementation takes equal advantage of ascending and
 189      * descending order in its input array, and can take advantage of
 190      * ascending and descending order in different parts of the same
 191      * input array.  It is well-suited to merging two or more sorted arrays:
 192      * simply concatenate the arrays and sort the resulting array.
 193      *
 194      * <p>The implementation was adapted from Tim Peters's list sort for Python
 195      * (<a href="http://svn.python.org/projects/python/trunk/Objects/listsort.txt">
 196      * TimSort</a>).  It uses techiques from Peter McIlroy's "Optimistic
 197      * Sorting and Information Theoretic Complexity", in Proceedings of the
 198      * Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474,
 199      * January 1993.
 200      *
 201      * <p>This implementation dumps the specified list into an array, sorts
 202      * the array, and iterates over the list resetting each element
 203      * from the corresponding position in the array.  This avoids the
 204      * n<sup>2</sup> log(n) performance that would result from attempting
 205      * to sort a linked list in place.
 206      *
 207      * @param  list the list to be sorted.
 208      * @param  c the comparator to determine the order of the list.  A
 209      *        {@code null} value indicates that the elements' <i>natural
 210      *        ordering</i> should be used.
 211      * @throws ClassCastException if the list contains elements that are not
 212      *         <i>mutually comparable</i> using the specified comparator.
 213      * @throws UnsupportedOperationException if the specified list's
 214      *         list-iterator does not support the {@code set} operation.
 215      * @throws IllegalArgumentException (optional) if the comparator is
 216      *         found to violate the {@link Comparator} contract
 217      */
 218     public static <T> void sort(List<T> list, Comparator<? super T> c) {
 219         Object[] a = list.toArray();
 220         if(a.length <= 1) {
 221             return;
 222         }
 223         Arrays.sort(a, (Comparator)c);
 224         ListIterator<T> i = list.listIterator();
 225         for (int j=0; j<a.length; j++) {
 226             i.next();
 227             i.set((T) a[j]);
 228         }
 229     }
 230 
 231 
 232     /**
 233      * Searches the specified list for the specified object using the binary
 234      * search algorithm.  The list must be sorted into ascending order
 235      * according to the {@linkplain Comparable natural ordering} of its
 236      * elements (as by the {@link #sort(List)} method) prior to making this
 237      * call.  If it is not sorted, the results are undefined.  If the list
 238      * contains multiple elements equal to the specified object, there is no
 239      * guarantee which one will be found.
 240      *
 241      * <p>This method runs in log(n) time for a "random access" list (which
 242      * provides near-constant-time positional access).  If the specified list
 243      * does not implement the {@link RandomAccess} interface and is large,
 244      * this method will do an iterator-based binary search that performs
 245      * O(n) link traversals and O(log n) element comparisons.
 246      *
 247      * @param  list the list to be searched.
 248      * @param  key the key to be searched for.
 249      * @return the index of the search key, if it is contained in the list;
 250      *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The
 251      *         <i>insertion point</i> is defined as the point at which the
 252      *         key would be inserted into the list: the index of the first
 253      *         element greater than the key, or <tt>list.size()</tt> if all
 254      *         elements in the list are less than the specified key.  Note
 255      *         that this guarantees that the return value will be &gt;= 0 if
 256      *         and only if the key is found.
 257      * @throws ClassCastException if the list contains elements that are not
 258      *         <i>mutually comparable</i> (for example, strings and
 259      *         integers), or the search key is not mutually comparable
 260      *         with the elements of the list.
 261      */
 262     public static <T>
 263     int binarySearch(List<? extends Comparable<? super T>> list, T key) {
 264         if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
 265             return Collections.indexedBinarySearch(list, key);
 266         else
 267             return Collections.iteratorBinarySearch(list, key);
 268     }
 269 
 270     private static <T>
 271     int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key)
 272     {
 273         int low = 0;
 274         int high = list.size()-1;
 275 
 276         while (low <= high) {
 277             int mid = (low + high) >>> 1;
 278             Comparable<? super T> midVal = list.get(mid);
 279             int cmp = midVal.compareTo(key);
 280 
 281             if (cmp < 0)
 282                 low = mid + 1;
 283             else if (cmp > 0)
 284                 high = mid - 1;
 285             else
 286                 return mid; // key found
 287         }
 288         return -(low + 1);  // key not found
 289     }
 290 
 291     private static <T>
 292     int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)
 293     {
 294         int low = 0;
 295         int high = list.size()-1;
 296         ListIterator<? extends Comparable<? super T>> i = list.listIterator();
 297 
 298         while (low <= high) {
 299             int mid = (low + high) >>> 1;
 300             Comparable<? super T> midVal = get(i, mid);
 301             int cmp = midVal.compareTo(key);
 302 
 303             if (cmp < 0)
 304                 low = mid + 1;
 305             else if (cmp > 0)
 306                 high = mid - 1;
 307             else
 308                 return mid; // key found
 309         }
 310         return -(low + 1);  // key not found
 311     }
 312 
 313     /**
 314      * Gets the ith element from the given list by repositioning the specified
 315      * list listIterator.
 316      */
 317     private static <T> T get(ListIterator<? extends T> i, int index) {
 318         T obj = null;
 319         int pos = i.nextIndex();
 320         if (pos <= index) {
 321             do {
 322                 obj = i.next();
 323             } while (pos++ < index);
 324         } else {
 325             do {
 326                 obj = i.previous();
 327             } while (--pos > index);
 328         }
 329         return obj;
 330     }
 331 
 332     /**
 333      * Searches the specified list for the specified object using the binary
 334      * search algorithm.  The list must be sorted into ascending order
 335      * according to the specified comparator (as by the
 336      * {@link #sort(List, Comparator) sort(List, Comparator)}
 337      * method), prior to making this call.  If it is
 338      * not sorted, the results are undefined.  If the list contains multiple
 339      * elements equal to the specified object, there is no guarantee which one
 340      * will be found.
 341      *
 342      * <p>This method runs in log(n) time for a "random access" list (which
 343      * provides near-constant-time positional access).  If the specified list
 344      * does not implement the {@link RandomAccess} interface and is large,
 345      * this method will do an iterator-based binary search that performs
 346      * O(n) link traversals and O(log n) element comparisons.
 347      *
 348      * @param  list the list to be searched.
 349      * @param  key the key to be searched for.
 350      * @param  c the comparator by which the list is ordered.
 351      *         A <tt>null</tt> value indicates that the elements'
 352      *         {@linkplain Comparable natural ordering} should be used.
 353      * @return the index of the search key, if it is contained in the list;
 354      *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The
 355      *         <i>insertion point</i> is defined as the point at which the
 356      *         key would be inserted into the list: the index of the first
 357      *         element greater than the key, or <tt>list.size()</tt> if all
 358      *         elements in the list are less than the specified key.  Note
 359      *         that this guarantees that the return value will be &gt;= 0 if
 360      *         and only if the key is found.
 361      * @throws ClassCastException if the list contains elements that are not
 362      *         <i>mutually comparable</i> using the specified comparator,
 363      *         or the search key is not mutually comparable with the
 364      *         elements of the list using this comparator.
 365      */
 366     public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
 367         if (c==null)
 368             return binarySearch((List) list, key);
 369 
 370         if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
 371             return Collections.indexedBinarySearch(list, key, c);
 372         else
 373             return Collections.iteratorBinarySearch(list, key, c);
 374     }
 375 
 376     private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
 377         int low = 0;
 378         int high = l.size()-1;
 379 
 380         while (low <= high) {
 381             int mid = (low + high) >>> 1;
 382             T midVal = l.get(mid);
 383             int cmp = c.compare(midVal, key);
 384 
 385             if (cmp < 0)
 386                 low = mid + 1;
 387             else if (cmp > 0)
 388                 high = mid - 1;
 389             else
 390                 return mid; // key found
 391         }
 392         return -(low + 1);  // key not found
 393     }
 394 
 395     private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
 396         int low = 0;
 397         int high = l.size()-1;
 398         ListIterator<? extends T> i = l.listIterator();
 399 
 400         while (low <= high) {
 401             int mid = (low + high) >>> 1;
 402             T midVal = get(i, mid);
 403             int cmp = c.compare(midVal, key);
 404 
 405             if (cmp < 0)
 406                 low = mid + 1;
 407             else if (cmp > 0)
 408                 high = mid - 1;
 409             else
 410                 return mid; // key found
 411         }
 412         return -(low + 1);  // key not found
 413     }
 414 
 415     private interface SelfComparable extends Comparable<SelfComparable> {}
 416 
 417 
 418     /**
 419      * Reverses the order of the elements in the specified list.<p>
 420      *
 421      * This method runs in linear time.
 422      *
 423      * @param  list the list whose elements are to be reversed.
 424      * @throws UnsupportedOperationException if the specified list or
 425      *         its list-iterator does not support the <tt>set</tt> operation.
 426      */
 427     public static void reverse(List<?> list) {
 428         int size = list.size();
 429         if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {
 430             for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)
 431                 swap(list, i, j);
 432         } else {
 433             ListIterator fwd = list.listIterator();
 434             ListIterator rev = list.listIterator(size);
 435             for (int i=0, mid=list.size()>>1; i<mid; i++) {
 436                 Object tmp = fwd.next();
 437                 fwd.set(rev.previous());
 438                 rev.set(tmp);
 439             }
 440         }
 441     }
 442 
 443     /**
 444      * Randomly permutes the specified list using a default source of
 445      * randomness.  All permutations occur with approximately equal
 446      * likelihood.<p>
 447      *
 448      * The hedge "approximately" is used in the foregoing description because
 449      * default source of randomness is only approximately an unbiased source
 450      * of independently chosen bits. If it were a perfect source of randomly
 451      * chosen bits, then the algorithm would choose permutations with perfect
 452      * uniformity.<p>
 453      *
 454      * This implementation traverses the list backwards, from the last element
 455      * up to the second, repeatedly swapping a randomly selected element into
 456      * the "current position".  Elements are randomly selected from the
 457      * portion of the list that runs from the first element to the current
 458      * position, inclusive.<p>
 459      *
 460      * This method runs in linear time.  If the specified list does not
 461      * implement the {@link RandomAccess} interface and is large, this
 462      * implementation dumps the specified list into an array before shuffling
 463      * it, and dumps the shuffled array back into the list.  This avoids the
 464      * quadratic behavior that would result from shuffling a "sequential
 465      * access" list in place.
 466      *
 467      * @param  list the list to be shuffled.
 468      * @throws UnsupportedOperationException if the specified list or
 469      *         its list-iterator does not support the <tt>set</tt> operation.
 470      */
 471     public static void shuffle(List<?> list) {
 472         Random rnd = r;
 473         if (rnd == null)
 474             r = rnd = new Random();
 475         shuffle(list, rnd);
 476     }
 477     private static Random r;
 478 
 479     /**
 480      * Randomly permute the specified list using the specified source of
 481      * randomness.  All permutations occur with equal likelihood
 482      * assuming that the source of randomness is fair.<p>
 483      *
 484      * This implementation traverses the list backwards, from the last element
 485      * up to the second, repeatedly swapping a randomly selected element into
 486      * the "current position".  Elements are randomly selected from the
 487      * portion of the list that runs from the first element to the current
 488      * position, inclusive.<p>
 489      *
 490      * This method runs in linear time.  If the specified list does not
 491      * implement the {@link RandomAccess} interface and is large, this
 492      * implementation dumps the specified list into an array before shuffling
 493      * it, and dumps the shuffled array back into the list.  This avoids the
 494      * quadratic behavior that would result from shuffling a "sequential
 495      * access" list in place.
 496      *
 497      * @param  list the list to be shuffled.
 498      * @param  rnd the source of randomness to use to shuffle the list.
 499      * @throws UnsupportedOperationException if the specified list or its
 500      *         list-iterator does not support the <tt>set</tt> operation.
 501      */
 502     public static void shuffle(List<?> list, Random rnd) {
 503         int size = list.size();
 504         if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
 505             for (int i=size; i>1; i--)
 506                 swap(list, i-1, rnd.nextInt(i));
 507         } else {
 508             Object arr[] = list.toArray();
 509 
 510             // Shuffle array
 511             for (int i=size; i>1; i--)
 512                 swap(arr, i-1, rnd.nextInt(i));
 513 
 514             // Dump array back into list
 515             ListIterator it = list.listIterator();
 516             for (int i=0; i<arr.length; i++) {
 517                 it.next();
 518                 it.set(arr[i]);
 519             }
 520         }
 521     }
 522 
 523     /**
 524      * Swaps the elements at the specified positions in the specified list.
 525      * (If the specified positions are equal, invoking this method leaves
 526      * the list unchanged.)
 527      *
 528      * @param list The list in which to swap elements.
 529      * @param i the index of one element to be swapped.
 530      * @param j the index of the other element to be swapped.
 531      * @throws IndexOutOfBoundsException if either <tt>i</tt> or <tt>j</tt>
 532      *         is out of range (i &lt; 0 || i &gt;= list.size()
 533      *         || j &lt; 0 || j &gt;= list.size()).
 534      * @since 1.4
 535      */
 536     public static void swap(List<?> list, int i, int j) {
 537         final List l = list;
 538         l.set(i, l.set(j, l.get(i)));
 539     }
 540 
 541     /**
 542      * Swaps the two specified elements in the specified array.
 543      */
 544     private static void swap(Object[] arr, int i, int j) {
 545         Object tmp = arr[i];
 546         arr[i] = arr[j];
 547         arr[j] = tmp;
 548     }
 549 
 550     /**
 551      * Replaces all of the elements of the specified list with the specified
 552      * element. <p>
 553      *
 554      * This method runs in linear time.
 555      *
 556      * @param  list the list to be filled with the specified element.
 557      * @param  obj The element with which to fill the specified list.
 558      * @throws UnsupportedOperationException if the specified list or its
 559      *         list-iterator does not support the <tt>set</tt> operation.
 560      */
 561     public static <T> void fill(List<? super T> list, T obj) {
 562         int size = list.size();
 563 
 564         if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
 565             for (int i=0; i<size; i++)
 566                 list.set(i, obj);
 567         } else {
 568             ListIterator<? super T> itr = list.listIterator();
 569             for (int i=0; i<size; i++) {
 570                 itr.next();
 571                 itr.set(obj);
 572             }
 573         }
 574     }
 575 
 576     /**
 577      * Copies all of the elements from one list into another.  After the
 578      * operation, the index of each copied element in the destination list
 579      * will be identical to its index in the source list.  The destination
 580      * list must be at least as long as the source list.  If it is longer, the
 581      * remaining elements in the destination list are unaffected. <p>
 582      *
 583      * This method runs in linear time.
 584      *
 585      * @param  dest The destination list.
 586      * @param  src The source list.
 587      * @throws IndexOutOfBoundsException if the destination list is too small
 588      *         to contain the entire source List.
 589      * @throws UnsupportedOperationException if the destination list's
 590      *         list-iterator does not support the <tt>set</tt> operation.
 591      */
 592     public static <T> void copy(List<? super T> dest, List<? extends T> src) {
 593         int srcSize = src.size();
 594         if (srcSize > dest.size())
 595             throw new IndexOutOfBoundsException("Source does not fit in dest");
 596 
 597         if (srcSize < COPY_THRESHOLD ||
 598             (src instanceof RandomAccess && dest instanceof RandomAccess)) {
 599             for (int i=0; i<srcSize; i++)
 600                 dest.set(i, src.get(i));
 601         } else {
 602             ListIterator<? super T> di=dest.listIterator();
 603             ListIterator<? extends T> si=src.listIterator();
 604             for (int i=0; i<srcSize; i++) {
 605                 di.next();
 606                 di.set(si.next());
 607             }
 608         }
 609     }
 610 
 611     /**
 612      * Returns the minimum element of the given collection, according to the
 613      * <i>natural ordering</i> of its elements.  All elements in the
 614      * collection must implement the <tt>Comparable</tt> interface.
 615      * Furthermore, all elements in the collection must be <i>mutually
 616      * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a
 617      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
 618      * <tt>e2</tt> in the collection).<p>
 619      *
 620      * This method iterates over the entire collection, hence it requires
 621      * time proportional to the size of the collection.
 622      *
 623      * @param  coll the collection whose minimum element is to be determined.
 624      * @return the minimum element of the given collection, according
 625      *         to the <i>natural ordering</i> of its elements.
 626      * @throws ClassCastException if the collection contains elements that are
 627      *         not <i>mutually comparable</i> (for example, strings and
 628      *         integers).
 629      * @throws NoSuchElementException if the collection is empty.
 630      * @see Comparable
 631      */
 632     public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {
 633         Iterator<? extends T> i = coll.iterator();
 634         T candidate = i.next();
 635 
 636         while (i.hasNext()) {
 637             T next = i.next();
 638             if (next.compareTo(candidate) < 0)
 639                 candidate = next;
 640         }
 641         return candidate;
 642     }
 643 
 644     /**
 645      * Returns the minimum element of the given collection, according to the
 646      * order induced by the specified comparator.  All elements in the
 647      * collection must be <i>mutually comparable</i> by the specified
 648      * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a
 649      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
 650      * <tt>e2</tt> in the collection).<p>
 651      *
 652      * This method iterates over the entire collection, hence it requires
 653      * time proportional to the size of the collection.
 654      *
 655      * @param  coll the collection whose minimum element is to be determined.
 656      * @param  comp the comparator with which to determine the minimum element.
 657      *         A <tt>null</tt> value indicates that the elements' <i>natural
 658      *         ordering</i> should be used.
 659      * @return the minimum element of the given collection, according
 660      *         to the specified comparator.
 661      * @throws ClassCastException if the collection contains elements that are
 662      *         not <i>mutually comparable</i> using the specified comparator.
 663      * @throws NoSuchElementException if the collection is empty.
