1 /*
   2  * Copyright (c) 1998, 2010, 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.lang.ref.WeakReference;
  28 import java.lang.ref.ReferenceQueue;
  29 
  30 
  31 /**
  32  * Hash table based implementation of the <tt>Map</tt> interface, with
  33  * <em>weak keys</em>.
  34  * An entry in a <tt>WeakHashMap</tt> will automatically be removed when
  35  * its key is no longer in ordinary use.  More precisely, the presence of a
  36  * mapping for a given key will not prevent the key from being discarded by the
  37  * garbage collector, that is, made finalizable, finalized, and then reclaimed.
  38  * When a key has been discarded its entry is effectively removed from the map,
  39  * so this class behaves somewhat differently from other <tt>Map</tt>
  40  * implementations.
  41  *
  42  * <p> Both null values and the null key are supported. This class has
  43  * performance characteristics similar to those of the <tt>HashMap</tt>
  44  * class, and has the same efficiency parameters of <em>initial capacity</em>
  45  * and <em>load factor</em>.
  46  *
  47  * <p> Like most collection classes, this class is not synchronized.
  48  * A synchronized <tt>WeakHashMap</tt> may be constructed using the
  49  * {@link Collections#synchronizedMap Collections.synchronizedMap}
  50  * method.
  51  *
  52  * <p> This class is intended primarily for use with key objects whose
  53  * <tt>equals</tt> methods test for object identity using the
  54  * <tt>==</tt> operator.  Once such a key is discarded it can never be
  55  * recreated, so it is impossible to do a lookup of that key in a
  56  * <tt>WeakHashMap</tt> at some later time and be surprised that its entry
  57  * has been removed.  This class will work perfectly well with key objects
  58  * whose <tt>equals</tt> methods are not based upon object identity, such
  59  * as <tt>String</tt> instances.  With such recreatable key objects,
  60  * however, the automatic removal of <tt>WeakHashMap</tt> entries whose
  61  * keys have been discarded may prove to be confusing.
  62  *
  63  * <p> The behavior of the <tt>WeakHashMap</tt> class depends in part upon
  64  * the actions of the garbage collector, so several familiar (though not
  65  * required) <tt>Map</tt> invariants do not hold for this class.  Because
  66  * the garbage collector may discard keys at any time, a
  67  * <tt>WeakHashMap</tt> may behave as though an unknown thread is silently
  68  * removing entries.  In particular, even if you synchronize on a
  69  * <tt>WeakHashMap</tt> instance and invoke none of its mutator methods, it
  70  * is possible for the <tt>size</tt> method to return smaller values over
  71  * time, for the <tt>isEmpty</tt> method to return <tt>false</tt> and
  72  * then <tt>true</tt>, for the <tt>containsKey</tt> method to return
  73  * <tt>true</tt> and later <tt>false</tt> for a given key, for the
  74  * <tt>get</tt> method to return a value for a given key but later return
  75  * <tt>null</tt>, for the <tt>put</tt> method to return
  76  * <tt>null</tt> and the <tt>remove</tt> method to return
  77  * <tt>false</tt> for a key that previously appeared to be in the map, and
  78  * for successive examinations of the key set, the value collection, and
  79  * the entry set to yield successively smaller numbers of elements.
  80  *
  81  * <p> Each key object in a <tt>WeakHashMap</tt> is stored indirectly as
  82  * the referent of a weak reference.  Therefore a key will automatically be
  83  * removed only after the weak references to it, both inside and outside of the
  84  * map, have been cleared by the garbage collector.
  85  *
  86  * <p> <strong>Implementation note:</strong> The value objects in a
  87  * <tt>WeakHashMap</tt> are held by ordinary strong references.  Thus care
  88  * should be taken to ensure that value objects do not strongly refer to their
  89  * own keys, either directly or indirectly, since that will prevent the keys
  90  * from being discarded.  Note that a value object may refer indirectly to its
  91  * key via the <tt>WeakHashMap</tt> itself; that is, a value object may
  92  * strongly refer to some other key object whose associated value object, in
  93  * turn, strongly refers to the key of the first value object.  One way

  94  * to deal with this is to wrap values themselves within
  95  * <tt>WeakReferences</tt> before
  96  * inserting, as in: <tt>m.put(key, new WeakReference(value))</tt>,
  97  * and then unwrapping upon each <tt>get</tt>.
