001/* WeakHashMap -- a hashtable that keeps only weak references
002   to its keys, allowing the virtual machine to reclaim them
003   Copyright (C) 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
004
005This file is part of GNU Classpath.
006
007GNU Classpath is free software; you can redistribute it and/or modify
008it under the terms of the GNU General Public License as published by
009the Free Software Foundation; either version 2, or (at your option)
010any later version.
011
012GNU Classpath is distributed in the hope that it will be useful, but
013WITHOUT ANY WARRANTY; without even the implied warranty of
014MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
015General Public License for more details.
016
017You should have received a copy of the GNU General Public License
018along with GNU Classpath; see the file COPYING.  If not, write to the
019Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02002110-1301 USA.
021
022Linking this library statically or dynamically with other modules is
023making a combined work based on this library.  Thus, the terms and
024conditions of the GNU General Public License cover the whole
025combination.
026
027As a special exception, the copyright holders of this library give you
028permission to link this library with independent modules to produce an
029executable, regardless of the license terms of these independent
030modules, and to copy and distribute the resulting executable under
031terms of your choice, provided that you also meet, for each linked
032independent module, the terms and conditions of the license of that
033module.  An independent module is a module which is not derived from
034or based on this library.  If you modify this library, you may extend
035this exception to your version of the library, but you are not
036obligated to do so.  If you do not wish to do so, delete this
037exception statement from your version. */
038
039
040package java.util;
041
042import java.lang.ref.ReferenceQueue;
043import java.lang.ref.WeakReference;
044
045/**
046 * A weak hash map has only weak references to the key. This means that it
047 * allows the key to be garbage collected if it is not used otherwise. If
048 * this happens, the entry will eventually disappear from the map,
049 * asynchronously.
050 *
051 * <p>A weak hash map makes most sense when the keys doesn't override the
052 * <code>equals</code> method: If there is no other reference to the
053 * key nobody can ever look up the key in this table and so the entry
054 * can be removed.  This table also works when the <code>equals</code>
055 * method is overloaded, such as String keys, but you should be prepared
056 * to deal with some entries disappearing spontaneously.
057 *
058 * <p>Other strange behaviors to be aware of: The size of this map may
059 * spontaneously shrink (even if you use a synchronized map and synchronize
060 * it); it behaves as if another thread removes entries from this table
061 * without synchronization.  The entry set returned by <code>entrySet</code>
062 * has similar phenomenons: The size may spontaneously shrink, or an
063 * entry, that was in the set before, suddenly disappears.
064 *
065 * <p>A weak hash map is not meant for caches; use a normal map, with
066 * soft references as values instead, or try {@link LinkedHashMap}.
067 *
068 * <p>The weak hash map supports null values and null keys.  The null key
069 * is never deleted from the map (except explictly of course). The
070 * performance of the methods are similar to that of a hash map.
071 *
072 * <p>The value objects are strongly referenced by this table.  So if a
073 * value object maintains a strong reference to the key (either direct
074 * or indirect) the key will never be removed from this map.  According
075 * to Sun, this problem may be fixed in a future release.  It is not
076 * possible to do it with the jdk 1.2 reference model, though.
077 *
078 * @author Jochen Hoenicke
079 * @author Eric Blake (ebb9@email.byu.edu)
080 * @author Tom Tromey (tromey@redhat.com)
081 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
082 *
083 * @see HashMap
084 * @see WeakReference
085 * @see LinkedHashMap
086 * @since 1.2
087 * @status updated to 1.4 (partial 1.5)
088 */
089public class WeakHashMap<K,V> extends AbstractMap<K,V> 
090{
091  // WARNING: WeakHashMap is a CORE class in the bootstrap cycle. See the
092  // comments in vm/reference/java/lang/Runtime for implications of this fact.
093
094  /**
095   * The default capacity for an instance of HashMap.
096   * Sun's documentation mildly suggests that this (11) is the correct
097   * value.
098   */
099  private static final int DEFAULT_CAPACITY = 11;
100
101  /**
102   * The default load factor of a HashMap.
103   */
104  private static final float DEFAULT_LOAD_FACTOR = 0.75F;
105
106  /**
107   * This is used instead of the key value <i>null</i>.  It is needed
108   * to distinguish between an null key and a removed key.