 664      * @see Comparable
 665      */
 666     public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
 667         if (comp==null)
 668             return (T)min((Collection<SelfComparable>) (Collection) coll);
 669 
 670         Iterator<? extends T> i = coll.iterator();
 671         T candidate = i.next();
 672 
 673         while (i.hasNext()) {
 674             T next = i.next();
 675             if (comp.compare(next, 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      * <i>natural ordering</i> of its elements.  All elements in the
 684      * collection must implement the <tt>Comparable</tt> interface.
 685      * Furthermore, all elements in the collection must be <i>mutually
 686      * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a
 687      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
 688      * <tt>e2</tt> in the collection).<p>
 689      *
 690      * This method iterates over the entire collection, hence it requires
 691      * time proportional to the size of the collection.
 692      *
 693      * @param  coll the collection whose maximum element is to be determined.
 694      * @return the maximum element of the given collection, according
 695      *         to the <i>natural ordering</i> of its elements.
 696      * @throws ClassCastException if the collection contains elements that are
 697      *         not <i>mutually comparable</i> (for example, strings and
 698      *         integers).
 699      * @throws NoSuchElementException if the collection is empty.
 700      * @see Comparable
 701      */
 702     public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {
 703         Iterator<? extends T> i = coll.iterator();
 704         T candidate = i.next();
 705 
 706         while (i.hasNext()) {
 707             T next = i.next();
 708             if (next.compareTo(candidate) > 0)
 709                 candidate = next;
 710         }
 711         return candidate;
 712     }
 713 
 714     /**
 715      * Returns the maximum element of the given collection, according to the
 716      * order induced by the specified comparator.  All elements in the
 717      * collection must be <i>mutually comparable</i> by the specified
 718      * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a
 719      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
 720      * <tt>e2</tt> in the collection).<p>
 721      *
 722      * This method iterates over the entire collection, hence it requires
 723      * time proportional to the size of the collection.
 724      *
 725      * @param  coll the collection whose maximum element is to be determined.
 726      * @param  comp the comparator with which to determine the maximum element.
 727      *         A <tt>null</tt> value indicates that the elements' <i>natural
 728      *        ordering</i> should be used.
 729      * @return the maximum element of the given collection, according
 730      *         to the specified comparator.
 731      * @throws ClassCastException if the collection contains elements that are
 732      *         not <i>mutually comparable</i> using the specified comparator.
 733      * @throws NoSuchElementException if the collection is empty.
 734      * @see Comparable
 735      */
 736     public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
 737         if (comp==null)
 738             return (T)max((Collection<SelfComparable>) (Collection) coll);
 739 
 740         Iterator<? extends T> i = coll.iterator();
 741         T candidate = i.next();
 742 
 743         while (i.hasNext()) {
 744             T next = i.next();
 745             if (comp.compare(next, candidate) > 0)
 746                 candidate = next;
 747         }
 748         return candidate;
 749     }
 750 
 751     /**
 752      * Rotates the elements in the specified list by the specified distance.
 753      * After calling this method, the element at index <tt>i</tt> will be
 754      * the element previously at index <tt>(i - distance)</tt> mod
 755      * <tt>list.size()</tt>, for all values of <tt>i</tt> between <tt>0</tt>
 756      * and <tt>list.size()-1</tt>, inclusive.  (This method has no effect on
 757      * the size of the list.)
 758      *
 759      * <p>For example, suppose <tt>list</tt> comprises<tt> [t, a, n, k, s]</tt>.
 760      * After invoking <tt>Collections.rotate(list, 1)</tt> (or
 761      * <tt>Collections.rotate(list, -4)</tt>), <tt>list</tt> will comprise
 762      * <tt>[s, t, a, n, k]</tt>.
 763      *
 764      * <p>Note that this method can usefully be applied to sublists to
 765      * move one or more elements within a list while preserving the
 766      * order of the remaining elements.  For example, the following idiom
 767      * moves the element at index <tt>j</tt> forward to position
 768      * <tt>k</tt> (which must be greater than or equal to <tt>j</tt>):
 769      * <pre>
 770      *     Collections.rotate(list.subList(j, k+1), -1);
 771      * </pre>
 772      * To make this concrete, suppose <tt>list</tt> comprises
 773      * <tt>[a, b, c, d, e]</tt>.  To move the element at index <tt>1</tt>
 774      * (<tt>b</tt>) forward two positions, perform the following invocation:
 775      * <pre>
 776      *     Collections.rotate(l.subList(1, 4), -1);
 777      * </pre>
 778      * The resulting list is <tt>[a, c, d, b, e]</tt>.
 779      *
 780      * <p>To move more than one element forward, increase the absolute value
 781      * of the rotation distance.  To move elements backward, use a positive
 782      * shift distance.
 783      *
 784      * <p>If the specified list is small or implements the {@link
 785      * RandomAccess} interface, this implementation exchanges the first
 786      * element into the location it should go, and then repeatedly exchanges
 787      * the displaced element into the location it should go until a displaced
 788      * element is swapped into the first element.  If necessary, the process
 789      * is repeated on the second and successive elements, until the rotation
 790      * is complete.  If the specified list is large and doesn't implement the
 791      * <tt>RandomAccess</tt> interface, this implementation breaks the
 792      * list into two sublist views around index <tt>-distance mod size</tt>.
 793      * Then the {@link #reverse(List)} method is invoked on each sublist view,
 794      * and finally it is invoked on the entire list.  For a more complete
 795      * description of both algorithms, see Section 2.3 of Jon Bentley's
 796      * <i>Programming Pearls</i> (Addison-Wesley, 1986).
 797      *
 798      * @param list the list to be rotated.
 799      * @param distance the distance to rotate the list.  There are no
 800      *        constraints on this value; it may be zero, negative, or
 801      *        greater than <tt>list.size()</tt>.
 802      * @throws UnsupportedOperationException if the specified list or
 803      *         its list-iterator does not support the <tt>set</tt> operation.
 804      * @since 1.4
 805      */
 806     public static void rotate(List<?> list, int distance) {
 807         if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD)
 808             rotate1(list, distance);
 809         else
 810             rotate2(list, distance);
 811     }
 812 
 813     private static <T> void rotate1(List<T> list, int distance) {
 814         int size = list.size();
 815         if (size == 0)
 816             return;
 817         distance = distance % size;
 818         if (distance < 0)
 819             distance += size;
 820         if (distance == 0)
 821             return;
 822 
 823         for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {
 824             T displaced = list.get(cycleStart);
 825             int i = cycleStart;
 826             do {
 827                 i += distance;
 828                 if (i >= size)
 829                     i -= size;
 830                 displaced = list.set(i, displaced);
 831                 nMoved ++;
 832             } while (i != cycleStart);
 833         }
 834     }
 835 
 836     private static void rotate2(List<?> list, int distance) {
 837         int size = list.size();
 838         if (size == 0)
 839             return;
 840         int mid =  -distance % size;
 841         if (mid < 0)
 842             mid += size;
 843         if (mid == 0)
 844             return;
 845 
 846         reverse(list.subList(0, mid));
 847         reverse(list.subList(mid, size));
 848         reverse(list);
 849     }
 850 
 851     /**
 852      * Replaces all occurrences of one specified value in a list with another.
 853      * More formally, replaces with <tt>newVal</tt> each element <tt>e</tt>
 854      * in <tt>list</tt> such that
 855      * <tt>(oldVal==null ? e==null : oldVal.equals(e))</tt>.
 856      * (This method has no effect on the size of the list.)
 857      *
 858      * @param list the list in which replacement is to occur.
 859      * @param oldVal the old value to be replaced.
 860      * @param newVal the new value with which <tt>oldVal</tt> is to be
 861      *        replaced.
 862      * @return <tt>true</tt> if <tt>list</tt> contained one or more elements
 863      *         <tt>e</tt> such that
 864      *         <tt>(oldVal==null ?  e==null : oldVal.equals(e))</tt>.
 865      * @throws UnsupportedOperationException if the specified list or
 866      *         its list-iterator does not support the <tt>set</tt> operation.
 867      * @since  1.4
 868      */
 869     public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
 870         boolean result = false;
 871         int size = list.size();
 872         if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
 873             if (oldVal==null) {
 874                 for (int i=0; i<size; i++) {
 875                     if (list.get(i)==null) {
 876                         list.set(i, newVal);
 877                         result = true;
 878                     }
 879                 }
 880             } else {
 881                 for (int i=0; i<size; i++) {
 882                     if (oldVal.equals(list.get(i))) {
 883                         list.set(i, newVal);
 884                         result = true;
 885                     }
 886                 }
 887             }
 888         } else {
 889             ListIterator<T> itr=list.listIterator();
 890             if (oldVal==null) {
 891                 for (int i=0; i<size; i++) {
 892                     if (itr.next()==null) {
 893                         itr.set(newVal);
 894                         result = true;
 895                     }
 896                 }
 897             } else {
 898                 for (int i=0; i<size; i++) {
 899                     if (oldVal.equals(itr.next())) {
 900                         itr.set(newVal);
 901                         result = true;
 902                     }
 903                 }
 904             }
 905         }
 906         return result;
 907     }
 908 
 909     /**
 910      * Returns the starting position of the first occurrence of the specified
 911      * target list within the specified source list, or -1 if there is no
 912      * such occurrence.  More formally, returns the lowest index <tt>i</tt>
 913      * such that <tt>source.subList(i, i+target.size()).equals(target)</tt>,
 914      * or -1 if there is no such index.  (Returns -1 if
 915      * <tt>target.size() > source.size()</tt>.)
 916      *
 917      * <p>This implementation uses the "brute force" technique of scanning
 918      * over the source list, looking for a match with the target at each
 919      * location in turn.
 920      *
 921      * @param source the list in which to search for the first occurrence
 922      *        of <tt>target</tt>.
 923      * @param target the list to search for as a subList of <tt>source</tt>.
 924      * @return the starting position of the first occurrence of the specified
 925      *         target list within the specified source list, or -1 if there
 926      *         is no such occurrence.
 927      * @since  1.4
 928      */
 929     public static int indexOfSubList(List<?> source, List<?> target) {
 930         int sourceSize = source.size();
 931         int targetSize = target.size();
 932         int maxCandidate = sourceSize - targetSize;
 933 
 934         if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
 935             (source instanceof RandomAccess&&target instanceof RandomAccess)) {
 936         nextCand:
 937             for (int candidate = 0; candidate <= maxCandidate; candidate++) {
 938                 for (int i=0, j=candidate; i<targetSize; i++, j++)
 939                     if (!eq(target.get(i), source.get(j)))
 940                         continue nextCand;  // Element mismatch, try next cand
 941                 return candidate;  // All elements of candidate matched target
 942             }
 943         } else {  // Iterator version of above algorithm
 944             ListIterator<?> si = source.listIterator();
 945         nextCand:
 946             for (int candidate = 0; candidate <= maxCandidate; candidate++) {
 947                 ListIterator<?> ti = target.listIterator();
 948                 for (int i=0; i<targetSize; i++) {
 949                     if (!eq(ti.next(), si.next())) {
 950                         // Back up source iterator to next candidate
 951                         for (int j=0; j<i; j++)
 952                             si.previous();
 953                         continue nextCand;
 954                     }
 955                 }
 956                 return candidate;
 957             }
 958         }
 959         return -1;  // No candidate matched the target
 960     }
 961 
 962     /**
 963      * Returns the starting position of the last occurrence of the specified
 964      * target list within the specified source list, or -1 if there is no such
 965      * occurrence.  More formally, returns the highest index <tt>i</tt>
 966      * such that <tt>source.subList(i, i+target.size()).equals(target)</tt>,
 967      * or -1 if there is no such index.  (Returns -1 if
 968      * <tt>target.size() > source.size()</tt>.)
 969      *
 970      * <p>This implementation uses the "brute force" technique of iterating
 971      * over the source list, looking for a match with the target at each
 972      * location in turn.
 973      *
 974      * @param source the list in which to search for the last occurrence
 975      *        of <tt>target</tt>.
 976      * @param target the list to search for as a subList of <tt>source</tt>.
 977      * @return the starting position of the last occurrence of the specified
 978      *         target list within the specified source list, or -1 if there
 979      *         is no such occurrence.
 980      * @since  1.4
 981      */
 982     public static int lastIndexOfSubList(List<?> source, List<?> target) {
 983         int sourceSize = source.size();
 984         int targetSize = target.size();
 985         int maxCandidate = sourceSize - targetSize;
 986 
 987         if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
 988             source instanceof RandomAccess) {   // Index access version
 989         nextCand:
 990             for (int candidate = maxCandidate; candidate >= 0; candidate--) {
 991                 for (int i=0, j=candidate; i<targetSize; i++, j++)
 992                     if (!eq(target.get(i), source.get(j)))
 993                         continue nextCand;  // Element mismatch, try next cand
 994                 return candidate;  // All elements of candidate matched target
 995             }
 996         } else {  // Iterator version of above algorithm
 997             if (maxCandidate < 0)
 998                 return -1;
 999             ListIterator<?> si = source.listIterator(maxCandidate);
1000         nextCand:
1001             for (int candidate = maxCandidate; candidate >= 0; candidate--) {
1002                 ListIterator<?> ti = target.listIterator();
1003                 for (int i=0; i<targetSize; i++) {
1004                     if (!eq(ti.next(), si.next())) {
1005                         if (candidate != 0) {
1006                             // Back up source iterator to next candidate
1007                             for (int j=0; j<=i+1; j++)
1008                                 si.previous();
1009                         }
1010                         continue nextCand;
1011                     }
1012                 }
1013                 return candidate;
1014             }
1015         }
1016         return -1;  // No candidate matched the target
1017     }
1018 
1019 
1020     // Unmodifiable Wrappers
1021 
1022     /**
1023      * Returns an unmodifiable view of the specified collection.  This method
1024      * allows modules to provide users with "read-only" access to internal
1025      * collections.  Query operations on the returned collection "read through"
1026      * to the specified collection, and attempts to modify the returned
1027      * collection, whether direct or via its iterator, result in an
1028      * <tt>UnsupportedOperationException</tt>.<p>
1029      *
1030      * The returned collection does <i>not</i> pass the hashCode and equals
1031      * operations through to the backing collection, but relies on
1032      * <tt>Object</tt>'s <tt>equals</tt> and <tt>hashCode</tt> methods.  This
1033      * is necessary to preserve the contracts of these operations in the case
1034      * that the backing collection is a set or a list.<p>
1035      *
1036      * The returned collection will be serializable if the specified collection
1037      * is serializable.
1038      *
1039      * @param  c the collection for which an unmodifiable view is to be
1040      *         returned.
1041      * @return an unmodifiable view of the specified collection.
1042      */
1043     public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
1044         return new UnmodifiableCollection<>(c);
1045     }
1046 
1047     /**
1048      * @serial include
1049      */
1050     static class UnmodifiableCollection<E> implements Collection<E>, Serializable {
1051         private static final long serialVersionUID = 1820017752578914078L;
1052 
1053         final Collection<? extends E> c;
1054 
1055         UnmodifiableCollection(Collection<? extends E> c) {
1056             if (c==null)
1057                 throw new NullPointerException();
1058             this.c = c;
1059         }
1060 
1061         public int size()                   {return c.size();}
1062         public boolean isEmpty()            {return c.isEmpty();}
1063         public boolean contains(Object o)   {return c.contains(o);}
1064         public Object[] toArray()           {return c.toArray();}
1065         public <T> T[] toArray(T[] a)       {return c.toArray(a);}
1066         public String toString()            {return c.toString();}
1067 
1068         public Iterator<E> iterator() {
1069             return new Iterator<E>() {
1070                 private final Iterator<? extends E> i = c.iterator();
1071 
1072                 public boolean hasNext() {return i.hasNext();}
1073                 public E next()          {return i.next();}
1074                 public void remove() {
1075                     throw new UnsupportedOperationException();
1076                 }
1077             };
1078         }
1079 
1080         public boolean add(E e) {
1081             throw new UnsupportedOperationException();
1082         }
1083         public boolean remove(Object o) {
1084             throw new UnsupportedOperationException();
1085         }
1086 
1087         public boolean containsAll(Collection<?> coll) {
1088             return c.containsAll(coll);
1089         }
1090         public boolean addAll(Collection<? extends E> coll) {
1091             throw new UnsupportedOperationException();
1092         }
1093         public boolean removeAll(Collection<?> coll) {
1094             throw new UnsupportedOperationException();
1095         }
1096         public boolean retainAll(Collection<?> coll) {
1097             throw new UnsupportedOperationException();
1098         }
1099         public void clear() {
1100             throw new UnsupportedOperationException();
1101         }
1102     }
1103 
1104     /**
1105      * Returns an unmodifiable view of the specified set.  This method allows
1106      * modules to provide users with "read-only" access to internal sets.
1107      * Query operations on the returned set "read through" to the specified
1108      * set, and attempts to modify the returned set, whether direct or via its
1109      * iterator, result in an <tt>UnsupportedOperationException</tt>.<p>
1110      *
1111      * The returned set will be serializable if the specified set
1112      * is serializable.
1113      *
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 unmodifiable view of the specified sorted set.  This method
1135      * allows modules to provide users with "read-only" access to internal
1136      * sorted sets.  Query operations on the returned sorted set "read
1137      * through" to the specified sorted set.  Attempts to modify the returned
1138      * sorted set, whether direct, via its iterator, or via its
1139      * <tt>subSet</tt>, <tt>headSet</tt>, or <tt>tailSet</tt> views, result in
1140      * an <tt>UnsupportedOperationException</tt>.<p>
1141      *
1142      * The returned sorted set will be serializable if the specified sorted set
1143      * is serializable.
1144      *
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 unmodifiable view of the specified list.  This method allows
1182      * modules to provide users with "read-only" access to internal
1183      * lists.  Query operations on the returned list "read through" to the
1184      * specified list, and attempts to modify the returned list, whether
1185      * direct or via its iterator, result in an
1186      * <tt>UnsupportedOperationException</tt>.<p>
1187      *
1188      * The returned list will be serializable if the specified list
1189      * is serializable. Similarly, the returned list will implement
1190      * {@link RandomAccess} if the specified list does.