  98  *
  99  * <p>The iterators returned by the <tt>iterator</tt> method of the collections
 100  * returned by all of this class's "collection view methods" are
 101  * <i>fail-fast</i>: if the map is structurally modified at any time after the
 102  * iterator is created, in any way except through the iterator's own
 103  * <tt>remove</tt> method, the iterator will throw a {@link
 104  * ConcurrentModificationException}.  Thus, in the face of concurrent
 105  * modification, the iterator fails quickly and cleanly, rather than risking
 106  * arbitrary, non-deterministic behavior at an undetermined time in the future.
 107  *
 108  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 109  * as it is, generally speaking, impossible to make any hard guarantees in the
 110  * presence of unsynchronized concurrent modification.  Fail-fast iterators
 111  * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 112  * Therefore, it would be wrong to write a program that depended on this
 113  * exception for its correctness:  <i>the fail-fast behavior of iterators
 114  * should be used only to detect bugs.</i>
 115  *
 116  * <p>This class is a member of the
 117  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 118  * Java Collections Framework</a>.
 119  *
 120  * @param <K> the type of keys maintained by this map
 121  * @param <V> the type of mapped values
 122  *
 123  * @author      Doug Lea
 124  * @author      Josh Bloch
 125  * @author      Mark Reinhold
 126  * @since       1.2
 127  * @see         java.util.HashMap
 128  * @see         java.lang.ref.WeakReference
 129  */
 130 public class WeakHashMap<K,V>
 131     extends AbstractMap<K,V>
 132     implements Map<K,V> {
 133 
 134     /**
 135      * The default initial capacity -- MUST be a power of two.
 136      */
 137     private static final int DEFAULT_INITIAL_CAPACITY = 16;
 138 
 139     /**
 140      * The maximum capacity, used if a higher value is implicitly specified
 141      * by either of the constructors with arguments.
 142      * MUST be a power of two <= 1<<30.
 143      */
 144     private static final int MAXIMUM_CAPACITY = 1 << 30;
 145 
 146     /**
 147      * The load factor used when none specified in constructor.
 148      */
 149     private static final float DEFAULT_LOAD_FACTOR = 0.75f;
 150 
 151     /**
 152      * The table, resized as necessary. Length MUST Always be a power of two.
 153      */
 154     Entry<K,V>[] table;
 155 
 156     /**
 157      * The number of key-value mappings contained in this weak hash map.
 158      */
 159     private int size;
 160 
 161     /**
 162      * The next size value at which to resize (capacity * load factor).
 163      */
 164     private int threshold;
 165 
 166     /**
 167      * The load factor for the hash table.
 168      */
 169     private final float loadFactor;
 170 
 171     /**
 172      * Reference queue for cleared WeakEntries
 173      */
 174     private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
 175 
 176     /**
 177      * The number of times this WeakHashMap has been structurally modified.
 178      * Structural modifications are those that change the number of
 179      * mappings in the map or otherwise modify its internal structure
 180      * (e.g., rehash).  This field is used to make iterators on
 181      * Collection-views of the map fail-fast.
 182      *
 183      * @see ConcurrentModificationException
 184      */
 185     int modCount;
 186 
 187     @SuppressWarnings("unchecked")
 188     private Entry<K,V>[] newTable(int n) {
 189         return (Entry<K,V>[]) new Entry<?,?>[n];
 190     }
 191 
 192     /**
 193      * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
 194      * capacity and the given load factor.
 195      *
 196      * @param  initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
 197      * @param  loadFactor      The load factor of the <tt>WeakHashMap</tt>
 198      * @throws IllegalArgumentException if the initial capacity is negative,
 199      *         or if the load factor is nonpositive.