109   */
110  // Package visible for use by nested classes.
111  static final Object NULL_KEY = new Object()
112  {
113    /**
114     * Sets the hashCode to 0, since that's what null would map to.
115     * @return the hash code 0
116     */
117    public int hashCode()
118    {
119      return 0;
120    }
121
122    /**
123     * Compares this key to the given object. Normally, an object should
124     * NEVER compare equal to null, but since we don't publicize NULL_VALUE,
125     * it saves bytecode to do so here.
126     * @return true iff o is this or null
127     */
128    public boolean equals(Object o)
129    {
130      return null == o || this == o;
131    }
132  };
133
134  /**
135   * The reference queue where our buckets (which are WeakReferences) are
136   * registered to.
137   */
138  private final ReferenceQueue queue;
139
140  /**
141   * The number of entries in this hash map.
142   */
143  // Package visible for use by nested classes.
144  int size;
145
146  /**
147   * The load factor of this WeakHashMap.  This is the maximum ratio of
148   * size versus number of buckets.  If size grows the number of buckets
149   * must grow, too.
150   */
151  private float loadFactor;
152
153  /**
154   * The rounded product of the capacity (i.e. number of buckets) and
155   * the load factor. When the number of elements exceeds the
156   * threshold, the HashMap calls <code>rehash()</code>.
157   */
158  private int threshold;
159
160  /**
161   * The number of structural modifications.  This is used by
162   * iterators, to see if they should fail.  This doesn't count
163   * the silent key removals, when a weak reference is cleared
164   * by the garbage collection.  Instead the iterators must make
165   * sure to have strong references to the entries they rely on.
166   */
167  // Package visible for use by nested classes.
168  int modCount;
169
170  /**
171   * The entry set.  There is only one instance per hashmap, namely
172   * theEntrySet.  Note that the entry set may silently shrink, just
173   * like the WeakHashMap.
174   */
175  private final class WeakEntrySet extends AbstractSet
176  {
177    /**
178     * Non-private constructor to reduce bytecode emitted.
179     */
180    WeakEntrySet()
181    {
182    }
183
184    /**
185     * Returns the size of this set.
186     *
187     * @return the set size
188     */
189    public int size()
190    {
191      return size;
192    }
193
194    /**
195     * Returns an iterator for all entries.
196     *
197     * @return an Entry iterator
198     */
199    public Iterator iterator()
200    {
201      return new Iterator()
202      {
203        /**
204         * The entry that was returned by the last
205         * <code>next()</code> call.  This is also the entry whose
206         * bucket should be removed by the <code>remove</code> call. <br>
207         *
208         * It is null, if the <code>next</code> method wasn't
209         * called yet, or if the entry was already removed.  <br>
210         *
211         * Remembering this entry here will also prevent it from
212         * being removed under us, since the entry strongly refers
213         * to the key.
214         */
215        WeakBucket.WeakEntry lastEntry;
216
217        /**
218         * The entry that will be returned by the next
219         * <code>next()</code> call.  It is <code>null</code> if there
220         * is no further entry. <br>
221         *
222         * Remembering this entry here will also prevent it from
223         * being removed under us, since the entry strongly refers
224         * to the key.
225         */
226        WeakBucket.WeakEntry nextEntry = findNext(null);
227
228        /**
229         * The known number of modification to the list, if it differs
230         * from the real number, we throw an exception.
231         */
232        int knownMod = modCount;
233
234        /**
235         * Check the known number of modification to the number of
236         * modifications of the table.  If it differs from the real
237         * number, we throw an exception.
238         * @throws ConcurrentModificationException if the number
239         *         of modifications doesn't match.
240         */
241        private void checkMod()
242        {
243          // This method will get inlined.
244          cleanQueue();
245          if (knownMod != modCount)
246            throw new ConcurrentModificationException(knownMod + " != "
247                                                      + modCount);
248        }
249
250        /**
251         * Get a strong reference to the next entry after
252         * lastBucket.
253         * @param lastEntry the previous bucket, or null if we should
254         * get the first entry.
255         * @return the next entry.