1191      *
1192      * @param  list the list for which an unmodifiable view is to be returned.
1193      * @return an unmodifiable view of the specified list.
1194      */
1195     public static <T> List<T> unmodifiableList(List<? extends T> list) {
1196         return (list instanceof RandomAccess ?
1197                 new UnmodifiableRandomAccessList<>(list) :
1198                 new UnmodifiableList<>(list));
1199     }
1200 
1201     /**
1202      * @serial include
1203      */
1204     static class UnmodifiableList<E> extends UnmodifiableCollection<E>
1205                                   implements List<E> {
1206         private static final long serialVersionUID = -283967356065247728L;
1207         final List<? extends E> list;
1208 
1209         UnmodifiableList(List<? extends E> list) {
1210             super(list);
1211             this.list = list;
1212         }
1213 
1214         public boolean equals(Object o) {return o == this || list.equals(o);}
1215         public int hashCode()           {return list.hashCode();}
1216 
1217         public E get(int index) {return list.get(index);}
1218         public E set(int index, E element) {
1219             throw new UnsupportedOperationException();
1220         }
1221         public void add(int index, E element) {
1222             throw new UnsupportedOperationException();
1223         }
1224         public E remove(int index) {
1225             throw new UnsupportedOperationException();
1226         }
1227         public int indexOf(Object o)            {return list.indexOf(o);}
1228         public int lastIndexOf(Object o)        {return list.lastIndexOf(o);}
1229         public boolean addAll(int index, Collection<? extends E> c) {
1230             throw new UnsupportedOperationException();
1231         }
1232         public ListIterator<E> listIterator()   {return listIterator(0);}
1233 
1234         public ListIterator<E> listIterator(final int index) {
1235             return new ListIterator<E>() {
1236                 private final ListIterator<? extends E> i
1237                     = list.listIterator(index);
1238 
1239                 public boolean hasNext()     {return i.hasNext();}
1240                 public E next()              {return i.next();}
1241                 public boolean hasPrevious() {return i.hasPrevious();}
1242                 public E previous()          {return i.previous();}
1243                 public int nextIndex()       {return i.nextIndex();}
1244                 public int previousIndex()   {return i.previousIndex();}
1245 
1246                 public void remove() {
1247                     throw new UnsupportedOperationException();
1248                 }
1249                 public void set(E e) {
1250                     throw new UnsupportedOperationException();
1251                 }
1252                 public void add(E e) {
1253                     throw new UnsupportedOperationException();
1254                 }
1255             };
1256         }
1257 
1258         public List<E> subList(int fromIndex, int toIndex) {
1259             return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
1260         }
1261 
1262         /**
1263          * UnmodifiableRandomAccessList instances are serialized as
1264          * UnmodifiableList instances to allow them to be deserialized
1265          * in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
1266          * This method inverts the transformation.  As a beneficial
1267          * side-effect, it also grafts the RandomAccess marker onto
1268          * UnmodifiableList instances that were serialized in pre-1.4 JREs.
1269          *
1270          * Note: Unfortunately, UnmodifiableRandomAccessList instances
1271          * serialized in 1.4.1 and deserialized in 1.4 will become
1272          * UnmodifiableList instances, as this method was missing in 1.4.
1273          */
1274         private Object readResolve() {
1275             return (list instanceof RandomAccess
1276                     ? new UnmodifiableRandomAccessList<>(list)
1277                     : this);
1278         }
1279     }
1280 
1281     /**
1282      * @serial include
1283      */
1284     static class UnmodifiableRandomAccessList<E> extends UnmodifiableList<E>
1285                                               implements RandomAccess
1286     {
1287         UnmodifiableRandomAccessList(List<? extends E> list) {
1288             super(list);
1289         }
1290 
1291         public List<E> subList(int fromIndex, int toIndex) {
1292             return new UnmodifiableRandomAccessList<>(
1293                 list.subList(fromIndex, toIndex));
1294         }
1295 
1296         private static final long serialVersionUID = -2542308836966382001L;
1297 
1298         /**
1299          * Allows instances to be deserialized in pre-1.4 JREs (which do
1300          * not have UnmodifiableRandomAccessList).  UnmodifiableList has
1301          * a readResolve method that inverts this transformation upon
1302          * deserialization.
1303          */
1304         private Object writeReplace() {
1305             return new UnmodifiableList<>(list);
1306         }
1307     }
1308 
1309     /**
1310      * Returns an unmodifiable view of the specified map.  This method
1311      * allows modules to provide users with "read-only" access to internal
1312      * maps.  Query operations on the returned map "read through"
1313      * to the specified map, and attempts to modify the returned
1314      * map, whether direct or via its collection views, result in an
1315      * <tt>UnsupportedOperationException</tt>.<p>
1316      *
1317      * The returned map will be serializable if the specified map
1318      * is serializable.
1319      *
1320      * @param  m the map for which an unmodifiable view is to be returned.
1321      * @return an unmodifiable view of the specified map.
1322      */
1323     public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m) {
1324         return new UnmodifiableMap<>(m);
1325     }
1326 
1327     /**
1328      * @serial include
1329      */
1330     private static class UnmodifiableMap<K,V> implements Map<K,V>, Serializable {
1331         private static final long serialVersionUID = -1034234728574286014L;
1332 
1333         private final Map<? extends K, ? extends V> m;
1334 
1335         UnmodifiableMap(Map<? extends K, ? extends V> m) {
1336             if (m==null)
1337                 throw new NullPointerException();
1338             this.m = m;
1339         }
1340 
1341         public int size()                        {return m.size();}
1342         public boolean isEmpty()                 {return m.isEmpty();}
1343         public boolean containsKey(Object key)   {return m.containsKey(key);}
1344         public boolean containsValue(Object val) {return m.containsValue(val);}
1345         public V get(Object key)                 {return m.get(key);}
1346 
1347         public V put(K key, V value) {
1348             throw new UnsupportedOperationException();
1349         }
1350         public V remove(Object key) {
1351             throw new UnsupportedOperationException();
1352         }
1353         public void putAll(Map<? extends K, ? extends V> m) {
1354             throw new UnsupportedOperationException();
1355         }
1356         public void clear() {
1357             throw new UnsupportedOperationException();
1358         }
1359 
1360         private transient Set<K> keySet = null;
1361         private transient Set<Map.Entry<K,V>> entrySet = null;
1362         private transient Collection<V> values = null;
1363 
1364         public Set<K> keySet() {
1365             if (keySet==null)
1366                 keySet = unmodifiableSet(m.keySet());
1367             return keySet;
1368         }
1369 
1370         public Set<Map.Entry<K,V>> entrySet() {
1371             if (entrySet==null)
1372                 entrySet = new UnmodifiableEntrySet<>(m.entrySet());
1373             return entrySet;
1374         }
1375 
1376         public Collection<V> values() {
1377             if (values==null)
1378                 values = unmodifiableCollection(m.values());
1379             return values;
1380         }
1381 
1382         public boolean equals(Object o) {return o == this || m.equals(o);}
1383         public int hashCode()           {return m.hashCode();}
1384         public String toString()        {return m.toString();}
1385 
1386         /**
1387          * We need this class in addition to UnmodifiableSet as
1388          * Map.Entries themselves permit modification of the backing Map
1389          * via their setValue operation.  This class is subtle: there are
1390          * many possible attacks that must be thwarted.
1391          *
1392          * @serial include
1393          */
1394         static class UnmodifiableEntrySet<K,V>
1395             extends UnmodifiableSet<Map.Entry<K,V>> {
1396             private static final long serialVersionUID = 7854390611657943733L;
1397 
1398             UnmodifiableEntrySet(Set<? extends Map.Entry<? extends K, ? extends V>> s) {
1399                 super((Set)s);
1400             }
1401             public Iterator<Map.Entry<K,V>> iterator() {
1402                 return new Iterator<Map.Entry<K,V>>() {
1403                     private final Iterator<? extends Map.Entry<? extends K, ? extends V>> i = c.iterator();
1404 
1405                     public boolean hasNext() {
1406                         return i.hasNext();
1407                     }
1408                     public Map.Entry<K,V> next() {
1409                         return new UnmodifiableEntry<>(i.next());
1410                     }
1411                     public void remove() {
1412                         throw new UnsupportedOperationException();
1413                     }
1414                 };
1415             }
1416 
1417             public Object[] toArray() {
1418                 Object[] a = c.toArray();
1419                 for (int i=0; i<a.length; i++)
1420                     a[i] = new UnmodifiableEntry<>((Map.Entry<K,V>)a[i]);
1421                 return a;
1422             }
1423 
1424             public <T> T[] toArray(T[] a) {
1425                 // We don't pass a to c.toArray, to avoid window of
1426                 // vulnerability wherein an unscrupulous multithreaded client
1427                 // could get his hands on raw (unwrapped) Entries from c.
1428                 Object[] arr = c.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));
1429 
1430                 for (int i=0; i<arr.length; i++)
1431                     arr[i] = new UnmodifiableEntry<>((Map.Entry<K,V>)arr[i]);
1432 
1433                 if (arr.length > a.length)
1434                     return (T[])arr;
1435 
1436                 System.arraycopy(arr, 0, a, 0, arr.length);
1437                 if (a.length > arr.length)
1438                     a[arr.length] = null;
1439                 return a;
1440             }
1441 
1442             /**
1443              * This method is overridden to protect the backing set against
1444              * an object with a nefarious equals function that senses
1445              * that the equality-candidate is Map.Entry and calls its
1446              * setValue method.
1447              */
1448             public boolean contains(Object o) {
1449                 if (!(o instanceof Map.Entry))
1450                     return false;
1451                 return c.contains(
1452                     new UnmodifiableEntry<>((Map.Entry<?,?>) o));
1453             }
1454 
1455             /**
1456              * The next two methods are overridden to protect against
1457              * an unscrupulous List whose contains(Object o) method senses
1458              * when o is a Map.Entry, and calls o.setValue.
1459              */
1460             public boolean containsAll(Collection<?> coll) {
1461                 for (Object e : coll) {
1462                     if (!contains(e)) // Invokes safe contains() above
1463                         return false;
1464                 }
1465                 return true;
1466             }
1467             public boolean equals(Object o) {
1468                 if (o == this)
1469                     return true;
1470 
1471                 if (!(o instanceof Set))
1472                     return false;
1473                 Set s = (Set) o;
1474                 if (s.size() != c.size())
1475                     return false;
1476                 return containsAll(s); // Invokes safe containsAll() above
1477             }
1478 
1479             /**
1480              * This "wrapper class" serves two purposes: it prevents
1481              * the client from modifying the backing Map, by short-circuiting
1482              * the setValue method, and it protects the backing Map against
1483              * an ill-behaved Map.Entry that attempts to modify another
1484              * Map Entry when asked to perform an equality check.
1485              */
1486             private static class UnmodifiableEntry<K,V> implements Map.Entry<K,V> {
1487                 private Map.Entry<? extends K, ? extends V> e;
1488 
1489                 UnmodifiableEntry(Map.Entry<? extends K, ? extends V> e) {this.e = e;}
1490 
1491                 public K getKey()        {return e.getKey();}
1492                 public V getValue()      {return e.getValue();}
1493                 public V setValue(V value) {
1494                     throw new UnsupportedOperationException();
1495                 }
1496                 public int hashCode()    {return e.hashCode();}
1497                 public boolean equals(Object o) {
1498                     if (this == o)
1499                         return true;
1500                     if (!(o instanceof Map.Entry))
1501                         return false;
1502                     Map.Entry t = (Map.Entry)o;
1503                     return eq(e.getKey(),   t.getKey()) &&
1504                            eq(e.getValue(), t.getValue());
1505                 }
1506                 public String toString() {return e.toString();}
1507             }
1508         }
1509     }
1510 
1511     /**
1512      * Returns an unmodifiable view of the specified sorted map.  This method
1513      * allows modules to provide users with "read-only" access to internal
1514      * sorted maps.  Query operations on the returned sorted map "read through"
1515      * to the specified sorted map.  Attempts to modify the returned
1516      * sorted map, whether direct, via its collection views, or via its
1517      * <tt>subMap</tt>, <tt>headMap</tt>, or <tt>tailMap</tt> views, result in
1518      * an <tt>UnsupportedOperationException</tt>.<p>
1519      *
1520      * The returned sorted map will be serializable if the specified sorted map
1521      * is serializable.
1522      *
1523      * @param m the sorted map for which an unmodifiable view is to be
1524      *        returned.
1525      * @return an unmodifiable view of the specified sorted map.
1526      */
1527     public static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) {
1528         return new UnmodifiableSortedMap<>(m);
1529     }
1530 
1531     /**
1532      * @serial include
1533      */
1534     static class UnmodifiableSortedMap<K,V>
1535           extends UnmodifiableMap<K,V>
1536           implements SortedMap<K,V>, Serializable {
1537         private static final long serialVersionUID = -8806743815996713206L;
1538 
1539         private final SortedMap<K, ? extends V> sm;
1540 
1541         UnmodifiableSortedMap(SortedMap<K, ? extends V> m) {super(m); sm = m;}
1542 
1543         public Comparator<? super K> comparator() {return sm.comparator();}
1544 
1545         public SortedMap<K,V> subMap(K fromKey, K toKey) {
1546             return new UnmodifiableSortedMap<>(sm.subMap(fromKey, toKey));
1547         }
1548         public SortedMap<K,V> headMap(K toKey) {
1549             return new UnmodifiableSortedMap<>(sm.headMap(toKey));
1550         }
1551         public SortedMap<K,V> tailMap(K fromKey) {
1552             return new UnmodifiableSortedMap<>(sm.tailMap(fromKey));
1553         }
1554 
1555         public K firstKey()           {return sm.firstKey();}
1556         public K lastKey()            {return sm.lastKey();}
1557     }
1558 
1559 
1560     // Synch Wrappers
1561 
1562     /**
1563      * Returns a synchronized (thread-safe) collection backed by the specified
1564      * collection.  In order to guarantee serial access, it is critical that
1565      * <strong>all</strong> access to the backing collection is accomplished
1566      * through the returned collection.<p>
1567      *
1568      * It is imperative that the user manually synchronize on the returned
1569      * collection when iterating over it:
1570      * <pre>
1571      *  Collection c = Collections.synchronizedCollection(myCollection);
1572      *     ...
1573      *  synchronized (c) {
1574      *      Iterator i = c.iterator(); // Must be in the synchronized block
1575      *      while (i.hasNext())
1576      *         foo(i.next());
1577      *  }
1578      * </pre>
1579      * Failure to follow this advice may result in non-deterministic behavior.
1580      *
1581      * <p>The returned collection does <i>not</i> pass the <tt>hashCode</tt>
1582      * and <tt>equals</tt> operations through to the backing collection, but
1583      * relies on <tt>Object</tt>'s equals and hashCode methods.  This is
1584      * necessary to preserve the contracts of these operations in the case
1585      * that the backing collection is a set or a list.<p>
1586      *
1587      * The returned collection will be serializable if the specified collection
1588      * is serializable.
1589      *
1590      * @param  c the collection to be "wrapped" in a synchronized collection.
1591      * @return a synchronized view of the specified collection.
1592      */
1593     public static <T> Collection<T> synchronizedCollection(Collection<T> c) {
1594         return new SynchronizedCollection<>(c);
1595     }
1596 
1597     static <T> Collection<T> synchronizedCollection(Collection<T> c, Object mutex) {
1598         return new SynchronizedCollection<>(c, mutex);
1599     }
1600 
1601     /**
1602      * @serial include
1603      */
1604     static class SynchronizedCollection<E> implements Collection<E>, Serializable {
1605         private static final long serialVersionUID = 3053995032091335093L;
1606 
1607         final Collection<E> c;  // Backing Collection
1608         final Object mutex;     // Object on which to synchronize
1609 
1610         SynchronizedCollection(Collection<E> c) {
1611             if (c==null)
1612                 throw new NullPointerException();
1613             this.c = c;
1614             mutex = this;
1615         }
1616         SynchronizedCollection(Collection<E> c, Object mutex) {
1617             this.c = c;
1618             this.mutex = mutex;
1619         }
1620 
1621         public int size() {
1622             synchronized (mutex) {return c.size();}
1623         }
1624         public boolean isEmpty() {
1625             synchronized (mutex) {return c.isEmpty();}
1626         }
1627         public boolean contains(Object o) {
1628             synchronized (mutex) {return c.contains(o);}
1629         }
1630         public Object[] toArray() {
1631             synchronized (mutex) {return c.toArray();}
1632         }
1633         public <T> T[] toArray(T[] a) {
1634             synchronized (mutex) {return c.toArray(a);}
1635         }
1636 
1637         public Iterator<E> iterator() {
1638             return c.iterator(); // Must be manually synched by user!
1639         }
1640 
1641         public boolean add(E e) {
1642             synchronized (mutex) {return c.add(e);}
1643         }
1644         public boolean remove(Object o) {
1645             synchronized (mutex) {return c.remove(o);}
1646         }
1647 
1648         public boolean containsAll(Collection<?> coll) {
1649             synchronized (mutex) {return c.containsAll(coll);}
1650         }
1651         public boolean addAll(Collection<? extends E> coll) {
1652             synchronized (mutex) {return c.addAll(coll);}
1653         }
1654         public boolean removeAll(Collection<?> coll) {
1655             synchronized (mutex) {return c.removeAll(coll);}
1656         }
1657         public boolean retainAll(Collection<?> coll) {
1658             synchronized (mutex) {return c.retainAll(coll);}
1659         }
1660         public void clear() {
1661             synchronized (mutex) {c.clear();}
1662         }
1663         public String toString() {
1664             synchronized (mutex) {return c.toString();}
1665         }
1666         private void writeObject(ObjectOutputStream s) throws IOException {
1667             synchronized (mutex) {s.defaultWriteObject();}
1668         }
1669     }
1670 
1671     /**
1672      * Returns a synchronized (thread-safe) set backed by the specified
1673      * set.  In order to guarantee serial access, it is critical that
1674      * <strong>all</strong> access to the backing set is accomplished
1675      * through the returned set.<p>
1676      *
1677      * It is imperative that the user manually synchronize on the returned
1678      * set when iterating over it:
1679      * <pre>
1680      *  Set s = Collections.synchronizedSet(new HashSet());
1681      *      ...