 200      */
 201     public WeakHashMap(int initialCapacity, float loadFactor) {
 202         if (initialCapacity < 0)
 203             throw new IllegalArgumentException("Illegal Initial Capacity: "+
 204                                                initialCapacity);
 205         if (initialCapacity > MAXIMUM_CAPACITY)
 206             initialCapacity = MAXIMUM_CAPACITY;
 207 
 208         if (loadFactor <= 0 || Float.isNaN(loadFactor))
 209             throw new IllegalArgumentException("Illegal Load factor: "+
 210                                                loadFactor);
 211         int capacity = 1;
 212         while (capacity < initialCapacity)
 213             capacity <<= 1;
 214         table = newTable(capacity);
 215         this.loadFactor = loadFactor;
 216         threshold = (int)(capacity * loadFactor);
 217     }
 218 
 219     /**
 220      * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
 221      * capacity and the default load factor (0.75).
 222      *
 223      * @param  initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
 224      * @throws IllegalArgumentException if the initial capacity is negative
 225      */
 226     public WeakHashMap(int initialCapacity) {
 227         this(initialCapacity, DEFAULT_LOAD_FACTOR);
 228     }
 229 
 230     /**
 231      * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial
 232      * capacity (16) and load factor (0.75).
 233      */
 234     public WeakHashMap() {
 235         this.loadFactor = DEFAULT_LOAD_FACTOR;
 236         threshold = DEFAULT_INITIAL_CAPACITY;
 237         table = newTable(DEFAULT_INITIAL_CAPACITY);
 238     }
 239 
 240     /**
 241      * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the
 242      * specified map.  The <tt>WeakHashMap</tt> is created with the default
 243      * load factor (0.75) and an initial capacity sufficient to hold the
 244      * mappings in the specified map.
 245      *
 246      * @param   m the map whose mappings are to be placed in this map
 247      * @throws  NullPointerException if the specified map is null
 248      * @since   1.3
 249      */
 250     public WeakHashMap(Map<? extends K, ? extends V> m) {
 251         this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, 16),
 252              DEFAULT_LOAD_FACTOR);
 253         putAll(m);
 254     }
 255 
 256     // internal utilities
 257 
 258     /**
 259      * Value representing null keys inside tables.
 260      */
 261     private static final Object NULL_KEY = new Object();
 262 
 263     /**
 264      * Use NULL_KEY for key if it is null.
 265      */
 266     private static Object maskNull(Object key) {
 267         return (key == null) ? NULL_KEY : key;
 268     }
 269 
 270     /**
 271      * Returns internal representation of null key back to caller as null.
 272      */
 273     static Object unmaskNull(Object key) {
 274         return (key == NULL_KEY) ? null : key;
 275     }
 276 
 277     /**
 278      * Checks for equality of non-null reference x and possibly-null y.  By
 279      * default uses Object.equals.
 280      */
 281     private static boolean eq(Object x, Object y) {
 282         return x == y || x.equals(y);
 283     }
 284 
 285     /**
 286      * Returns index for hash code h.
 287      */
 288     private static int indexFor(int h, int length) {
 289         return h & (length-1);
 290     }
 291 
 292     /**
 293      * Expunges stale entries from the table.
 294      */
 295     private void expungeStaleEntries() {
 296         for (Object x; (x = queue.poll()) != null; ) {
 297             synchronized (queue) {
 298                 @SuppressWarnings("unchecked")
 299                     Entry<K,V> e = (Entry<K,V>) x;
 300                 int i = indexFor(e.hash, table.length);
 301 
 302                 Entry<K,V> prev = table[i];
 303                 Entry<K,V> p = prev;
 304                 while (p != null) {
 305                     Entry<K,V> next = p.next;
 306                     if (p == e) {
 307                         if (prev == e)
 308                             table[i] = next;
 309                         else
 310                             prev.next = next;
 311                         // Must not null out e.next;
 312                         // stale entries may be in use by a HashIterator
 313                         e.value = null; // Help GC
 314                         size--;
 315                         break;
 316                     }
 317                     prev = p;
 318                     p = next;
 319                 }
 320             }
 321         }
 322     }
 323 
 324     /**
 325      * Returns the table after first expunging stale entries.
 326      */
 327     private Entry<K,V>[] getTable() {
 328         expungeStaleEntries();
 329         return table;
 330     }
 331 
 332     /**
 333      * Returns the number of key-value mappings in this map.
 334      * This result is a snapshot, and may not reflect unprocessed
 335      * entries that will be removed before next attempted access
 336      * because they are no longer referenced.