256         */
257        private WeakBucket.WeakEntry findNext(WeakBucket.WeakEntry lastEntry)
258        {
259          int slot;
260          WeakBucket nextBucket;
261          if (lastEntry != null)
262            {
263              nextBucket = lastEntry.getBucket().next;
264              slot = lastEntry.getBucket().slot;
265            }
266          else
267            {
268              nextBucket = buckets[0];
269              slot = 0;
270            }
271
272          while (true)
273            {
274              while (nextBucket != null)
275                {
276                  WeakBucket.WeakEntry entry = nextBucket.getEntry();
277                  if (entry != null)
278                    // This is the next entry.
279                    return entry;
280
281                  // Entry was cleared, try next.
282                  nextBucket = nextBucket.next;
283                }
284
285              slot++;
286              if (slot == buckets.length)
287                // No more buckets, we are through.
288                return null;
289
290              nextBucket = buckets[slot];
291            }
292        }
293
294        /**
295         * Checks if there are more entries.
296         * @return true, iff there are more elements.
297         */
298        public boolean hasNext()
299        {
300          return nextEntry != null;
301        }
302
303        /**
304         * Returns the next entry.
305         * @return the next entry.
306         * @throws ConcurrentModificationException if the hash map was
307         *         modified.
308         * @throws NoSuchElementException if there is no entry.
309         */
310        public Object next()
311        {
312          checkMod();
313          if (nextEntry == null)
314            throw new NoSuchElementException();
315          lastEntry = nextEntry;
316          nextEntry = findNext(lastEntry);
317          return lastEntry;
318        }
319
320        /**
321         * Removes the last returned entry from this set.  This will
322         * also remove the bucket of the underlying weak hash map.
323         * @throws ConcurrentModificationException if the hash map was
324         *         modified.
325         * @throws IllegalStateException if <code>next()</code> was
326         *         never called or the element was already removed.
327         */
328        public void remove()
329        {
330          checkMod();
331          if (lastEntry == null)
332            throw new IllegalStateException();
333          modCount++;
334          internalRemove(lastEntry.getBucket());
335          lastEntry = null;
336          knownMod++;
337        }
338      };
339    }
340  }
341
342  /**
343   * A bucket is a weak reference to the key, that contains a strong
344   * reference to the value, a pointer to the next bucket and its slot
345   * number. <br>
346   *
347   * It would be cleaner to have a WeakReference as field, instead of
348   * extending it, but if a weak reference gets cleared, we only get
349   * the weak reference (by queue.poll) and wouldn't know where to
350   * look for this reference in the hashtable, to remove that entry.
351   *
352   * @author Jochen Hoenicke
353   */
354  private static class WeakBucket<K, V> extends WeakReference<K>
355  {
356    /**
357     * The value of this entry.  The key is stored in the weak
358     * reference that we extend.
359     */
360    V value;
361
362    /**
363     * The next bucket describing another entry that uses the same
364     * slot.
365     */
366    WeakBucket<K, V> next;
367
368    /**
369     * The slot of this entry. This should be
370     * <code>Math.abs(key.hashCode() % buckets.length)</code>.
371     *
372     * But since the key may be silently removed we have to remember
373     * the slot number.
374     *
375     * If this bucket was removed the slot is -1.  This marker will
376     * prevent the bucket from being removed twice.
377     */
378    int slot;
379
380    /**
381     * Creates a new bucket for the given key/value pair and the specified
382     * slot.
383     * @param key the key
384     * @param queue the queue the weak reference belongs to
385     * @param value the value
386     * @param slot the slot.  This must match the slot where this bucket
387     *        will be enqueued.
388     */
389    public WeakBucket(K key, ReferenceQueue queue, V value,
390                      int slot)
391    {
392      super(key, queue);
393      this.value = value;
394      this.slot = slot;
395    }
396
397    /**
398     * This class gives the <code>Entry</code> representation of the
399     * current bucket.  It also keeps a strong reference to the
400     * key; bad things may happen otherwise.
401     */
402    class WeakEntry implements Map.Entry<K, V>
403    {
404      /**
405       * The strong ref to the key.
406       */
407      K key;
408
409      /**
410       * Creates a new entry for the key.
411       * @param key the key
412       */
413      public WeakEntry(K key)
414      {
415        this.key = key;
416      }
417
418      /**
419       * Returns the underlying bucket.