1682      *  synchronized (s) {
1683      *      Iterator i = s.iterator(); // Must be in the synchronized block
1684      *      while (i.hasNext())
1685      *          foo(i.next());
1686      *  }
1687      * </pre>
1688      * Failure to follow this advice may result in non-deterministic behavior.
1689      *
1690      * <p>The returned set will be serializable if the specified set is
1691      * serializable.
1692      *
1693      * @param  s the set to be "wrapped" in a synchronized set.
1694      * @return a synchronized view of the specified set.
1695      */
1696     public static <T> Set<T> synchronizedSet(Set<T> s) {
1697         return new SynchronizedSet<>(s);
1698     }
1699 
1700     static <T> Set<T> synchronizedSet(Set<T> s, Object mutex) {
1701         return new SynchronizedSet<>(s, mutex);
1702     }
1703 
1704     /**
1705      * @serial include
1706      */
1707     static class SynchronizedSet<E>
1708           extends SynchronizedCollection<E>
1709           implements Set<E> {
1710         private static final long serialVersionUID = 487447009682186044L;
1711 
1712         SynchronizedSet(Set<E> s) {
1713             super(s);
1714         }
1715         SynchronizedSet(Set<E> s, Object mutex) {
1716             super(s, mutex);
1717         }
1718 
1719         public boolean equals(Object o) {
1720             if (this == o)
1721                 return true;
1722             synchronized (mutex) {return c.equals(o);}
1723         }
1724         public int hashCode() {
1725             synchronized (mutex) {return c.hashCode();}
1726         }
1727     }
1728 
1729     /**
1730      * Returns a synchronized (thread-safe) sorted set backed by the specified
1731      * sorted set.  In order to guarantee serial access, it is critical that
1732      * <strong>all</strong> access to the backing sorted set is accomplished
1733      * through the returned sorted set (or its views).<p>
1734      *
1735      * It is imperative that the user manually synchronize on the returned
1736      * sorted set when iterating over it or any of its <tt>subSet</tt>,
1737      * <tt>headSet</tt>, or <tt>tailSet</tt> views.
1738      * <pre>
1739      *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
1740      *      ...
1741      *  synchronized (s) {
1742      *      Iterator i = s.iterator(); // Must be in the synchronized block
1743      *      while (i.hasNext())
1744      *          foo(i.next());
1745      *  }
1746      * </pre>
1747      * or:
1748      * <pre>
1749      *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
1750      *  SortedSet s2 = s.headSet(foo);
1751      *      ...
1752      *  synchronized (s) {  // Note: s, not s2!!!
1753      *      Iterator i = s2.iterator(); // Must be in the synchronized block
1754      *      while (i.hasNext())
1755      *          foo(i.next());
1756      *  }
1757      * </pre>
1758      * Failure to follow this advice may result in non-deterministic behavior.
1759      *
1760      * <p>The returned sorted set will be serializable if the specified
1761      * sorted set is serializable.
1762      *
1763      * @param  s the sorted set to be "wrapped" in a synchronized sorted set.
1764      * @return a synchronized view of the specified sorted set.
1765      */
1766     public static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) {
1767         return new SynchronizedSortedSet<>(s);
1768     }
1769 
1770     /**
1771      * @serial include
1772      */
1773     static class SynchronizedSortedSet<E>
1774         extends SynchronizedSet<E>
1775         implements SortedSet<E>
1776     {
1777         private static final long serialVersionUID = 8695801310862127406L;
1778 
1779         private final SortedSet<E> ss;
1780 
1781         SynchronizedSortedSet(SortedSet<E> s) {
1782             super(s);
1783             ss = s;
1784         }
1785         SynchronizedSortedSet(SortedSet<E> s, Object mutex) {
1786             super(s, mutex);
1787             ss = s;
1788         }
1789 
1790         public Comparator<? super E> comparator() {
1791             synchronized (mutex) {return ss.comparator();}
1792         }
1793 
1794         public SortedSet<E> subSet(E fromElement, E toElement) {
1795             synchronized (mutex) {
1796                 return new SynchronizedSortedSet<>(
1797                     ss.subSet(fromElement, toElement), mutex);
1798             }
1799         }
1800         public SortedSet<E> headSet(E toElement) {
1801             synchronized (mutex) {
1802                 return new SynchronizedSortedSet<>(ss.headSet(toElement), mutex);
1803             }
1804         }
1805         public SortedSet<E> tailSet(E fromElement) {
1806             synchronized (mutex) {
1807                return new SynchronizedSortedSet<>(ss.tailSet(fromElement),mutex);
1808             }
1809         }
1810 
1811         public E first() {
1812             synchronized (mutex) {return ss.first();}
1813         }
1814         public E last() {
1815             synchronized (mutex) {return ss.last();}
1816         }
1817     }
1818 
1819     /**
1820      * Returns a synchronized (thread-safe) list backed by the specified
1821      * list.  In order to guarantee serial access, it is critical that
1822      * <strong>all</strong> access to the backing list is accomplished
1823      * through the returned list.<p>
1824      *
1825      * It is imperative that the user manually synchronize on the returned
1826      * list when iterating over it:
1827      * <pre>
1828      *  List list = Collections.synchronizedList(new ArrayList());
1829      *      ...
1830      *  synchronized (list) {
1831      *      Iterator i = list.iterator(); // Must be in synchronized block
1832      *      while (i.hasNext())
1833      *          foo(i.next());
1834      *  }
1835      * </pre>
1836      * Failure to follow this advice may result in non-deterministic behavior.
1837      *
1838      * <p>The returned list will be serializable if the specified list is
1839      * serializable.
1840      *
1841      * @param  list the list to be "wrapped" in a synchronized list.
1842      * @return a synchronized view of the specified list.
1843      */
1844     public static <T> List<T> synchronizedList(List<T> list) {
1845         return (list instanceof RandomAccess ?
1846                 new SynchronizedRandomAccessList<>(list) :
1847                 new SynchronizedList<>(list));
1848     }
1849 
1850     static <T> List<T> synchronizedList(List<T> list, Object mutex) {
1851         return (list instanceof RandomAccess ?
1852                 new SynchronizedRandomAccessList<>(list, mutex) :
1853                 new SynchronizedList<>(list, mutex));
1854     }
1855 
1856     /**
1857      * @serial include
1858      */
1859     static class SynchronizedList<E>
1860         extends SynchronizedCollection<E>
1861         implements List<E> {
1862         private static final long serialVersionUID = -7754090372962971524L;
1863 
1864         final List<E> list;
1865 
1866         SynchronizedList(List<E> list) {
1867             super(list);
1868             this.list = list;
1869         }
1870         SynchronizedList(List<E> list, Object mutex) {
1871             super(list, mutex);
1872             this.list = list;
1873         }
1874 
1875         public boolean equals(Object o) {
1876             if (this == o)
1877                 return true;
1878             synchronized (mutex) {return list.equals(o);}
1879         }
1880         public int hashCode() {
1881             synchronized (mutex) {return list.hashCode();}
1882         }
1883 
1884         public E get(int index) {
1885             synchronized (mutex) {return list.get(index);}
1886         }
1887         public E set(int index, E element) {
1888             synchronized (mutex) {return list.set(index, element);}
1889         }
1890         public void add(int index, E element) {
1891             synchronized (mutex) {list.add(index, element);}
1892         }
1893         public E remove(int index) {
1894             synchronized (mutex) {return list.remove(index);}
1895         }
1896 
1897         public int indexOf(Object o) {
1898             synchronized (mutex) {return list.indexOf(o);}
1899         }
1900         public int lastIndexOf(Object o) {
1901             synchronized (mutex) {return list.lastIndexOf(o);}
1902         }
1903 
1904         public boolean addAll(int index, Collection<? extends E> c) {
1905             synchronized (mutex) {return list.addAll(index, c);}
1906         }
1907 
1908         public ListIterator<E> listIterator() {
1909             return list.listIterator(); // Must be manually synched by user
1910         }
1911 
1912         public ListIterator<E> listIterator(int index) {
1913             return list.listIterator(index); // Must be manually synched by user
1914         }
1915 
1916         public List<E> subList(int fromIndex, int toIndex) {
1917             synchronized (mutex) {
1918                 return new SynchronizedList<>(list.subList(fromIndex, toIndex),
1919                                             mutex);
1920             }
1921         }
1922 
1923         /**
1924          * SynchronizedRandomAccessList instances are serialized as
1925          * SynchronizedList instances to allow them to be deserialized
1926          * in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).
1927          * This method inverts the transformation.  As a beneficial
1928          * side-effect, it also grafts the RandomAccess marker onto
1929          * SynchronizedList instances that were serialized in pre-1.4 JREs.
1930          *
1931          * Note: Unfortunately, SynchronizedRandomAccessList instances
1932          * serialized in 1.4.1 and deserialized in 1.4 will become
1933          * SynchronizedList instances, as this method was missing in 1.4.
1934          */
1935         private Object readResolve() {
1936             return (list instanceof RandomAccess
1937                     ? new SynchronizedRandomAccessList<>(list)
1938                     : this);
1939         }
1940     }
1941 
1942     /**
1943      * @serial include
1944      */
1945     static class SynchronizedRandomAccessList<E>
1946         extends SynchronizedList<E>
1947         implements RandomAccess {
1948 
1949         SynchronizedRandomAccessList(List<E> list) {
1950             super(list);
1951         }
1952 
1953         SynchronizedRandomAccessList(List<E> list, Object mutex) {
1954             super(list, mutex);
1955         }
1956 
1957         public List<E> subList(int fromIndex, int toIndex) {
1958             synchronized (mutex) {
1959                 return new SynchronizedRandomAccessList<>(
1960                     list.subList(fromIndex, toIndex), mutex);
1961             }
1962         }
1963 
1964         private static final long serialVersionUID = 1530674583602358482L;
1965 
1966         /**
1967          * Allows instances to be deserialized in pre-1.4 JREs (which do
1968          * not have SynchronizedRandomAccessList).  SynchronizedList has
1969          * a readResolve method that inverts this transformation upon
1970          * deserialization.
1971          */
1972         private Object writeReplace() {
1973             return new SynchronizedList<>(list);
1974         }
1975     }
1976 
1977     /**
1978      * Returns a synchronized (thread-safe) map backed by the specified
1979      * map.  In order to guarantee serial access, it is critical that
1980      * <strong>all</strong> access to the backing map is accomplished
1981      * through the returned map.<p>
1982      *
1983      * It is imperative that the user manually synchronize on the returned
1984      * map when iterating over any of its collection views:
1985      * <pre>
1986      *  Map m = Collections.synchronizedMap(new HashMap());
1987      *      ...
1988      *  Set s = m.keySet();  // Needn't be in synchronized block
1989      *      ...
1990      *  synchronized (m) {  // Synchronizing on m, not s!
1991      *      Iterator i = s.iterator(); // Must be in synchronized block
1992      *      while (i.hasNext())
1993      *          foo(i.next());
1994      *  }
1995      * </pre>
1996      * Failure to follow this advice may result in non-deterministic behavior.
1997      *
1998      * <p>The returned map will be serializable if the specified map is
1999      * serializable.
2000      *
2001      * @param  m the map to be "wrapped" in a synchronized map.
2002      * @return a synchronized view of the specified map.
2003      */
2004     public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
2005         return new SynchronizedMap<>(m);
2006     }
2007 
2008     /**
2009      * @serial include
2010      */
2011     private static class SynchronizedMap<K,V>
2012         implements Map<K,V>, Serializable {
2013         private static final long serialVersionUID = 1978198479659022715L;
2014 
2015         private final Map<K,V> m;     // Backing Map
2016         final Object      mutex;        // Object on which to synchronize
2017 
2018         SynchronizedMap(Map<K,V> m) {
2019             if (m==null)
2020                 throw new NullPointerException();
2021             this.m = m;
2022             mutex = this;
2023         }
2024 
2025         SynchronizedMap(Map<K,V> m, Object mutex) {
2026             this.m = m;
2027             this.mutex = mutex;
2028         }
2029 
2030         public int size() {
2031             synchronized (mutex) {return m.size();}
2032         }
2033         public boolean isEmpty() {
2034             synchronized (mutex) {return m.isEmpty();}
2035         }
2036         public boolean containsKey(Object key) {
2037             synchronized (mutex) {return m.containsKey(key);}
2038         }
2039         public boolean containsValue(Object value) {
2040             synchronized (mutex) {return m.containsValue(value);}
2041         }
2042         public V get(Object key) {
2043             synchronized (mutex) {return m.get(key);}
2044         }
2045 
2046         public V put(K key, V value) {
2047             synchronized (mutex) {return m.put(key, value);}
2048         }
2049         public V remove(Object key) {
2050             synchronized (mutex) {return m.remove(key);}
2051         }
2052         public void putAll(Map<? extends K, ? extends V> map) {
2053             synchronized (mutex) {m.putAll(map);}
2054         }
2055         public void clear() {
2056             synchronized (mutex) {m.clear();}
2057         }
2058 
2059         private transient Set<K> keySet = null;
2060         private transient Set<Map.Entry<K,V>> entrySet = null;
2061         private transient Collection<V> values = null;
2062 
2063         public Set<K> keySet() {
2064             synchronized (mutex) {
2065                 if (keySet==null)
2066                     keySet = new SynchronizedSet<>(m.keySet(), mutex);
2067                 return keySet;
2068             }
2069         }
2070 
2071         public Set<Map.Entry<K,V>> entrySet() {
2072             synchronized (mutex) {
2073                 if (entrySet==null)
2074                     entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
2075                 return entrySet;
2076             }
2077         }
2078 
2079         public Collection<V> values() {
2080             synchronized (mutex) {
2081                 if (values==null)
2082                     values = new SynchronizedCollection<>(m.values(), mutex);
2083                 return values;
2084             }
2085         }
2086 
2087         public boolean equals(Object o) {
2088             if (this == o)
2089                 return true;
2090             synchronized (mutex) {return m.equals(o);}
2091         }
2092         public int hashCode() {
2093             synchronized (mutex) {return m.hashCode();}
2094         }
2095         public String toString() {
2096             synchronized (mutex) {return m.toString();}
2097         }
2098         private void writeObject(ObjectOutputStream s) throws IOException {
2099             synchronized (mutex) {s.defaultWriteObject();}
2100         }
2101     }
2102 
2103     /**
2104      * Returns a synchronized (thread-safe) sorted map backed by the specified
2105      * sorted map.  In order to guarantee serial access, it is critical that
2106      * <strong>all</strong> access to the backing sorted map is accomplished
2107      * through the returned sorted map (or its views).<p>
2108      *
2109      * It is imperative that the user manually synchronize on the returned
2110      * sorted map when iterating over any of its collection views, or the
2111      * collections views of any of its <tt>subMap</tt>, <tt>headMap</tt> or
2112      * <tt>tailMap</tt> views.
2113      * <pre>
2114      *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
2115      *      ...
2116      *  Set s = m.keySet();  // Needn't be in synchronized block
2117      *      ...
2118      *  synchronized (m) {  // Synchronizing on m, not s!
2119      *      Iterator i = s.iterator(); // Must be in synchronized block
2120      *      while (i.hasNext())
2121      *          foo(i.next());
2122      *  }
2123      * </pre>
2124      * or:
2125      * <pre>
2126      *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
2127      *  SortedMap m2 = m.subMap(foo, bar);
2128      *      ...
2129      *  Set s2 = m2.keySet();  // Needn't be in synchronized block
2130      *      ...
2131      *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
2132      *      Iterator i = s.iterator(); // Must be in synchronized block
2133      *      while (i.hasNext())
2134      *          foo(i.next());
2135      *  }
2136      * </pre>
2137      * Failure to follow this advice may result in non-deterministic behavior.
2138      *
2139      * <p>The returned sorted map will be serializable if the specified
2140      * sorted map is serializable.
2141      *
2142      * @param  m the sorted map to be "wrapped" in a synchronized sorted map.
2143      * @return a synchronized view of the specified sorted map.
2144      */
2145     public static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m) {
2146         return new SynchronizedSortedMap<>(m);
2147     }
2148 
2149 
2150     /**
2151      * @serial include
2152      */
2153     static class SynchronizedSortedMap<K,V>
2154         extends SynchronizedMap<K,V>
2155         implements SortedMap<K,V>
2156     {
2157         private static final long serialVersionUID = -8798146769416483793L;
2158 
2159         private final SortedMap<K,V> sm;
2160 
2161         SynchronizedSortedMap(SortedMap<K,V> m) {
2162             super(m);
2163             sm = m;
2164         }
2165         SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {
2166             super(m, mutex);
2167             sm = m;
2168         }
2169 
2170         public Comparator<? super K> comparator() {
2171             synchronized (mutex) {return sm.comparator();}
2172         }
2173 
2174         public SortedMap<K,V> subMap(K fromKey, K toKey) {
2175             synchronized (mutex) {
2176                 return new SynchronizedSortedMap<>(
2177                     sm.subMap(fromKey, toKey), mutex);
2178             }
2179         }
2180         public SortedMap<K,V> headMap(K toKey) {
2181             synchronized (mutex) {
2182                 return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);
2183             }
2184         }
2185         public SortedMap<K,V> tailMap(K fromKey) {
2186             synchronized (mutex) {
2187                return new SynchronizedSortedMap<>(sm.tailMap(fromKey),mutex);
2188             }
2189         }
2190 
2191         public K firstKey() {
2192             synchronized (mutex) {return sm.firstKey();}
2193         }
2194         public K lastKey() {
2195             synchronized (mutex) {return sm.lastKey();}
2196         }
2197     }
2198 
2199     // Dynamically typesafe collection wrappers
2200 
2201     /**
2202      * Returns a dynamically typesafe view of the specified collection.