 337      */
 338     public int size() {
 339         if (size == 0)
 340             return 0;
 341         expungeStaleEntries();
 342         return size;
 343     }
 344 
 345     /**
 346      * Returns <tt>true</tt> if this map contains no key-value mappings.
 347      * This result is a snapshot, and may not reflect unprocessed
 348      * entries that will be removed before next attempted access
 349      * because they are no longer referenced.
 350      */
 351     public boolean isEmpty() {
 352         return size() == 0;
 353     }
 354 
 355     /**
 356      * Returns the value to which the specified key is mapped,
 357      * or {@code null} if this map contains no mapping for the key.
 358      *
 359      * <p>More formally, if this map contains a mapping from a key
 360      * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
 361      * key.equals(k))}, then this method returns {@code v}; otherwise
 362      * it returns {@code null}.  (There can be at most one such mapping.)
 363      *
 364      * <p>A return value of {@code null} does not <i>necessarily</i>
 365      * indicate that the map contains no mapping for the key; it's also
 366      * possible that the map explicitly maps the key to {@code null}.
 367      * The {@link #containsKey containsKey} operation may be used to
 368      * distinguish these two cases.
 369      *
 370      * @see #put(Object, Object)
 371      */
 372     public V get(Object key) {
 373         Object k = maskNull(key);
 374         int h = HashMap.hash(k.hashCode());
 375         Entry<K,V>[] tab = getTable();
 376         int index = indexFor(h, tab.length);
 377         Entry<K,V> e = tab[index];
 378         while (e != null) {
 379             if (e.hash == h && eq(k, e.get()))
 380                 return e.value;
 381             e = e.next;
 382         }
 383         return null;
 384     }
 385 
 386     /**
 387      * Returns <tt>true</tt> if this map contains a mapping for the
 388      * specified key.
 389      *
 390      * @param  key   The key whose presence in this map is to be tested
 391      * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;
 392      *         <tt>false</tt> otherwise
 393      */
 394     public boolean containsKey(Object key) {
 395         return getEntry(key) != null;
 396     }
 397 
 398     /**
 399      * Returns the entry associated with the specified key in this map.
 400      * Returns null if the map contains no mapping for this key.
 401      */
 402     Entry<K,V> getEntry(Object key) {
 403         Object k = maskNull(key);
 404         int h = HashMap.hash(k.hashCode());
 405         Entry<K,V>[] tab = getTable();
 406         int index = indexFor(h, tab.length);
 407         Entry<K,V> e = tab[index];
 408         while (e != null && !(e.hash == h && eq(k, e.get())))
 409             e = e.next;
 410         return e;
 411     }
 412 
 413     /**
 414      * Associates the specified value with the specified key in this map.
 415      * If the map previously contained a mapping for this key, the old
 416      * value is replaced.
 417      *
 418      * @param key key with which the specified value is to be associated.
 419      * @param value value to be associated with the specified key.
 420      * @return the previous value associated with <tt>key</tt>, or
 421      *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 422      *         (A <tt>null</tt> return can also indicate that the map
 423      *         previously associated <tt>null</tt> with <tt>key</tt>.)
 424      */
 425     public V put(K key, V value) {
 426         Object k = maskNull(key);
 427         int h = HashMap.hash(k.hashCode());
 428         Entry<K,V>[] tab = getTable();
 429         int i = indexFor(h, tab.length);
 430 
 431         for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
 432             if (h == e.hash && eq(k, e.get())) {
 433                 V oldValue = e.value;
 434                 if (value != oldValue)
 435                     e.value = value;
 436                 return oldValue;
 437             }
 438         }
 439 
 440         modCount++;
 441         Entry<K,V> e = tab[i];
 442         tab[i] = new Entry<>(k, value, queue, h, e);
 443         if (++size >= threshold)
 444             resize(tab.length * 2);
 445         return null;
 446     }
 447 
 448     /**
 449      * Rehashes the contents of this map into a new array with a
 450      * larger capacity.  This method is called automatically when the
 451      * number of keys in this map reaches its threshold.
 452      *
 453      * If current capacity is MAXIMUM_CAPACITY, this method does not
 454      * resize the map, but sets threshold to Integer.MAX_VALUE.
 455      * This has the effect of preventing future calls.