420       * @return the owning bucket
421       */
422      public WeakBucket getBucket()
423      {
424        return WeakBucket.this;
425      }
426
427      /**
428       * Returns the key.
429       * @return the key
430       */
431      public K getKey()
432      {
433        return key == NULL_KEY ? null : key;
434      }
435
436      /**
437       * Returns the value.
438       * @return the value
439       */
440      public V getValue()
441      {
442        return value;
443      }
444
445      /**
446       * This changes the value.  This change takes place in
447       * the underlying hash map.
448       * @param newVal the new value
449       * @return the old value
450       */
451      public V setValue(V newVal)
452      {
453        V oldVal = value;
454        value = newVal;
455        return oldVal;
456      }
457
458      /**
459       * The hashCode as specified in the Entry interface.
460       * @return the hash code
461       */
462      public int hashCode()
463      {
464        return key.hashCode() ^ WeakHashMap.hashCode(value);
465      }
466
467      /**
468       * The equals method as specified in the Entry interface.
469       * @param o the object to compare to
470       * @return true iff o represents the same key/value pair
471       */
472      public boolean equals(Object o)
473      {
474        if (o instanceof Map.Entry)
475          {
476            Map.Entry e = (Map.Entry) o;
477            return WeakHashMap.equals(getKey(), e.getKey())
478              && WeakHashMap.equals(value, e.getValue());
479          }
480        return false;
481      }
482
483      public String toString()
484      {
485        return getKey() + "=" + value;
486      }
487    }
488
489    /**
490     * This returns the entry stored in this bucket, or null, if the
491     * bucket got cleared in the mean time.
492     * @return the Entry for this bucket, if it exists
493     */
494    WeakEntry getEntry()
495    {
496      final K key = this.get();
497      if (key == null)
498        return null;
499      return new WeakEntry(key);
500    }
501  }
502
503  /**
504   * The entry set returned by <code>entrySet()</code>.
505   */
506  private final WeakEntrySet theEntrySet;
507
508  /**
509   * The hash buckets.  These are linked lists. Package visible for use in
510   * nested classes.
511   */
512  WeakBucket[] buckets;
513
514  /**
515   * Creates a new weak hash map with default load factor and default
516   * capacity.
517   */
518  public WeakHashMap()
519  {
520    this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
521  }
522
523  /**
524   * Creates a new weak hash map with default load factor and the given
525   * capacity.
526   * @param initialCapacity the initial capacity
527   * @throws IllegalArgumentException if initialCapacity is negative
528   */
529  public WeakHashMap(int initialCapacity)
530  {
531    this(initialCapacity, DEFAULT_LOAD_FACTOR);
532  }
533
534  /**
535   * Creates a new weak hash map with the given initial capacity and
536   * load factor.
537   * @param initialCapacity the initial capacity.
538   * @param loadFactor the load factor (see class description of HashMap).
539   * @throws IllegalArgumentException if initialCapacity is negative, or
540   *         loadFactor is non-positive
541   */
542  public WeakHashMap(int initialCapacity, float loadFactor)
543  {
544    // Check loadFactor for NaN as well.
545    if (initialCapacity < 0 || ! (loadFactor > 0))
546      throw new IllegalArgumentException();
547    if (initialCapacity == 0)
548      initialCapacity = 1;
549    this.loadFactor = loadFactor;
550    threshold = (int) (initialCapacity * loadFactor);
551    theEntrySet = new WeakEntrySet();
552    queue = new ReferenceQueue();
553    buckets = new WeakBucket[initialCapacity];
554  }
555
556  /**
557   * Construct a new WeakHashMap with the same mappings as the given map.
558   * The WeakHashMap has a default load factor of 0.75.
559   *
560   * @param m the map to copy
561   * @throws NullPointerException if m is null
562   * @since 1.3
563   */
564  public WeakHashMap(Map<? extends K, ? extends V> m)
565  {
566    this(m.size(), DEFAULT_LOAD_FACTOR);
567    putAll(m);
568  }
569
570  /**
571   * Simply hashes a non-null Object to its array index.
572   * @param key the key to hash
573   * @return its slot number
574   */
575  private int hash(Object key)
576  {
577    return Math.abs(key.hashCode() % buckets.length);
578  }
579
580  /**
581   * Cleans the reference queue.  This will poll all references (which
582   * are WeakBuckets) from the queue and remove them from this map.