2203      * Any attempt to insert an element of the wrong type will result in an
2204      * immediate {@link ClassCastException}.  Assuming a collection
2205      * contains no incorrectly typed elements prior to the time a
2206      * dynamically typesafe view is generated, and that all subsequent
2207      * access to the collection takes place through the view, it is
2208      * <i>guaranteed</i> that the collection cannot contain an incorrectly
2209      * typed element.
2210      *
2211      * <p>The generics mechanism in the language provides compile-time
2212      * (static) type checking, but it is possible to defeat this mechanism
2213      * with unchecked casts.  Usually this is not a problem, as the compiler
2214      * issues warnings on all such unchecked operations.  There are, however,
2215      * times when static type checking alone is not sufficient.  For example,
2216      * suppose a collection is passed to a third-party library and it is
2217      * imperative that the library code not corrupt the collection by
2218      * inserting an element of the wrong type.
2219      *
2220      * <p>Another use of dynamically typesafe views is debugging.  Suppose a
2221      * program fails with a {@code ClassCastException}, indicating that an
2222      * incorrectly typed element was put into a parameterized collection.
2223      * Unfortunately, the exception can occur at any time after the erroneous
2224      * element is inserted, so it typically provides little or no information
2225      * as to the real source of the problem.  If the problem is reproducible,
2226      * one can quickly determine its source by temporarily modifying the
2227      * program to wrap the collection with a dynamically typesafe view.
2228      * For example, this declaration:
2229      *  <pre> {@code
2230      *     Collection<String> c = new HashSet<String>();
2231      * }</pre>
2232      * may be replaced temporarily by this one:
2233      *  <pre> {@code
2234      *     Collection<String> c = Collections.checkedCollection(
2235      *         new HashSet<String>(), String.class);
2236      * }</pre>
2237      * Running the program again will cause it to fail at the point where
2238      * an incorrectly typed element is inserted into the collection, clearly
2239      * identifying the source of the problem.  Once the problem is fixed, the
2240      * modified declaration may be reverted back to the original.
2241      *
2242      * <p>The returned collection does <i>not</i> pass the hashCode and equals
2243      * operations through to the backing collection, but relies on
2244      * {@code Object}'s {@code equals} and {@code hashCode} methods.  This
2245      * is necessary to preserve the contracts of these operations in the case
2246      * that the backing collection is a set or a list.
2247      *
2248      * <p>The returned collection will be serializable if the specified
2249      * collection is serializable.
2250      *
2251      * <p>Since {@code null} is considered to be a value of any reference
2252      * type, the returned collection permits insertion of null elements
2253      * whenever the backing collection does.
2254      *
2255      * @param c the collection for which a dynamically typesafe view is to be
2256      *          returned
2257      * @param type the type of element that {@code c} is permitted to hold
2258      * @return a dynamically typesafe view of the specified collection
2259      * @since 1.5
2260      */
2261     public static <E> Collection<E> checkedCollection(Collection<E> c,
2262                                                       Class<E> type) {
2263         return new CheckedCollection<>(c, type);
2264     }
2265 
2266     @SuppressWarnings("unchecked")
2267     static <T> T[] zeroLengthArray(Class<T> type) {
2268         return (T[]) Array.newInstance(type, 0);
2269     }
2270 
2271     /**
2272      * @serial include
2273      */
2274     static class CheckedCollection<E> implements Collection<E>, Serializable {
2275         private static final long serialVersionUID = 1578914078182001775L;
2276 
2277         final Collection<E> c;
2278         final Class<E> type;
2279 
2280         void typeCheck(Object o) {
2281             if (o != null && !type.isInstance(o))
2282                 throw new ClassCastException(badElementMsg(o));
2283         }
2284 
2285         private String badElementMsg(Object o) {
2286             return "Attempt to insert " + o.getClass() +
2287                 " element into collection with element type " + type;
2288         }
2289 
2290         CheckedCollection(Collection<E> c, Class<E> type) {
2291             if (c==null || type == null)
2292                 throw new NullPointerException();
2293             this.c = c;
2294             this.type = type;
2295         }
2296 
2297         public int size()                 { return c.size(); }
2298         public boolean isEmpty()          { return c.isEmpty(); }
2299         public boolean contains(Object o) { return c.contains(o); }
2300         public Object[] toArray()         { return c.toArray(); }
2301         public <T> T[] toArray(T[] a)     { return c.toArray(a); }
2302         public String toString()          { return c.toString(); }
2303         public boolean remove(Object o)   { return c.remove(o); }
2304         public void clear()               {        c.clear(); }
2305 
2306         public boolean containsAll(Collection<?> coll) {
2307             return c.containsAll(coll);
2308         }
2309         public boolean removeAll(Collection<?> coll) {
2310             return c.removeAll(coll);
2311         }
2312         public boolean retainAll(Collection<?> coll) {
2313             return c.retainAll(coll);
2314         }
2315 
2316         public Iterator<E> iterator() {
2317             final Iterator<E> it = c.iterator();
2318             return new Iterator<E>() {
2319                 public boolean hasNext() { return it.hasNext(); }
2320                 public E next()          { return it.next(); }
2321                 public void remove()     {        it.remove(); }};
2322         }
2323 
2324         public boolean add(E e) {
2325             typeCheck(e);
2326             return c.add(e);
2327         }
2328 
2329         private E[] zeroLengthElementArray = null; // Lazily initialized
2330 
2331         private E[] zeroLengthElementArray() {
2332             return zeroLengthElementArray != null ? zeroLengthElementArray :
2333                 (zeroLengthElementArray = zeroLengthArray(type));
2334         }
2335 
2336         @SuppressWarnings("unchecked")
2337         Collection<E> checkedCopyOf(Collection<? extends E> coll) {
2338             Object[] a = null;
2339             try {
2340                 E[] z = zeroLengthElementArray();
2341                 a = coll.toArray(z);
2342                 // Defend against coll violating the toArray contract
2343                 if (a.getClass() != z.getClass())
2344                     a = Arrays.copyOf(a, a.length, z.getClass());
2345             } catch (ArrayStoreException ignore) {
2346                 // To get better and consistent diagnostics,
2347                 // we call typeCheck explicitly on each element.
2348                 // We call clone() to defend against coll retaining a
2349                 // reference to the returned array and storing a bad
2350                 // element into it after it has been type checked.
2351                 a = coll.toArray().clone();
2352                 for (Object o : a)
2353                     typeCheck(o);
2354             }
2355             // A slight abuse of the type system, but safe here.
2356             return (Collection<E>) Arrays.asList(a);
2357         }
2358 
2359         public boolean addAll(Collection<? extends E> coll) {
2360             // Doing things this way insulates us from concurrent changes
2361             // in the contents of coll and provides all-or-nothing
2362             // semantics (which we wouldn't get if we type-checked each
2363             // element as we added it)
2364             return c.addAll(checkedCopyOf(coll));
2365         }
2366     }
2367 
2368     /**
2369      * Returns a dynamically typesafe view of the specified queue.
2370      * Any attempt to insert an element of the wrong type will result in
2371      * an immediate {@link ClassCastException}.  Assuming a queue contains
2372      * no incorrectly typed elements prior to the time a dynamically typesafe
2373      * view is generated, and that all subsequent access to the queue
2374      * takes place through the view, it is <i>guaranteed</i> that the
2375      * queue cannot contain an incorrectly typed element.
2376      *
2377      * <p>A discussion of the use of dynamically typesafe views may be
2378      * found in the documentation for the {@link #checkedCollection
2379      * checkedCollection} method.
2380      *
2381      * <p>The returned queue will be serializable if the specified queue
2382      * is serializable.
2383      *
2384      * <p>Since {@code null} is considered to be a value of any reference
2385      * type, the returned queue permits insertion of {@code null} elements
2386      * whenever the backing queue does.
2387      *
2388      * @param queue the queue for which a dynamically typesafe view is to be
2389      *             returned
2390      * @param type the type of element that {@code queue} is permitted to hold
2391      * @return a dynamically typesafe view of the specified queue
2392      * @since 1.8
2393      */
2394     public static <E> Queue<E> checkedQueue(Queue<E> queue, Class<E> type) {
2395         return new CheckedQueue<>(queue, type);
2396     }
2397 
2398     /**
2399      * @serial include
2400      */
2401     static class CheckedQueue<E>
2402         extends CheckedCollection<E>
2403         implements Queue<E>, Serializable
2404     {
2405         private static final long serialVersionUID = 1433151992604707767L;
2406         final Queue<E> queue;
2407 
2408         CheckedQueue(Queue<E> queue, Class<E> elementType) {
2409             super(queue, elementType);
2410             this.queue = queue;
2411         }
2412 
2413         public E element()              {return queue.element();}
2414         public boolean equals(Object o) {return o == this || c.equals(o);}
2415         public int hashCode()           {return c.hashCode();}
2416         public E peek()                 {return queue.peek();}
2417         public E poll()                 {return queue.poll();}
2418         public E remove()               {return queue.remove();}
2419 
2420         public boolean offer(E e) {
2421             typeCheck(e);
2422             return add(e);
2423         }
2424     }
2425 
2426     /**
2427      * Returns a dynamically typesafe view of the specified set.
2428      * Any attempt to insert an element of the wrong type will result in
2429      * an immediate {@link ClassCastException}.  Assuming a set contains
2430      * no incorrectly typed elements prior to the time a dynamically typesafe
2431      * view is generated, and that all subsequent access to the set
2432      * takes place through the view, it is <i>guaranteed</i> that the
2433      * set cannot contain an incorrectly typed element.
2434      *
2435      * <p>A discussion of the use of dynamically typesafe views may be
2436      * found in the documentation for the {@link #checkedCollection
2437      * checkedCollection} method.
2438      *
2439      * <p>The returned set will be serializable if the specified set is
2440      * serializable.
2441      *
2442      * <p>Since {@code null} is considered to be a value of any reference
2443      * type, the returned set permits insertion of null elements whenever
2444      * the backing set does.
2445      *
2446      * @param s the set for which a dynamically typesafe view is to be
2447      *          returned
2448      * @param type the type of element that {@code s} is permitted to hold
2449      * @return a dynamically typesafe view of the specified set
2450      * @since 1.5
2451      */
2452     public static <E> Set<E> checkedSet(Set<E> s, Class<E> type) {
2453         return new CheckedSet<>(s, type);
2454     }
2455 
2456     /**
2457      * @serial include
2458      */
2459     static class CheckedSet<E> extends CheckedCollection<E>
2460                                  implements Set<E>, Serializable
2461     {
2462         private static final long serialVersionUID = 4694047833775013803L;
2463 
2464         CheckedSet(Set<E> s, Class<E> elementType) { super(s, elementType); }
2465 
2466         public boolean equals(Object o) { return o == this || c.equals(o); }
2467         public int hashCode()           { return c.hashCode(); }
2468     }
2469 
2470     /**
2471      * Returns a dynamically typesafe view of the specified sorted set.
2472      * Any attempt to insert an element of the wrong type will result in an
2473      * immediate {@link ClassCastException}.  Assuming a sorted set
2474      * contains no incorrectly typed elements prior to the time a
2475      * dynamically typesafe view is generated, and that all subsequent
2476      * access to the sorted set takes place through the view, it is
2477      * <i>guaranteed</i> that the sorted set cannot contain an incorrectly
2478      * typed element.
2479      *
2480      * <p>A discussion of the use of dynamically typesafe views may be
2481      * found in the documentation for the {@link #checkedCollection
2482      * checkedCollection} method.
2483      *
2484      * <p>The returned sorted set will be serializable if the specified sorted
2485      * set is serializable.
2486      *
2487      * <p>Since {@code null} is considered to be a value of any reference
2488      * type, the returned sorted set permits insertion of null elements
2489      * whenever the backing sorted set does.
2490      *
2491      * @param s the sorted set for which a dynamically typesafe view is to be
2492      *          returned
2493      * @param type the type of element that {@code s} is permitted to hold
2494      * @return a dynamically typesafe view of the specified sorted set
2495      * @since 1.5
2496      */
2497     public static <E> SortedSet<E> checkedSortedSet(SortedSet<E> s,
2498                                                     Class<E> type) {
2499         return new CheckedSortedSet<>(s, type);
2500     }
2501 
2502     /**
2503      * @serial include
2504      */
2505     static class CheckedSortedSet<E> extends CheckedSet<E>
2506         implements SortedSet<E>, Serializable
2507     {
2508         private static final long serialVersionUID = 1599911165492914959L;
2509         private final SortedSet<E> ss;
2510 
2511         CheckedSortedSet(SortedSet<E> s, Class<E> type) {
2512             super(s, type);
2513             ss = s;
2514         }
2515 
2516         public Comparator<? super E> comparator() { return ss.comparator(); }
2517         public E first()                   { return ss.first(); }
2518         public E last()                    { return ss.last(); }
2519 
2520         public SortedSet<E> subSet(E fromElement, E toElement) {
2521             return checkedSortedSet(ss.subSet(fromElement,toElement), type);
2522         }
2523         public SortedSet<E> headSet(E toElement) {
2524             return checkedSortedSet(ss.headSet(toElement), type);
2525         }
2526         public SortedSet<E> tailSet(E fromElement) {
2527             return checkedSortedSet(ss.tailSet(fromElement), type);
2528         }
2529     }
2530 
2531     /**
2532      * Returns a dynamically typesafe view of the specified list.
2533      * Any attempt to insert an element of the wrong type will result in
2534      * an immediate {@link ClassCastException}.  Assuming a list contains
2535      * no incorrectly typed elements prior to the time a dynamically typesafe
2536      * view is generated, and that all subsequent access to the list
2537      * takes place through the view, it is <i>guaranteed</i> that the
2538      * list cannot contain an incorrectly typed element.
2539      *
2540      * <p>A discussion of the use of dynamically typesafe views may be
2541      * found in the documentation for the {@link #checkedCollection
2542      * checkedCollection} method.
2543      *
2544      * <p>The returned list will be serializable if the specified list
2545      * is serializable.
2546      *
2547      * <p>Since {@code null} is considered to be a value of any reference
2548      * type, the returned list permits insertion of null elements whenever
2549      * the backing list does.
2550      *
2551      * @param list the list for which a dynamically typesafe view is to be
2552      *             returned
2553      * @param type the type of element that {@code list} is permitted to hold
2554      * @return a dynamically typesafe view of the specified list
2555      * @since 1.5
2556      */
2557     public static <E> List<E> checkedList(List<E> list, Class<E> type) {
2558         return (list instanceof RandomAccess ?
2559                 new CheckedRandomAccessList<>(list, type) :
2560                 new CheckedList<>(list, type));
2561     }
2562 
2563     /**
2564      * @serial include
2565      */
2566     static class CheckedList<E>
2567         extends CheckedCollection<E>
2568         implements List<E>
2569     {
2570         private static final long serialVersionUID = 65247728283967356L;
2571         final List<E> list;
2572 
2573         CheckedList(List<E> list, Class<E> type) {
2574             super(list, type);
2575             this.list = list;
2576         }
2577 
2578         public boolean equals(Object o)  { return o == this || list.equals(o); }
2579         public int hashCode()            { return list.hashCode(); }
2580         public E get(int index)          { return list.get(index); }
2581         public E remove(int index)       { return list.remove(index); }
2582         public int indexOf(Object o)     { return list.indexOf(o); }
2583         public int lastIndexOf(Object o) { return list.lastIndexOf(o); }
2584 
2585         public E set(int index, E element) {
2586             typeCheck(element);
2587             return list.set(index, element);
2588         }
2589 
2590         public void add(int index, E element) {
2591             typeCheck(element);
2592             list.add(index, element);
2593         }
2594 
2595         public boolean addAll(int index, Collection<? extends E> c) {
2596             return list.addAll(index, checkedCopyOf(c));
2597         }
2598         public ListIterator<E> listIterator()   { return listIterator(0); }
2599 
2600         public ListIterator<E> listIterator(final int index) {
2601             final ListIterator<E> i = list.listIterator(index);
2602 
2603             return new ListIterator<E>() {
2604                 public boolean hasNext()     { return i.hasNext(); }
2605                 public E next()              { return i.next(); }
2606                 public boolean hasPrevious() { return i.hasPrevious(); }
2607                 public E previous()          { return i.previous(); }
2608                 public int nextIndex()       { return i.nextIndex(); }
2609                 public int previousIndex()   { return i.previousIndex(); }
2610                 public void remove()         {        i.remove(); }
2611 
2612                 public void set(E e) {
2613                     typeCheck(e);
2614                     i.set(e);
2615                 }
2616 
2617                 public void add(E e) {
2618                     typeCheck(e);
2619                     i.add(e);
2620                 }
2621             };
2622         }
2623 
2624         public List<E> subList(int fromIndex, int toIndex) {
2625             return new CheckedList<>(list.subList(fromIndex, toIndex), type);
2626         }
2627     }
2628 
2629     /**
2630      * @serial include
2631      */
2632     static class CheckedRandomAccessList<E> extends CheckedList<E>
2633                                             implements RandomAccess
2634     {
2635         private static final long serialVersionUID = 1638200125423088369L;
2636 
2637         CheckedRandomAccessList(List<E> list, Class<E> type) {
2638             super(list, type);
2639         }
2640 
2641         public List<E> subList(int fromIndex, int toIndex) {
2642             return new CheckedRandomAccessList<>(
2643                 list.subList(fromIndex, toIndex), type);
2644         }
2645     }
2646 
2647     /**
2648      * Returns a dynamically typesafe view of the specified map.
2649      * Any attempt to insert a mapping whose key or value have the wrong
2650      * type will result in an immediate {@link ClassCastException}.
2651      * Similarly, any attempt to modify the value currently associated with
2652      * a key will result in an immediate {@link ClassCastException},
2653      * whether the modification is attempted directly through the map
2654      * itself, or through a {@link Map.Entry} instance obtained from the
2655      * map's {@link Map#entrySet() entry set} view.