 456      *
 457      * @param newCapacity the new capacity, MUST be a power of two;
 458      *        must be greater than current capacity unless current
 459      *        capacity is MAXIMUM_CAPACITY (in which case value
 460      *        is irrelevant).
 461      */
 462     void resize(int newCapacity) {
 463         Entry<K,V>[] oldTable = getTable();
 464         int oldCapacity = oldTable.length;
 465         if (oldCapacity == MAXIMUM_CAPACITY) {
 466             threshold = Integer.MAX_VALUE;
 467             return;
 468         }
 469 
 470         Entry<K,V>[] newTable = newTable(newCapacity);
 471         transfer(oldTable, newTable);
 472         table = newTable;
 473 
 474         /*
 475          * If ignoring null elements and processing ref queue caused massive
 476          * shrinkage, then restore old table.  This should be rare, but avoids
 477          * unbounded expansion of garbage-filled tables.
 478          */
 479         if (size >= threshold / 2) {
 480             threshold = (int)(newCapacity * loadFactor);
 481         } else {
 482             expungeStaleEntries();
 483             transfer(newTable, oldTable);
 484             table = oldTable;
 485         }
 486     }
 487 
 488     /** Transfers all entries from src to dest tables */
 489     private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest) {
 490         for (int j = 0; j < src.length; ++j) {
 491             Entry<K,V> e = src[j];
 492             src[j] = null;
 493             while (e != null) {
 494                 Entry<K,V> next = e.next;
 495                 Object key = e.get();
 496                 if (key == null) {
 497                     e.next = null;  // Help GC
 498                     e.value = null; //  "   "
 499                     size--;
 500                 } else {
 501                     int i = indexFor(e.hash, dest.length);
 502                     e.next = dest[i];
 503                     dest[i] = e;
 504                 }
 505                 e = next;
 506             }
 507         }
 508     }
 509 
 510     /**
 511      * Copies all of the mappings from the specified map to this map.
 512      * These mappings will replace any mappings that this map had for any
 513      * of the keys currently in the specified map.
 514      *
 515      * @param m mappings to be stored in this map.
 516      * @throws  NullPointerException if the specified map is null.
 517      */
 518     public void putAll(Map<? extends K, ? extends V> m) {
 519         int numKeysToBeAdded = m.size();
 520         if (numKeysToBeAdded == 0)
 521             return;
 522 
 523         /*
 524          * Expand the map if the map if the number of mappings to be added
 525          * is greater than or equal to threshold.  This is conservative; the
 526          * obvious condition is (m.size() + size) >= threshold, but this
 527          * condition could result in a map with twice the appropriate capacity,
 528          * if the keys to be added overlap with the keys already in this map.
 529          * By using the conservative calculation, we subject ourself
 530          * to at most one extra resize.
 531          */
 532         if (numKeysToBeAdded > threshold) {
 533             int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
 534             if (targetCapacity > MAXIMUM_CAPACITY)
 535                 targetCapacity = MAXIMUM_CAPACITY;
 536             int newCapacity = table.length;
 537             while (newCapacity < targetCapacity)
 538                 newCapacity <<= 1;
 539             if (newCapacity > table.length)
 540                 resize(newCapacity);
 541         }
 542 
 543         for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
 544             put(e.getKey(), e.getValue());
 545     }
 546 
 547     /**
 548      * Removes the mapping for a key from this weak hash map if it is present.
 549      * More formally, if this map contains a mapping from key <tt>k</tt> to
 550      * value <tt>v</tt> such that <code>(key==null ?  k==null :
 551      * key.equals(k))</code>, that mapping is removed.  (The map can contain
 552      * at most one such mapping.)
 553      *
 554      * <p>Returns the value to which this map previously associated the key,
 555      * or <tt>null</tt> if the map contained no mapping for the key.  A
 556      * return value of <tt>null</tt> does not <i>necessarily</i> indicate
 557      * that the map contained no mapping for the key; it's also possible
 558      * that the map explicitly mapped the key to <tt>null</tt>.
 559      *
 560      * <p>The map will not contain a mapping for the specified key once the
 561      * call returns.