2656      *
2657      * <p>Assuming a map contains no incorrectly typed keys or values
2658      * prior to the time a dynamically typesafe view is generated, and
2659      * that all subsequent access to the map takes place through the view
2660      * (or one of its collection views), it is <i>guaranteed</i> that the
2661      * map cannot contain an incorrectly typed key or value.
2662      *
2663      * <p>A discussion of the use of dynamically typesafe views may be
2664      * found in the documentation for the {@link #checkedCollection
2665      * checkedCollection} method.
2666      *
2667      * <p>The returned map will be serializable if the specified map is
2668      * serializable.
2669      *
2670      * <p>Since {@code null} is considered to be a value of any reference
2671      * type, the returned map permits insertion of null keys or values
2672      * whenever the backing map does.
2673      *
2674      * @param m the map for which a dynamically typesafe view is to be
2675      *          returned
2676      * @param keyType the type of key that {@code m} is permitted to hold
2677      * @param valueType the type of value that {@code m} is permitted to hold
2678      * @return a dynamically typesafe view of the specified map
2679      * @since 1.5
2680      */
2681     public static <K, V> Map<K, V> checkedMap(Map<K, V> m,
2682                                               Class<K> keyType,
2683                                               Class<V> valueType) {
2684         return new CheckedMap<>(m, keyType, valueType);
2685     }
2686 
2687 
2688     /**
2689      * @serial include
2690      */
2691     private static class CheckedMap<K,V>
2692         implements Map<K,V>, Serializable
2693     {
2694         private static final long serialVersionUID = 5742860141034234728L;
2695 
2696         private final Map<K, V> m;
2697         final Class<K> keyType;
2698         final Class<V> valueType;
2699 
2700         private void typeCheck(Object key, Object value) {
2701             if (key != null && !keyType.isInstance(key))
2702                 throw new ClassCastException(badKeyMsg(key));
2703 
2704             if (value != null && !valueType.isInstance(value))
2705                 throw new ClassCastException(badValueMsg(value));
2706         }
2707 
2708         private String badKeyMsg(Object key) {
2709             return "Attempt to insert " + key.getClass() +
2710                 " key into map with key type " + keyType;
2711         }
2712 
2713         private String badValueMsg(Object value) {
2714             return "Attempt to insert " + value.getClass() +
2715                 " value into map with value type " + valueType;
2716         }
2717 
2718         CheckedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) {
2719             if (m == null || keyType == null || valueType == null)
2720                 throw new NullPointerException();
2721             this.m = m;
2722             this.keyType = keyType;
2723             this.valueType = valueType;
2724         }
2725 
2726         public int size()                      { return m.size(); }
2727         public boolean isEmpty()               { return m.isEmpty(); }
2728         public boolean containsKey(Object key) { return m.containsKey(key); }
2729         public boolean containsValue(Object v) { return m.containsValue(v); }
2730         public V get(Object key)               { return m.get(key); }
2731         public V remove(Object key)            { return m.remove(key); }
2732         public void clear()                    { m.clear(); }
2733         public Set<K> keySet()                 { return m.keySet(); }
2734         public Collection<V> values()          { return m.values(); }
2735         public boolean equals(Object o)        { return o == this || m.equals(o); }
2736         public int hashCode()                  { return m.hashCode(); }
2737         public String toString()               { return m.toString(); }
2738 
2739         public V put(K key, V value) {
2740             typeCheck(key, value);
2741             return m.put(key, value);
2742         }
2743 
2744         @SuppressWarnings("unchecked")
2745         public void putAll(Map<? extends K, ? extends V> t) {
2746             // Satisfy the following goals:
2747             // - good diagnostics in case of type mismatch
2748             // - all-or-nothing semantics
2749             // - protection from malicious t
2750             // - correct behavior if t is a concurrent map
2751             Object[] entries = t.entrySet().toArray();
2752             List<Map.Entry<K,V>> checked = new ArrayList<>(entries.length);
2753             for (Object o : entries) {
2754                 Map.Entry<?,?> e = (Map.Entry<?,?>) o;
2755                 Object k = e.getKey();
2756                 Object v = e.getValue();
2757                 typeCheck(k, v);
2758                 checked.add(
2759                     new AbstractMap.SimpleImmutableEntry<>((K) k, (V) v));
2760             }
2761             for (Map.Entry<K,V> e : checked)
2762                 m.put(e.getKey(), e.getValue());
2763         }
2764 
2765         private transient Set<Map.Entry<K,V>> entrySet = null;
2766 
2767         public Set<Map.Entry<K,V>> entrySet() {
2768             if (entrySet==null)
2769                 entrySet = new CheckedEntrySet<>(m.entrySet(), valueType);
2770             return entrySet;
2771         }
2772 
2773         /**
2774          * We need this class in addition to CheckedSet as Map.Entry permits
2775          * modification of the backing Map via the setValue operation.  This
2776          * class is subtle: there are many possible attacks that must be
2777          * thwarted.
2778          *
2779          * @serial exclude
2780          */
2781         static class CheckedEntrySet<K,V> implements Set<Map.Entry<K,V>> {
2782             private final Set<Map.Entry<K,V>> s;
2783             private final Class<V> valueType;
2784 
2785             CheckedEntrySet(Set<Map.Entry<K, V>> s, Class<V> valueType) {
2786                 this.s = s;
2787                 this.valueType = valueType;
2788             }
2789 
2790             public int size()        { return s.size(); }
2791             public boolean isEmpty() { return s.isEmpty(); }
2792             public String toString() { return s.toString(); }
2793             public int hashCode()    { return s.hashCode(); }
2794             public void clear()      {        s.clear(); }
2795 
2796             public boolean add(Map.Entry<K, V> e) {
2797                 throw new UnsupportedOperationException();
2798             }
2799             public boolean addAll(Collection<? extends Map.Entry<K, V>> coll) {
2800                 throw new UnsupportedOperationException();
2801             }
2802 
2803             public Iterator<Map.Entry<K,V>> iterator() {
2804                 final Iterator<Map.Entry<K, V>> i = s.iterator();
2805                 final Class<V> valueType = this.valueType;
2806 
2807                 return new Iterator<Map.Entry<K,V>>() {
2808                     public boolean hasNext() { return i.hasNext(); }
2809                     public void remove()     { i.remove(); }
2810 
2811                     public Map.Entry<K,V> next() {
2812                         return checkedEntry(i.next(), valueType);
2813                     }
2814                 };
2815             }
2816 
2817             @SuppressWarnings("unchecked")
2818             public Object[] toArray() {
2819                 Object[] source = s.toArray();
2820 
2821                 /*
2822                  * Ensure that we don't get an ArrayStoreException even if
2823                  * s.toArray returns an array of something other than Object
2824                  */
2825                 Object[] dest = (CheckedEntry.class.isInstance(
2826                     source.getClass().getComponentType()) ? source :
2827                                  new Object[source.length]);
2828 
2829                 for (int i = 0; i < source.length; i++)
2830                     dest[i] = checkedEntry((Map.Entry<K,V>)source[i],
2831                                            valueType);
2832                 return dest;
2833             }
2834 
2835             @SuppressWarnings("unchecked")
2836             public <T> T[] toArray(T[] a) {
2837                 // We don't pass a to s.toArray, to avoid window of
2838                 // vulnerability wherein an unscrupulous multithreaded client
2839                 // could get his hands on raw (unwrapped) Entries from s.
2840                 T[] arr = s.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));
2841 
2842                 for (int i=0; i<arr.length; i++)
2843                     arr[i] = (T) checkedEntry((Map.Entry<K,V>)arr[i],
2844                                               valueType);
2845                 if (arr.length > a.length)
2846                     return arr;
2847 
2848                 System.arraycopy(arr, 0, a, 0, arr.length);
2849                 if (a.length > arr.length)
2850                     a[arr.length] = null;
2851                 return a;
2852             }
2853 
2854             /**
2855              * This method is overridden to protect the backing set against
2856              * an object with a nefarious equals function that senses
2857              * that the equality-candidate is Map.Entry and calls its
2858              * setValue method.
2859              */
2860             public boolean contains(Object o) {
2861                 if (!(o instanceof Map.Entry))
2862                     return false;
2863                 Map.Entry<?,?> e = (Map.Entry<?,?>) o;
2864                 return s.contains(
2865                     (e instanceof CheckedEntry) ? e : checkedEntry(e, valueType));
2866             }
2867 
2868             /**
2869              * The bulk collection methods are overridden to protect
2870              * against an unscrupulous collection whose contains(Object o)
2871              * method senses when o is a Map.Entry, and calls o.setValue.
2872              */
2873             public boolean containsAll(Collection<?> c) {
2874                 for (Object o : c)
2875                     if (!contains(o)) // Invokes safe contains() above
2876                         return false;
2877                 return true;
2878             }
2879 
2880             public boolean remove(Object o) {
2881                 if (!(o instanceof Map.Entry))
2882                     return false;
2883                 return s.remove(new AbstractMap.SimpleImmutableEntry
2884                                 <>((Map.Entry<?,?>)o));
2885             }
2886 
2887             public boolean removeAll(Collection<?> c) {
2888                 return batchRemove(c, false);
2889             }
2890             public boolean retainAll(Collection<?> c) {
2891                 return batchRemove(c, true);
2892             }
2893             private boolean batchRemove(Collection<?> c, boolean complement) {
2894                 boolean modified = false;
2895                 Iterator<Map.Entry<K,V>> it = iterator();
2896                 while (it.hasNext()) {
2897                     if (c.contains(it.next()) != complement) {
2898                         it.remove();
2899                         modified = true;
2900                     }
2901                 }
2902                 return modified;
2903             }
2904 
2905             public boolean equals(Object o) {
2906                 if (o == this)
2907                     return true;
2908                 if (!(o instanceof Set))
2909                     return false;
2910                 Set<?> that = (Set<?>) o;
2911                 return that.size() == s.size()
2912                     && containsAll(that); // Invokes safe containsAll() above
2913             }
2914 
2915             static <K,V,T> CheckedEntry<K,V,T> checkedEntry(Map.Entry<K,V> e,
2916                                                             Class<T> valueType) {
2917                 return new CheckedEntry<>(e, valueType);
2918             }
2919 
2920             /**
2921              * This "wrapper class" serves two purposes: it prevents
2922              * the client from modifying the backing Map, by short-circuiting
2923              * the setValue method, and it protects the backing Map against
2924              * an ill-behaved Map.Entry that attempts to modify another
2925              * Map.Entry when asked to perform an equality check.
2926              */
2927             private static class CheckedEntry<K,V,T> implements Map.Entry<K,V> {
2928                 private final Map.Entry<K, V> e;
2929                 private final Class<T> valueType;
2930 
2931                 CheckedEntry(Map.Entry<K, V> e, Class<T> valueType) {
2932                     this.e = e;
2933                     this.valueType = valueType;
2934                 }
2935 
2936                 public K getKey()        { return e.getKey(); }
2937                 public V getValue()      { return e.getValue(); }
2938                 public int hashCode()    { return e.hashCode(); }
2939                 public String toString() { return e.toString(); }
2940 
2941                 public V setValue(V value) {
2942                     if (value != null && !valueType.isInstance(value))
2943                         throw new ClassCastException(badValueMsg(value));
2944                     return e.setValue(value);
2945                 }
2946 
2947                 private String badValueMsg(Object value) {
2948                     return "Attempt to insert " + value.getClass() +
2949                         " value into map with value type " + valueType;
2950                 }
2951 
2952                 public boolean equals(Object o) {
2953                     if (o == this)
2954                         return true;
2955                     if (!(o instanceof Map.Entry))
2956                         return false;
2957                     return e.equals(new AbstractMap.SimpleImmutableEntry
2958                                     <>((Map.Entry<?,?>)o));
2959                 }
2960             }
2961         }
2962     }
2963 
2964     /**
2965      * Returns a dynamically typesafe view of the specified sorted map.
2966      * Any attempt to insert a mapping whose key or value have the wrong
2967      * type will result in an immediate {@link ClassCastException}.
2968      * Similarly, any attempt to modify the value currently associated with
2969      * a key will result in an immediate {@link ClassCastException},
2970      * whether the modification is attempted directly through the map
2971      * itself, or through a {@link Map.Entry} instance obtained from the
2972      * map's {@link Map#entrySet() entry set} view.
2973      *
2974      * <p>Assuming a map contains no incorrectly typed keys or values
2975      * prior to the time a dynamically typesafe view is generated, and
2976      * that all subsequent access to the map takes place through the view
2977      * (or one of its collection views), it is <i>guaranteed</i> that the
2978      * map cannot contain an incorrectly typed key or value.
2979      *
2980      * <p>A discussion of the use of dynamically typesafe views may be
2981      * found in the documentation for the {@link #checkedCollection
2982      * checkedCollection} method.
2983      *
2984      * <p>The returned map will be serializable if the specified map is
2985      * serializable.
2986      *
2987      * <p>Since {@code null} is considered to be a value of any reference
2988      * type, the returned map permits insertion of null keys or values
2989      * whenever the backing map does.
2990      *
2991      * @param m the map for which a dynamically typesafe view is to be
2992      *          returned
2993      * @param keyType the type of key that {@code m} is permitted to hold
2994      * @param valueType the type of value that {@code m} is permitted to hold
2995      * @return a dynamically typesafe view of the specified map
2996      * @since 1.5
2997      */
2998     public static <K,V> SortedMap<K,V> checkedSortedMap(SortedMap<K, V> m,
2999                                                         Class<K> keyType,
3000                                                         Class<V> valueType) {
3001         return new CheckedSortedMap<>(m, keyType, valueType);
3002     }
3003 
3004     /**
3005      * @serial include
3006      */
3007     static class CheckedSortedMap<K,V> extends CheckedMap<K,V>
3008         implements SortedMap<K,V>, Serializable
3009     {
3010         private static final long serialVersionUID = 1599671320688067438L;
3011 
3012         private final SortedMap<K, V> sm;
3013 
3014         CheckedSortedMap(SortedMap<K, V> m,
3015                          Class<K> keyType, Class<V> valueType) {
3016             super(m, keyType, valueType);
3017             sm = m;
3018         }
3019 
3020         public Comparator<? super K> comparator() { return sm.comparator(); }
3021         public K firstKey()                       { return sm.firstKey(); }
3022         public K lastKey()                        { return sm.lastKey(); }
3023 
3024         public SortedMap<K,V> subMap(K fromKey, K toKey) {
3025             return checkedSortedMap(sm.subMap(fromKey, toKey),
3026                                     keyType, valueType);
3027         }
3028         public SortedMap<K,V> headMap(K toKey) {
3029             return checkedSortedMap(sm.headMap(toKey), keyType, valueType);
3030         }
3031         public SortedMap<K,V> tailMap(K fromKey) {
3032             return checkedSortedMap(sm.tailMap(fromKey), keyType, valueType);
3033         }
3034     }
3035 
3036     // Empty collections
3037 
3038     /**
3039      * Returns an iterator that has no elements.  More precisely,
3040      *
3041      * <ul compact>
3042      *
3043      * <li>{@link Iterator#hasNext hasNext} always returns {@code
3044      * false}.
3045      *
3046      * <li>{@link Iterator#next next} always throws {@link
3047      * NoSuchElementException}.
3048      *
3049      * <li>{@link Iterator#remove remove} always throws {@link
3050      * IllegalStateException}.
3051      *
3052      * </ul>
3053      *
3054      * <p>Implementations of this method are permitted, but not
3055      * required, to return the same object from multiple invocations.
3056      *
3057      * @return an empty iterator
3058      * @since 1.7
3059      */
3060     @SuppressWarnings("unchecked")
3061     public static <T> Iterator<T> emptyIterator() {
3062         return (Iterator<T>) EmptyIterator.EMPTY_ITERATOR;
3063     }
3064 
3065     private static class EmptyIterator<E> implements Iterator<E> {
3066         static final EmptyIterator<Object> EMPTY_ITERATOR
3067             = new EmptyIterator<>();
3068 
3069         public boolean hasNext() { return false; }
3070         public E next() { throw new NoSuchElementException(); }
3071         public void remove() { throw new IllegalStateException(); }
3072     }
3073 
3074     /**
3075      * Returns a list iterator that has no elements.  More precisely,
3076      *
3077      * <ul compact>
3078      *
3079      * <li>{@link Iterator#hasNext hasNext} and {@link
3080      * ListIterator#hasPrevious hasPrevious} always return {@code
3081      * false}.
3082      *
3083      * <li>{@link Iterator#next next} and {@link ListIterator#previous
3084      * previous} always throw {@link NoSuchElementException}.
3085      *
3086      * <li>{@link Iterator#remove remove} and {@link ListIterator#set
3087      * set} always throw {@link IllegalStateException}.
3088      *
3089      * <li>{@link ListIterator#add add} always throws {@link
3090      * UnsupportedOperationException}.
3091      *
3092      * <li>{@link ListIterator#nextIndex nextIndex} always returns
3093      * {@code 0} .
3094      *
3095      * <li>{@link ListIterator#previousIndex previousIndex} always
3096      * returns {@code -1}.
3097      *
3098      * </ul>
3099      *
3100      * <p>Implementations of this method are permitted, but not
3101      * required, to return the same object from multiple invocations.
3102      *
3103      * @return an empty list iterator
3104      * @since 1.7
3105      */
3106     @SuppressWarnings("unchecked")
3107     public static <T> ListIterator<T> emptyListIterator() {
3108         return (ListIterator<T>) EmptyListIterator.EMPTY_ITERATOR;
3109     }
3110 
3111     private static class EmptyListIterator<E>
3112         extends EmptyIterator<E>
3113         implements ListIterator<E>
3114     {
3115         static final EmptyListIterator<Object> EMPTY_ITERATOR
3116             = new EmptyListIterator<>();
3117 
3118         public boolean hasPrevious() { return false; }
3119         public E previous() { throw new NoSuchElementException(); }
3120         public int nextIndex()     { return 0; }
3121         public int previousIndex() { return -1; }
3122         public void set(E e) { throw new IllegalStateException(); }
3123         public void add(E e) { throw new UnsupportedOperationException(); }
3124     }
3125 
3126     /**
3127      * Returns an enumeration that has no elements.  More precisely,
3128      *
3129      * <ul compact>
3130      *
3131      * <li>{@link Enumeration#hasMoreElements hasMoreElements} always
3132      * returns {@code false}.
3133      *
3134      * <li> {@link Enumeration#nextElement nextElement} always throws
3135      * {@link NoSuchElementException}.
3136      *
3137      * </ul>
3138      *
3139      * <p>Implementations of this method are permitted, but not
3140      * required, to return the same object from multiple invocations.
3141      *
3142      * @return an empty enumeration
3143      * @since 1.7
3144      */
3145     @SuppressWarnings("unchecked")
3146     public static <T> Enumeration<T> emptyEnumeration() {
3147         return (Enumeration<T>) EmptyEnumeration.EMPTY_ENUMERATION;
3148     }
3149 
3150     private static class EmptyEnumeration<E> implements Enumeration<E> {
3151         static final EmptyEnumeration<Object> EMPTY_ENUMERATION
3152             = new EmptyEnumeration<>();
3153 
3154         public boolean hasMoreElements() { return false; }
3155         public E nextElement() { throw new NoSuchElementException(); }
3156     }
3157 
3158     /**
3159      * The empty set (immutable).  This set is serializable.
3160      *
3161      * @see #emptySet()
3162      */
3163     @SuppressWarnings("unchecked")
3164     public static final Set EMPTY_SET = new EmptySet<>();
3165 
3166     /**
3167      * Returns the empty set (immutable).  This set is serializable.
3168      * Unlike the like-named field, this method is parameterized.
3169      *
3170      * <p>This example illustrates the type-safe way to obtain an empty set:
3171      * <pre>
3172      *     Set&lt;String&gt; s = Collections.emptySet();
3173      * </pre>
3174      * Implementation note:  Implementations of this method need not
3175      * create a separate <tt>Set</tt> object for each call.   Using this
3176      * method is likely to have comparable cost to using the like-named
3177      * field.  (Unlike this method, the field does not provide type safety.)
3178      *
3179      * @see #EMPTY_SET
3180      * @since 1.5
3181      */
3182     @SuppressWarnings("unchecked")
3183     public static final <T> Set<T> emptySet() {
3184         return (Set<T>) EMPTY_SET;
3185     }
3186 
3187     /**
3188      * @serial include
3189      */
3190     private static class EmptySet<E>
3191         extends AbstractSet<E>
3192         implements Serializable
3193     {
3194         private static final long serialVersionUID = 1582296315990362920L;
3195 
3196         public Iterator<E> iterator() { return emptyIterator(); }
3197 
3198         public int size() {return 0;}
3199         public boolean isEmpty() {return true;}
3200 
3201         public boolean contains(Object obj) {return false;}
3202         public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
3203 
3204         public Object[] toArray() { return new Object[0]; }
3205 
3206         public <T> T[] toArray(T[] a) {
3207             if (a.length > 0)
3208                 a[0] = null;
3209             return a;
3210         }
3211 
3212         // Preserves singleton property
3213         private Object readResolve() {
3214             return EMPTY_SET;
3215         }
3216     }
3217 
3218     /**
3219      * Returns the empty sorted set (immutable).  This set is serializable.
3220      *
3221      * <p>This example illustrates the type-safe way to obtain an empty sorted
3222      * set:
3223      * <pre>
3224      *     SortedSet&lt;String&gt; s = Collections.emptySortedSet();
3225      * </pre>
3226      * Implementation note:  Implementations of this method need not
3227      * create a separate <tt>SortedSet</tt> object for each call.
3228      *
3229      * @since 1.8
3230      */
3231     @SuppressWarnings("unchecked")
3232     public static final <E> SortedSet<E> emptySortedSet() {
3233         return (SortedSet<E>) new EmptySortedSet<>();
3234     }
3235 
3236     /**
3237      * @serial include
3238      */
3239     private static class EmptySortedSet<E>
3240         extends AbstractSet<E>
3241         implements SortedSet<E>, Serializable
3242     {
3243         private static final long serialVersionUID = 6316515401502265487L;
3244         public Iterator<E> iterator() { return emptyIterator(); }
3245         public int size() {return 0;}
3246         public boolean isEmpty() {return true;}
3247         public boolean contains(Object obj) {return false;}
3248         public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
3249         public Object[] toArray() { return new Object[0]; }
3250 
3251         public <E> E[] toArray(E[] a) {
3252             if (a.length > 0)
3253                 a[0] = null;
3254             return a;
3255         }
3256 
3257         // Preserves singleton property
3258         private Object readResolve() {
3259             return new EmptySortedSet<>();
3260         }
3261 
3262         public Comparator comparator() {
3263             return null;
3264         }
3265 
3266         public SortedSet<E> subSet(Object fromElement, Object toElement) {
3267             Objects.requireNonNull(fromElement);
3268             Objects.requireNonNull(toElement);
3269 
3270             if (!(fromElement instanceof Comparable) ||
3271                     !(toElement instanceof Comparable))
3272             {
3273                 throw new ClassCastException();
3274             }
3275 
3276             if ((((Comparable)fromElement).compareTo(toElement) >= 0) ||
3277                     (((Comparable)toElement).compareTo(fromElement) < 0))
3278             {
3279                 throw new IllegalArgumentException();
3280             }
3281 
3282             return emptySortedSet();
3283         }
3284 
3285         public SortedSet<E> headSet(Object toElement) {
3286             Objects.requireNonNull(toElement);
3287 
3288             if (!(toElement instanceof Comparable)) {
3289                 throw new ClassCastException();
3290             }
3291 
3292             return emptySortedSet();
3293         }
3294 
3295         public SortedSet<E> tailSet(Object fromElement) {
3296             Objects.requireNonNull(fromElement);
3297 
3298             if (!(fromElement instanceof Comparable)) {
3299                 throw new ClassCastException();
3300             }
3301 
3302             return emptySortedSet();
3303         }
3304 
3305         public E first() {
3306             throw new NoSuchElementException();
3307         }
3308 
3309         public E last() {
3310             throw new NoSuchElementException();
3311         }
3312     }
3313 
3314     /**
3315      * The empty list (immutable).  This list is serializable.
3316      *
3317      * @see #emptyList()
3318      */
3319     @SuppressWarnings("unchecked")
3320     public static final List EMPTY_LIST = new EmptyList<>();
3321 
3322     /**
3323      * Returns the empty list (immutable).  This list is serializable.
3324      *
3325      * <p>This example illustrates the type-safe way to obtain an empty list:
3326      * <pre>
3327      *     List&lt;String&gt; s = Collections.emptyList();
3328      * </pre>
3329      * Implementation note:  Implementations of this method need not
3330      * create a separate <tt>List</tt> object for each call.   Using this
3331      * method is likely to have comparable cost to using the like-named
3332      * field.  (Unlike this method, the field does not provide type safety.)
3333      *
3334      * @see #EMPTY_LIST
3335      * @since 1.5
3336      */
3337     @SuppressWarnings("unchecked")
3338     public static final <T> List<T> emptyList() {
3339         return (List<T>) EMPTY_LIST;
3340     }
3341 
3342     /**
3343      * @serial include
3344      */
3345     private static class EmptyList<E>
3346         extends AbstractList<E>
3347         implements RandomAccess, Serializable {
3348         private static final long serialVersionUID = 8842843931221139166L;
3349 
3350         public Iterator<E> iterator() {
3351             return emptyIterator();
3352         }
3353         public ListIterator<E> listIterator() {
3354             return emptyListIterator();
3355         }
3356 
3357         public int size() {return 0;}
3358         public boolean isEmpty() {return true;}
3359 
3360         public boolean contains(Object obj) {return false;}
3361         public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
3362 
3363         public Object[] toArray() { return new Object[0]; }
3364 
3365         public <T> T[] toArray(T[] a) {
3366             if (a.length > 0)
3367                 a[0] = null;
3368             return a;
3369         }
3370 
3371         public E get(int index) {
3372             throw new IndexOutOfBoundsException("Index: "+index);
3373         }
3374 
3375         public boolean equals(Object o) {
3376             return (o instanceof List) && ((List<?>)o).isEmpty();
3377         }
3378 
3379         public int hashCode() { return 1; }
3380 
3381         // Preserves singleton property
3382         private Object readResolve() {
3383             return EMPTY_LIST;
3384         }
3385     }
3386 
3387     /**
3388      * The empty map (immutable).  This map is serializable.
3389      *
3390      * @see #emptyMap()
3391      * @since 1.3
3392      */
3393     @SuppressWarnings("unchecked")
3394     public static final Map EMPTY_MAP = new EmptyMap<>();
3395 
3396     /**
3397      * Returns the empty map (immutable).  This map is serializable.
3398      *
3399      * <p>This example illustrates the type-safe way to obtain an empty set:
3400      * <pre>
3401      *     Map&lt;String, Date&gt; s = Collections.emptyMap();
3402      * </pre>
3403      * Implementation note:  Implementations of this method need not
3404      * create a separate <tt>Map</tt> object for each call.   Using this
3405      * method is likely to have comparable cost to using the like-named
3406      * field.  (Unlike this method, the field does not provide type safety.)
3407      *
3408      * @see #EMPTY_MAP
3409      * @since 1.5
3410      */
3411     @SuppressWarnings("unchecked")
3412     public static final <K,V> Map<K,V> emptyMap() {
3413         return (Map<K,V>) EMPTY_MAP;
3414     }
3415 
3416     /**
3417      * @serial include
3418      */
3419     private static class EmptyMap<K,V>
3420         extends AbstractMap<K,V>
3421         implements Serializable
3422     {
3423         private static final long serialVersionUID = 6428348081105594320L;
3424 
3425         public int size()                          {return 0;}
3426         public boolean isEmpty()                   {return true;}
3427         public boolean containsKey(Object key)     {return false;}
3428         public boolean containsValue(Object value) {return false;}
3429         public V get(Object key)                   {return null;}
3430         public Set<K> keySet()                     {return emptySet();}
3431         public Collection<V> values()              {return emptySet();}
3432         public Set<Map.Entry<K,V>> entrySet()      {return emptySet();}
3433 
3434         public boolean equals(Object o) {
3435             return (o instanceof Map) && ((Map<?,?>)o).isEmpty();
3436         }
3437 
3438         public int hashCode()                      {return 0;}
3439 
3440         // Preserves singleton property
3441         private Object readResolve() {
3442             return EMPTY_MAP;
3443         }
3444     }
3445 
3446     // Singleton collections
3447 
3448     /**
3449      * Returns an immutable set containing only the specified object.
3450      * The returned set is serializable.
3451      *
3452      * @param o the sole object to be stored in the returned set.
3453      * @return an immutable set containing only the specified object.
3454      */
3455     public static <T> Set<T> singleton(T o) {
3456         return new SingletonSet<>(o);
3457     }
3458 
3459     static <E> Iterator<E> singletonIterator(final E e) {
3460         return new Iterator<E>() {
3461             private boolean hasNext = true;
3462             public boolean hasNext() {
3463                 return hasNext;
3464             }
3465             public E next() {
3466                 if (hasNext) {
3467                     hasNext = false;
3468                     return e;
3469                 }
3470                 throw new NoSuchElementException();
3471             }
3472             public void remove() {
3473                 throw new UnsupportedOperationException();
3474             }
3475         };
3476     }
3477 
3478     /**
3479      * @serial include
3480      */
3481     private static class SingletonSet<E>
3482         extends AbstractSet<E>
3483         implements Serializable
3484     {
3485         private static final long serialVersionUID = 3193687207550431679L;
3486 
3487         private final E element;
3488 
3489         SingletonSet(E e) {element = e;}
3490 
3491         public Iterator<E> iterator() {
3492             return singletonIterator(element);
3493         }
3494 
3495         public int size() {return 1;}
3496 
3497         public boolean contains(Object o) {return eq(o, element);}
3498     }
3499 
3500     /**
3501      * Returns an immutable list containing only the specified object.
3502      * The returned list is serializable.
3503      *
3504      * @param o the sole object to be stored in the returned list.
3505      * @return an immutable list containing only the specified object.
3506      * @since 1.3
3507      */
3508     public static <T> List<T> singletonList(T o) {
3509         return new SingletonList<>(o);
3510     }
3511 
3512     /**
3513      * @serial include
3514      */
3515     private static class SingletonList<E>
3516         extends AbstractList<E>
3517         implements RandomAccess, Serializable {
3518 
3519         private static final long serialVersionUID = 3093736618740652951L;
3520 
3521         private final E element;
3522 
3523         SingletonList(E obj)                {element = obj;}
3524 
3525         public Iterator<E> iterator() {
3526             return singletonIterator(element);
3527         }
3528 
3529         public int size()                   {return 1;}
3530 
3531         public boolean contains(Object obj) {return eq(obj, element);}
3532 
3533         public E get(int index) {
3534             if (index != 0)
3535               throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
3536             return element;
3537         }
3538     }
3539 
3540     /**
3541      * Returns an immutable map, mapping only the specified key to the
3542      * specified value.  The returned map is serializable.
3543      *
3544      * @param key the sole key to be stored in the returned map.
3545      * @param value the value to which the returned map maps <tt>key</tt>.
3546      * @return an immutable map containing only the specified key-value
3547      *         mapping.
3548      * @since 1.3
3549      */
3550     public static <K,V> Map<K,V> singletonMap(K key, V value) {
3551         return new SingletonMap<>(key, value);
3552     }
3553 
3554     /**
3555      * @serial include
3556      */
3557     private static class SingletonMap<K,V>
3558           extends AbstractMap<K,V>
3559           implements Serializable {
3560         private static final long serialVersionUID = -6979724477215052911L;
3561 
3562         private final K k;
3563         private final V v;
3564 
3565         SingletonMap(K key, V value) {
3566             k = key;
3567             v = value;
3568         }
3569 
3570         public int size()                          {return 1;}
3571 
3572         public boolean isEmpty()                   {return false;}
3573 
3574         public boolean containsKey(Object key)     {return eq(key, k);}
3575 
3576         public boolean containsValue(Object value) {return eq(value, v);}
3577 
3578         public V get(Object key)                   {return (eq(key, k) ? v : null);}
3579 
3580         private transient Set<K> keySet = null;
3581         private transient Set<Map.Entry<K,V>> entrySet = null;
3582         private transient Collection<V> values = null;
3583 
3584         public Set<K> keySet() {
3585             if (keySet==null)
3586                 keySet = singleton(k);
3587             return keySet;
3588         }
3589 
3590         public Set<Map.Entry<K,V>> entrySet() {
3591             if (entrySet==null)
3592                 entrySet = Collections.<Map.Entry<K,V>>singleton(
3593                     new SimpleImmutableEntry<>(k, v));
3594             return entrySet;
3595         }
3596 
3597         public Collection<V> values() {
3598             if (values==null)
3599                 values = singleton(v);
3600             return values;
3601         }
3602 
3603     }
3604 
3605     // Miscellaneous
3606 
3607     /**
3608      * Returns an immutable list consisting of <tt>n</tt> copies of the
3609      * specified object.  The newly allocated data object is tiny (it contains
3610      * a single reference to the data object).  This method is useful in
3611      * combination with the <tt>List.addAll</tt> method to grow lists.
3612      * The returned list is serializable.
3613      *
3614      * @param  n the number of elements in the returned list.
3615      * @param  o the element to appear repeatedly in the returned list.
3616      * @return an immutable list consisting of <tt>n</tt> copies of the
3617      *         specified object.
3618      * @throws IllegalArgumentException if {@code n < 0}
3619      * @see    List#addAll(Collection)
3620      * @see    List#addAll(int, Collection)
3621      */
3622     public static <T> List<T> nCopies(int n, T o) {
3623         if (n < 0)
3624             throw new IllegalArgumentException("List length = " + n);
3625         return new CopiesList<>(n, o);
3626     }
3627 
3628     /**
3629      * @serial include
3630      */
3631     private static class CopiesList<E>
3632         extends AbstractList<E>
3633         implements RandomAccess, Serializable
3634     {
3635         private static final long serialVersionUID = 2739099268398711800L;
3636 
3637         final int n;
3638         final E element;
3639 
3640         CopiesList(int n, E e) {
3641             assert n >= 0;
3642             this.n = n;
3643             element = e;
3644         }
3645 
3646         public int size() {
3647             return n;
3648         }
3649 
3650         public boolean contains(Object obj) {
3651             return n != 0 && eq(obj, element);
3652         }
3653 
3654         public int indexOf(Object o) {
3655             return contains(o) ? 0 : -1;
3656         }
3657 
3658         public int lastIndexOf(Object o) {
3659             return contains(o) ? n - 1 : -1;
3660         }
3661 
3662         public E get(int index) {
3663             if (index < 0 || index >= n)
3664                 throw new IndexOutOfBoundsException("Index: "+index+
3665                                                     ", Size: "+n);
3666             return element;
3667         }
3668 
3669         public Object[] toArray() {
3670             final Object[] a = new Object[n];
3671             if (element != null)
3672                 Arrays.fill(a, 0, n, element);
3673             return a;
3674         }
3675 
3676         public <T> T[] toArray(T[] a) {
3677             final int n = this.n;
3678             if (a.length < n) {
3679                 a = (T[])java.lang.reflect.Array
3680                     .newInstance(a.getClass().getComponentType(), n);
3681                 if (element != null)
3682                     Arrays.fill(a, 0, n, element);
3683             } else {
3684                 Arrays.fill(a, 0, n, element);
3685                 if (a.length > n)
3686                     a[n] = null;
3687             }
3688             return a;
3689         }
3690 
3691         public List<E> subList(int fromIndex, int toIndex) {
3692             if (fromIndex < 0)
3693                 throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
3694             if (toIndex > n)
3695                 throw new IndexOutOfBoundsException("toIndex = " + toIndex);
3696             if (fromIndex > toIndex)
3697                 throw new IllegalArgumentException("fromIndex(" + fromIndex +
3698                                                    ") > toIndex(" + toIndex + ")");
3699             return new CopiesList<>(toIndex - fromIndex, element);
3700         }
3701     }
3702 
3703     /**
3704      * Returns a comparator that imposes the reverse of the <em>natural
3705      * ordering</em> on a collection of objects that implement the
3706      * {@code Comparable} interface.  (The natural ordering is the ordering
3707      * imposed by the objects' own {@code compareTo} method.)  This enables a
3708      * simple idiom for sorting (or maintaining) collections (or arrays) of
3709      * objects that implement the {@code Comparable} interface in
3710      * reverse-natural-order.  For example, suppose {@code a} is an array of
3711      * strings. Then: <pre>
3712      *          Arrays.sort(a, Collections.reverseOrder());
3713      * </pre> sorts the array in reverse-lexicographic (alphabetical) order.<p>
3714      *
3715      * The returned comparator is serializable.