 562      *
 563      * @param key key whose mapping is to be removed from the map
 564      * @return the previous value associated with <tt>key</tt>, or
 565      *         <tt>null</tt> if there was no mapping for <tt>key</tt>
 566      */
 567     public V remove(Object key) {
 568         Object k = maskNull(key);
 569         int h = HashMap.hash(k.hashCode());
 570         Entry<K,V>[] tab = getTable();
 571         int i = indexFor(h, tab.length);
 572         Entry<K,V> prev = tab[i];
 573         Entry<K,V> e = prev;
 574 
 575         while (e != null) {
 576             Entry<K,V> next = e.next;
 577             if (h == e.hash && eq(k, e.get())) {
 578                 modCount++;
 579                 size--;
 580                 if (prev == e)
 581                     tab[i] = next;
 582                 else
 583                     prev.next = next;
 584                 return e.value;
 585             }
 586             prev = e;
 587             e = next;
 588         }
 589 
 590         return null;
 591     }
 592 
 593     /** Special version of remove needed by Entry set */
 594     boolean removeMapping(Object o) {
 595         if (!(o instanceof Map.Entry))
 596             return false;
 597         Entry<K,V>[] tab = getTable();
 598         Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
 599         Object k = maskNull(entry.getKey());
 600         int h = HashMap.hash(k.hashCode());
 601         int i = indexFor(h, tab.length);
 602         Entry<K,V> prev = tab[i];
 603         Entry<K,V> e = prev;
 604 
 605         while (e != null) {
 606             Entry<K,V> next = e.next;
 607             if (h == e.hash && e.equals(entry)) {
 608                 modCount++;
 609                 size--;
 610                 if (prev == e)
 611                     tab[i] = next;
 612                 else
 613                     prev.next = next;
 614                 return true;
 615             }
 616             prev = e;
 617             e = next;
 618         }
 619 
 620         return false;
 621     }
 622 
 623     /**
 624      * Removes all of the mappings from this map.
 625      * The map will be empty after this call returns.
 626      */
 627     public void clear() {
 628         // clear out ref queue. We don't need to expunge entries
 629         // since table is getting cleared.
 630         while (queue.poll() != null)
 631             ;
 632 
 633         modCount++;
 634         Arrays.fill(table, null);
 635         size = 0;
 636 
 637         // Allocation of array may have caused GC, which may have caused
 638         // additional entries to go stale.  Removing these entries from the
 639         // reference queue will make them eligible for reclamation.
 640         while (queue.poll() != null)
 641             ;
 642     }
 643 
 644     /**
 645      * Returns <tt>true</tt> if this map maps one or more keys to the
 646      * specified value.
 647      *
 648      * @param value value whose presence in this map is to be tested
 649      * @return <tt>true</tt> if this map maps one or more keys to the
 650      *         specified value
 651      */
 652     public boolean containsValue(Object value) {
 653         if (value==null)
 654             return containsNullValue();
 655 
 656         Entry<K,V>[] tab = getTable();
 657         for (int i = tab.length; i-- > 0;)
 658             for (Entry<K,V> e = tab[i]; e != null; e = e.next)
 659                 if (value.equals(e.value))
 660                     return true;
 661         return false;
 662     }
 663 
 664     /**
 665      * Special-case code for containsValue with null argument
 666      */
 667     private boolean containsNullValue() {
 668         Entry<K,V>[] tab = getTable();
 669         for (int i = tab.length; i-- > 0;)
 670             for (Entry<K,V> e = tab[i]; e != null; e = e.next)
 671                 if (e.value==null)
 672                     return true;
 673         return false;
 674     }
 675 
 676     /**
 677      * The entries in this hash table extend WeakReference, using its main ref
 678      * field as the key.
 679      */
 680     private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
 681         V value;
 682         final int hash;
 683         Entry<K,V> next;
 684 
 685         /**
 686          * Creates new entry.