3716      *
3717      * @return A comparator that imposes the reverse of the <i>natural
3718      *         ordering</i> on a collection of objects that implement
3719      *         the <tt>Comparable</tt> interface.
3720      * @see Comparable
3721      */
3722     public static <T> Comparator<T> reverseOrder() {
3723         return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
3724     }
3725 
3726     /**
3727      * @serial include
3728      */
3729     private static class ReverseComparator
3730         implements Comparator<Comparable<Object>>, Serializable {
3731 
3732         private static final long serialVersionUID = 7207038068494060240L;
3733 
3734         static final ReverseComparator REVERSE_ORDER
3735             = new ReverseComparator();
3736 
3737         public int compare(Comparable<Object> c1, Comparable<Object> c2) {
3738             return c2.compareTo(c1);
3739         }
3740 
3741         private Object readResolve() { return reverseOrder(); }
3742     }
3743 
3744     /**
3745      * Returns a comparator that imposes the reverse ordering of the specified
3746      * comparator.  If the specified comparator is {@code null}, this method is
3747      * equivalent to {@link #reverseOrder()} (in other words, it returns a
3748      * comparator that imposes the reverse of the <em>natural ordering</em> on
3749      * a collection of objects that implement the Comparable interface).
3750      *
3751      * <p>The returned comparator is serializable (assuming the specified
3752      * comparator is also serializable or {@code null}).
3753      *
3754      * @param cmp a comparator who's ordering is to be reversed by the returned
3755      * comparator or {@code null}
3756      * @return A comparator that imposes the reverse ordering of the
3757      *         specified comparator.
3758      * @since 1.5
3759      */
3760     public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
3761         if (cmp == null)
3762             return reverseOrder();
3763 
3764         if (cmp instanceof ReverseComparator2)
3765             return ((ReverseComparator2<T>)cmp).cmp;
3766 
3767         return new ReverseComparator2<>(cmp);
3768     }
3769 
3770     /**
3771      * @serial include
3772      */
3773     private static class ReverseComparator2<T> implements Comparator<T>,
3774         Serializable
3775     {
3776         private static final long serialVersionUID = 4374092139857L;
3777 
3778         /**
3779          * The comparator specified in the static factory.  This will never
3780          * be null, as the static factory returns a ReverseComparator
3781          * instance if its argument is null.
3782          *
3783          * @serial
3784          */
3785         final Comparator<T> cmp;
3786 
3787         ReverseComparator2(Comparator<T> cmp) {
3788             assert cmp != null;
3789             this.cmp = cmp;
3790         }
3791 
3792         public int compare(T t1, T t2) {
3793             return cmp.compare(t2, t1);
3794         }
3795 
3796         public boolean equals(Object o) {
3797             return (o == this) ||
3798                 (o instanceof ReverseComparator2 &&
3799                  cmp.equals(((ReverseComparator2)o).cmp));
3800         }
3801 
3802         public int hashCode() {
3803             return cmp.hashCode() ^ Integer.MIN_VALUE;
3804         }
3805     }
3806 
3807     /**
3808      * Returns an enumeration over the specified collection.  This provides
3809      * interoperability with legacy APIs that require an enumeration
3810      * as input.
3811      *
3812      * @param c the collection for which an enumeration is to be returned.
3813      * @return an enumeration over the specified collection.
3814      * @see Enumeration
3815      */
3816     public static <T> Enumeration<T> enumeration(final Collection<T> c) {
3817         return new Enumeration<T>() {
3818             private final Iterator<T> i = c.iterator();
3819 
3820             public boolean hasMoreElements() {
3821                 return i.hasNext();
3822             }
3823 
3824             public T nextElement() {
3825                 return i.next();
3826             }
3827         };
3828     }
3829 
3830     /**
3831      * Returns an array list containing the elements returned by the
3832      * specified enumeration in the order they are returned by the
3833      * enumeration.  This method provides interoperability between
3834      * legacy APIs that return enumerations and new APIs that require
3835      * collections.
3836      *
3837      * @param e enumeration providing elements for the returned
3838      *          array list
3839      * @return an array list containing the elements returned
3840      *         by the specified enumeration.
3841      * @since 1.4
3842      * @see Enumeration
3843      * @see ArrayList
3844      */
3845     public static <T> ArrayList<T> list(Enumeration<T> e) {
3846         ArrayList<T> l = new ArrayList<>();
3847         while (e.hasMoreElements())
3848             l.add(e.nextElement());
3849         return l;
3850     }
3851 
3852     /**
3853      * Returns true if the specified arguments are equal, or both null.
3854      */
3855     static boolean eq(Object o1, Object o2) {
3856         return o1==null ? o2==null : o1.equals(o2);
3857     }
3858 
3859     /**
3860      * Returns the number of elements in the specified collection equal to the
3861      * specified object.  More formally, returns the number of elements
3862      * <tt>e</tt> in the collection such that
3863      * <tt>(o == null ? e == null : o.equals(e))</tt>.
3864      *
3865      * @param c the collection in which to determine the frequency
3866      *     of <tt>o</tt>
3867      * @param o the object whose frequency is to be determined
3868      * @throws NullPointerException if <tt>c</tt> is null
3869      * @since 1.5
3870      */
3871     public static int frequency(Collection<?> c, Object o) {
3872         int result = 0;
3873         if (o == null) {
3874             for (Object e : c)
3875                 if (e == null)
3876                     result++;
3877         } else {
3878             for (Object e : c)
3879                 if (o.equals(e))
3880                     result++;
3881         }
3882         return result;
3883     }
3884 
3885     /**
3886      * Returns {@code true} if the two specified collections have no
3887      * elements in common.
3888      *
3889      * <p>Care must be exercised if this method is used on collections that
3890      * do not comply with the general contract for {@code Collection}.
3891      * Implementations may elect to iterate over either collection and test
3892      * for containment in the other collection (or to perform any equivalent
3893      * computation).  If either collection uses a nonstandard equality test
3894      * (as does a {@link SortedSet} whose ordering is not <em>compatible with
3895      * equals</em>, or the key set of an {@link IdentityHashMap}), both
3896      * collections must use the same nonstandard equality test, or the
3897      * result of this method is undefined.
3898      *
3899      * <p>Care must also be exercised when using collections that have
3900      * restrictions on the elements that they may contain. Collection
3901      * implementations are allowed to throw exceptions for any operation
3902      * involving elements they deem ineligible. For absolute safety the
3903      * specified collections should contain only elements which are
3904      * eligible elements for both collections.
3905      *
3906      * <p>Note that it is permissible to pass the same collection in both
3907      * parameters, in which case the method will return {@code true} if and
3908      * only if the collection is empty.
3909      *
3910      * @param c1 a collection
3911      * @param c2 a collection
3912      * @return {@code true} if the two specified collections have no
3913      * elements in common.
3914      * @throws NullPointerException if either collection is {@code null}.
3915      * @throws NullPointerException if one collection contains a {@code null}
3916      * element and {@code null} is not an eligible element for the other collection.
3917      * (<a href="Collection.html#optional-restrictions">optional</a>)
3918      * @throws ClassCastException if one collection contains an element that is
3919      * of a type which is ineligible for the other collection.
3920      * (<a href="Collection.html#optional-restrictions">optional</a>)
3921      * @since 1.5
3922      */
3923     public static boolean disjoint(Collection<?> c1, Collection<?> c2) {
3924         // The collection to be used for contains(). Preference is given to
3925         // the collection who's contains() has lower O() complexity.
3926         Collection<?> contains = c2;
3927         // The collection to be iterated. If the collections' contains() impl
3928         // are of different O() complexity, the collection with slower
3929         // contains() will be used for iteration. For collections who's
3930         // contains() are of the same complexity then best performance is
3931         // achieved by iterating the smaller collection.
3932         Collection<?> iterate = c1;
3933 
3934         // Performance optimization cases. The heuristics:
3935         //   1. Generally iterate over c1.
3936         //   2. If c1 is a Set then iterate over c2.
3937         //   3. If either collection is empty then result is always true.
3938         //   4. Iterate over the smaller Collection.
3939         if (c1 instanceof Set) {
3940             // Use c1 for contains as a Set's contains() is expected to perform
3941             // better than O(N/2)
3942             iterate = c2;
3943             contains = c1;
3944         } else if (!(c2 instanceof Set)) {
3945             // Both are mere Collections. Iterate over smaller collection.
3946             // Example: If c1 contains 3 elements and c2 contains 50 elements and
3947             // assuming contains() requires ceiling(N/2) comparisons then
3948             // checking for all c1 elements in c2 would require 75 comparisons
3949             // (3 * ceiling(50/2)) vs. checking all c2 elements in c1 requiring
3950             // 100 comparisons (50 * ceiling(3/2)).
3951             int c1size = c1.size();
3952             int c2size = c2.size();
3953             if (c1size == 0 || c2size == 0) {
3954                 // At least one collection is empty. Nothing will match.
3955                 return true;
3956             }
3957 
3958             if (c1size > c2size) {
3959                 iterate = c2;
3960                 contains = c1;
3961             }
3962         }
3963 
3964         for (Object e : iterate) {
3965             if (contains.contains(e)) {
3966                // Found a common element. Collections are not disjoint.
3967                 return false;
3968             }
3969         }
3970 
3971         // No common elements were found.
3972         return true;
3973     }
3974 
3975     /**
3976      * Adds all of the specified elements to the specified collection.
3977      * Elements to be added may be specified individually or as an array.
3978      * The behavior of this convenience method is identical to that of
3979      * <tt>c.addAll(Arrays.asList(elements))</tt>, but this method is likely
3980      * to run significantly faster under most implementations.
3981      *
3982      * <p>When elements are specified individually, this method provides a
3983      * convenient way to add a few elements to an existing collection:
3984      * <pre>
3985      *     Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
3986      * </pre>
3987      *
3988      * @param c the collection into which <tt>elements</tt> are to be inserted
3989      * @param elements the elements to insert into <tt>c</tt>
3990      * @return <tt>true</tt> if the collection changed as a result of the call
3991      * @throws UnsupportedOperationException if <tt>c</tt> does not support
3992      *         the <tt>add</tt> operation
3993      * @throws NullPointerException if <tt>elements</tt> contains one or more
3994      *         null values and <tt>c</tt> does not permit null elements, or
3995      *         if <tt>c</tt> or <tt>elements</tt> are <tt>null</tt>
3996      * @throws IllegalArgumentException if some property of a value in
3997      *         <tt>elements</tt> prevents it from being added to <tt>c</tt>
3998      * @see Collection#addAll(Collection)
3999      * @since 1.5
4000      */
4001     @SafeVarargs
4002     public static <T> boolean addAll(Collection<? super T> c, T... elements) {
4003         boolean result = false;
4004         for (T element : elements)
4005             result |= c.add(element);
4006         return result;
4007     }
4008 
4009     /**
4010      * Returns a set backed by the specified map.  The resulting set displays
4011      * the same ordering, concurrency, and performance characteristics as the
4012      * backing map.  In essence, this factory method provides a {@link Set}
4013      * implementation corresponding to any {@link Map} implementation.  There
4014      * is no need to use this method on a {@link Map} implementation that
4015      * already has a corresponding {@link Set} implementation (such as {@link
4016      * HashMap} or {@link TreeMap}).
4017      *
4018      * <p>Each method invocation on the set returned by this method results in
4019      * exactly one method invocation on the backing map or its <tt>keySet</tt>
4020      * view, with one exception.  The <tt>addAll</tt> method is implemented
4021      * as a sequence of <tt>put</tt> invocations on the backing map.
4022      *
4023      * <p>The specified map must be empty at the time this method is invoked,
4024      * and should not be accessed directly after this method returns.  These
4025      * conditions are ensured if the map is created empty, passed directly
4026      * to this method, and no reference to the map is retained, as illustrated
4027      * in the following code fragment:
4028      * <pre>
4029      *    Set&lt;Object&gt; weakHashSet = Collections.newSetFromMap(
4030      *        new WeakHashMap&lt;Object, Boolean&gt;());
4031      * </pre>
4032      *
4033      * @param map the backing map
4034      * @return the set backed by the map
4035      * @throws IllegalArgumentException if <tt>map</tt> is not empty
4036      * @since 1.6
4037      */
4038     public static <E> Set<E> newSetFromMap(Map<E, Boolean> map) {
4039         return new SetFromMap<>(map);
4040     }
4041 
4042     /**
4043      * @serial include
4044      */
4045     private static class SetFromMap<E> extends AbstractSet<E>
4046         implements Set<E>, Serializable
4047     {
4048         private final Map<E, Boolean> m;  // The backing map
4049         private transient Set<E> s;       // Its keySet
4050 
4051         SetFromMap(Map<E, Boolean> map) {
4052             if (!map.isEmpty())
4053                 throw new IllegalArgumentException("Map is non-empty");
4054             m = map;
4055             s = map.keySet();
4056         }
4057 
4058         public void clear()               {        m.clear(); }
4059         public int size()                 { return m.size(); }
4060         public boolean isEmpty()          { return m.isEmpty(); }
4061         public boolean contains(Object o) { return m.containsKey(o); }
4062         public boolean remove(Object o)   { return m.remove(o) != null; }
4063         public boolean add(E e) { return m.put(e, Boolean.TRUE) == null; }
4064         public Iterator<E> iterator()     { return s.iterator(); }
4065         public Object[] toArray()         { return s.toArray(); }
4066         public <T> T[] toArray(T[] a)     { return s.toArray(a); }
4067         public String toString()          { return s.toString(); }
4068         public int hashCode()             { return s.hashCode(); }
4069         public boolean equals(Object o)   { return o == this || s.equals(o); }
4070         public boolean containsAll(Collection<?> c) {return s.containsAll(c);}
4071         public boolean removeAll(Collection<?> c)   {return s.removeAll(c);}
4072         public boolean retainAll(Collection<?> c)   {return s.retainAll(c);}
4073         // addAll is the only inherited implementation
4074 
4075         private static final long serialVersionUID = 2454657854757543876L;
4076 
4077         private void readObject(java.io.ObjectInputStream stream)
4078             throws IOException, ClassNotFoundException
4079         {
4080             stream.defaultReadObject();
4081             s = m.keySet();
4082         }
4083     }
4084 
4085     /**
4086      * Returns a view of a {@link Deque} as a Last-in-first-out (Lifo)
4087      * {@link Queue}. Method <tt>add</tt> is mapped to <tt>push</tt>,
4088      * <tt>remove</tt> is mapped to <tt>pop</tt> and so on. This
4089      * view can be useful when you would like to use a method
4090      * requiring a <tt>Queue</tt> but you need Lifo ordering.
4091      *
4092      * <p>Each method invocation on the queue returned by this method
4093      * results in exactly one method invocation on the backing deque, with
4094      * one exception.  The {@link Queue#addAll addAll} method is
4095      * implemented as a sequence of {@link Deque#addFirst addFirst}
4096      * invocations on the backing deque.
4097      *
4098      * @param deque the deque
4099      * @return the queue
4100      * @since  1.6
4101      */
4102     public static <T> Queue<T> asLifoQueue(Deque<T> deque) {
4103         return new AsLIFOQueue<>(deque);
4104     }
4105 
4106     /**
4107      * @serial include
4108      */
4109     static class AsLIFOQueue<E> extends AbstractQueue<E>
4110         implements Queue<E>, Serializable {
4111         private static final long serialVersionUID = 1802017725587941708L;
4112         private final Deque<E> q;
4113         AsLIFOQueue(Deque<E> q)           { this.q = q; }
4114         public boolean add(E e)           { q.addFirst(e); return true; }
4115         public boolean offer(E e)         { return q.offerFirst(e); }
4116         public E poll()                   { return q.pollFirst(); }
4117         public E remove()                 { return q.removeFirst(); }
4118         public E peek()                   { return q.peekFirst(); }
4119         public E element()                { return q.getFirst(); }
4120         public void clear()               {        q.clear(); }
4121         public int size()                 { return q.size(); }
4122         public boolean isEmpty()          { return q.isEmpty(); }
4123         public boolean contains(Object o) { return q.contains(o); }
4124         public boolean remove(Object o)   { return q.remove(o); }
4125         public Iterator<E> iterator()     { return q.iterator(); }
4126         public Object[] toArray()         { return q.toArray(); }
4127         public <T> T[] toArray(T[] a)     { return q.toArray(a); }
4128         public String toString()          { return q.toString(); }
4129         public boolean containsAll(Collection<?> c) {return q.containsAll(c);}
4130         public boolean removeAll(Collection<?> c)   {return q.removeAll(c);}
4131         public boolean retainAll(Collection<?> c)   {return q.retainAll(c);}
4132         // We use inherited addAll; forwarding addAll would be wrong
4133     }
4134 }