 687          */
 688         Entry(Object key, V value,
 689               ReferenceQueue<Object> queue,
 690               int hash, Entry<K,V> next) {
 691             super(key, queue);
 692             this.value = value;
 693             this.hash  = hash;
 694             this.next  = next;
 695         }
 696 
 697         @SuppressWarnings("unchecked")
 698         public K getKey() {
 699             return (K) WeakHashMap.unmaskNull(get());
 700         }
 701 
 702         public V getValue() {
 703             return value;
 704         }
 705 
 706         public V setValue(V newValue) {
 707             V oldValue = value;
 708             value = newValue;
 709             return oldValue;
 710         }
 711 
 712         public boolean equals(Object o) {
 713             if (!(o instanceof Map.Entry))
 714                 return false;
 715             Map.Entry<?,?> e = (Map.Entry<?,?>)o;
 716             K k1 = getKey();
 717             Object k2 = e.getKey();
 718             if (k1 == k2 || (k1 != null && k1.equals(k2))) {
 719                 V v1 = getValue();
 720                 Object v2 = e.getValue();
 721                 if (v1 == v2 || (v1 != null && v1.equals(v2)))
 722                     return true;
 723             }
 724             return false;
 725         }
 726 
 727         public int hashCode() {
 728             K k = getKey();
 729             V v = getValue();
 730             return ((k==null ? 0 : k.hashCode()) ^
 731                     (v==null ? 0 : v.hashCode()));
 732         }
 733 
 734         public String toString() {
 735             return getKey() + "=" + getValue();
 736         }
 737     }
 738 
 739     private abstract class HashIterator<T> implements Iterator<T> {
 740         private int index;
 741         private Entry<K,V> entry = null;
 742         private Entry<K,V> lastReturned = null;
 743         private int expectedModCount = modCount;
 744 
 745         /**
 746          * Strong reference needed to avoid disappearance of key
 747          * between hasNext and next
 748          */
 749         private Object nextKey = null;
 750 
 751         /**
 752          * Strong reference needed to avoid disappearance of key
 753          * between nextEntry() and any use of the entry
 754          */
 755         private Object currentKey = null;
 756 
 757         HashIterator() {
 758             index = isEmpty() ? 0 : table.length;
 759         }
 760 
 761         public boolean hasNext() {
 762             Entry<K,V>[] t = table;
 763 
 764             while (nextKey == null) {
 765                 Entry<K,V> e = entry;
 766                 int i = index;
 767                 while (e == null && i > 0)
 768                     e = t[--i];
 769                 entry = e;
 770                 index = i;
 771                 if (e == null) {
 772                     currentKey = null;
 773                     return false;
 774                 }
 775                 nextKey = e.get(); // hold on to key in strong ref
 776                 if (nextKey == null)
 777                     entry = entry.next;
 778             }
 779             return true;
 780         }
 781 
 782         /** The common parts of next() across different types of iterators */
 783         protected Entry<K,V> nextEntry() {
 784             if (modCount != expectedModCount)
 785                 throw new ConcurrentModificationException();
 786             if (nextKey == null && !hasNext())
 787                 throw new NoSuchElementException();
 788 
 789             lastReturned = entry;
 790             entry = entry.next;
 791             currentKey = nextKey;
 792             nextKey = null;
 793             return lastReturned;
 794         }
 795 
 796         public void remove() {
 797             if (lastReturned == null)
 798                 throw new IllegalStateException();
 799             if (modCount != expectedModCount)
 800                 throw new ConcurrentModificationException();
 801 
 802             WeakHashMap.this.remove(currentKey);
 803             expectedModCount = modCount;
 804             lastReturned = null;
 805             currentKey = null;
 806         }
 807 
 808     }
 809 
 810     private class ValueIterator extends HashIterator<V> {
 811         public V next() {
 812             return nextEntry().value;
 813         }
 814     }
 815 
 816     private class KeyIterator extends HashIterator<K> {
 817         public K next() {
 818             return nextEntry().getKey();
 819         }
 820     }
 821 
 822     private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
 823         public Map.Entry<K,V> next() {
 824             return nextEntry();
 825         }
 826     }
 827 
 828     // Views
 829 
 830     private transient Set<Map.Entry<K,V>> entrySet = null;
 831 
 832     /**
 833      * Returns a {@link Set} view of the keys contained in this map.
 834      * The set is backed by the map, so changes to the map are
 835      * reflected in the set, and vice-versa.  If the map is modified
 836      * while an iteration over the set is in progress (except through
 837      * the iterator's own <tt>remove</tt> operation), the results of
 838      * the iteration are undefined.  The set supports element removal,
 839      * which removes the corresponding mapping from the map, via the
 840      * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
 841      * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
 842      * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
 843      * operations.
 844      */
 845     public Set<K> keySet() {
 846         Set<K> ks = keySet;
 847         return (ks != null ? ks : (keySet = new KeySet()));
 848     }
 849 
 850     private class KeySet extends AbstractSet<K> {
 851         public Iterator<K> iterator() {
 852             return new KeyIterator();
 853         }
 854 
 855         public int size() {
 856             return WeakHashMap.this.size();
 857         }
 858 
 859         public boolean contains(Object o) {
 860             return containsKey(o);
 861         }
 862 
 863         public boolean remove(Object o) {
 864             if (containsKey(o)) {
 865                 WeakHashMap.this.remove(o);
 866                 return true;
 867             }
 868             else
 869                 return false;
 870         }
 871 
 872         public void clear() {
 873             WeakHashMap.this.clear();
 874         }
 875     }
 876 
 877     /**
 878      * Returns a {@link Collection} view of the values contained in this map.
 879      * The collection is backed by the map, so changes to the map are
 880      * reflected in the collection, and vice-versa.  If the map is
 881      * modified while an iteration over the collection is in progress
 882      * (except through the iterator's own <tt>remove</tt> operation),
 883      * the results of the iteration are undefined.  The collection
 884      * supports element removal, which removes the corresponding
 885      * mapping from the map, via the <tt>Iterator.remove</tt>,
 886      * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
 887      * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
 888      * support the <tt>add</tt> or <tt>addAll</tt> operations.
 889      */
 890     public Collection<V> values() {
 891         Collection<V> vs = values;
 892         return (vs != null) ? vs : (values = new Values());
 893     }
 894 
 895     private class Values extends AbstractCollection<V> {
 896         public Iterator<V> iterator() {
 897             return new ValueIterator();
 898         }
 899 
 900         public int size() {
 901             return WeakHashMap.this.size();
 902         }
 903 
 904         public boolean contains(Object o) {
 905             return containsValue(o);
 906         }
 907 
 908         public void clear() {
 909             WeakHashMap.this.clear();
 910         }
 911     }
 912 
 913     /**
 914      * Returns a {@link Set} view of the mappings contained in this map.
 915      * The set is backed by the map, so changes to the map are
 916      * reflected in the set, and vice-versa.  If the map is modified
 917      * while an iteration over the set is in progress (except through
 918      * the iterator's own <tt>remove</tt> operation, or through the
 919      * <tt>setValue</tt> operation on a map entry returned by the
 920      * iterator) the results of the iteration are undefined.  The set
 921      * supports element removal, which removes the corresponding
 922      * mapping from the map, via the <tt>Iterator.remove</tt>,
 923      * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
 924      * <tt>clear</tt> operations.  It does not support the
 925      * <tt>add</tt> or <tt>addAll</tt> operations.
 926      */
 927     public Set<Map.Entry<K,V>> entrySet() {
 928         Set<Map.Entry<K,V>> es = entrySet;
 929         return es != null ? es : (entrySet = new EntrySet());
 930     }
 931 
 932     private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
 933         public Iterator<Map.Entry<K,V>> iterator() {
 934             return new EntryIterator();
 935         }
 936 
 937         public boolean contains(Object o) {
 938             if (!(o instanceof Map.Entry))
 939                 return false;
 940             Map.Entry<?,?> e = (Map.Entry<?,?>)o;
 941             Entry<K,V> candidate = getEntry(e.getKey());
 942             return candidate != null && candidate.equals(e);
 943         }
 944 
 945         public boolean remove(Object o) {
 946             return removeMapping(o);
 947         }
 948 
 949         public int size() {
 950             return WeakHashMap.this.size();
 951         }
 952 
 953         public void clear() {
 954             WeakHashMap.this.clear();
 955         }
 956 
 957         private List<Map.Entry<K,V>> deepCopy() {
 958             List<Map.Entry<K,V>> list = new ArrayList<>(size());
 959             for (Map.Entry<K,V> e : this)
 960                 list.add(new AbstractMap.SimpleEntry<>(e));
 961             return list;
 962         }
 963 
 964         public Object[] toArray() {
 965             return deepCopy().toArray();
 966         }
 967 
 968         public <T> T[] toArray(T[] a) {
 969             return deepCopy().toArray(a);
 970         }
 971     }
 972 }
--- EOF ---