583   * This will not change modCount, even if it modifies the map.  The
584   * iterators have to make sure that nothing bad happens.  <br>
585   *
586   * Currently the iterator maintains a strong reference to the key, so
587   * that is no problem.
588   */
589  // Package visible for use by nested classes.
590  void cleanQueue()
591  {
592    Object bucket = queue.poll();
593    while (bucket != null)
594      {
595        internalRemove((WeakBucket) bucket);
596        bucket = queue.poll();
597      }
598  }
599
600  /**
601   * Rehashes this hashtable.  This will be called by the
602   * <code>add()</code> method if the size grows beyond the threshold.
603   * It will grow the bucket size at least by factor two and allocates
604   * new buckets.
605   */
606  private void rehash()
607  {
608    WeakBucket[] oldBuckets = buckets;
609    int newsize = buckets.length * 2 + 1; // XXX should be prime.
610    threshold = (int) (newsize * loadFactor);
611    buckets = new WeakBucket[newsize];
612
613    // Now we have to insert the buckets again.
614    for (int i = 0; i < oldBuckets.length; i++)
615      {
616        WeakBucket bucket = oldBuckets[i];
617        WeakBucket nextBucket;
618        while (bucket != null)
619          {
620            nextBucket = bucket.next;
621
622            Object key = bucket.get();
623            if (key == null)
624              {
625                // This bucket should be removed; it is probably
626                // already on the reference queue.  We don't insert it
627                // at all, and mark it as cleared.
628                bucket.slot = -1;
629                size--;
630              }
631            else
632              {
633                // Add this bucket to its new slot.
634                int slot = hash(key);
635                bucket.slot = slot;
636                bucket.next = buckets[slot];
637                buckets[slot] = bucket;
638              }
639            bucket = nextBucket;
640          }
641      }
642  }
643
644  /**
645   * Finds the entry corresponding to key.  Since it returns an Entry
646   * it will also prevent the key from being removed under us.
647   * @param key the key, may be null
648   * @return The WeakBucket.WeakEntry or null, if the key wasn't found.
649   */
650  private WeakBucket.WeakEntry internalGet(Object key)
651  {
652    if (key == null)
653      key = NULL_KEY;
654    int slot = hash(key);
655    WeakBucket bucket = buckets[slot];
656    while (bucket != null)
657      {
658        WeakBucket.WeakEntry entry = bucket.getEntry();
659        if (entry != null && equals(key, entry.key))
660          return entry;
661
662        bucket = bucket.next;
663      }
664    return null;
665  }
666
667  /**
668   * Adds a new key/value pair to the hash map.
669   * @param key the key. This mustn't exists in the map. It may be null.
670   * @param value the value.
671   */
672  private void internalAdd(Object key, Object value)
673  {
674    if (key == null)
675      key = NULL_KEY;
676    int slot = hash(key);
677    WeakBucket bucket = new WeakBucket(key, queue, value, slot);
678    bucket.next = buckets[slot];
679    buckets[slot] = bucket;
680    size++;
681  }
682
683  /**
684   * Removes a bucket from this hash map, if it wasn't removed before
685   * (e.g. one time through rehashing and one time through reference queue).
686   * Package visible for use in nested classes.
687   *
688   * @param bucket the bucket to remove.
689   */
690  void internalRemove(WeakBucket bucket)
691  {
692    int slot = bucket.slot;
693    if (slot == -1)
694      // This bucket was already removed.
695      return;
696
697    // Mark the bucket as removed.  This is necessary, since the
698    // bucket may be enqueued later by the garbage collection, and
699    // internalRemove will be called a second time.
700    bucket.slot = -1;
701
702    WeakBucket prev = null;
703    WeakBucket next = buckets[slot];
704    while (next != bucket)
705      {
706         if (next == null) throw new InternalError("WeakHashMap in incosistent state");
707         prev = next; 
708         next = prev.next;
709      }
710    if (prev == null)
711      buckets[slot] = bucket.next;
712    else 
713      prev.next = bucket.next;
714
715    size--;
716  }
717
718  /**
719   * Returns the size of this hash map.  Note that the size() may shrink
720   * spontaneously, if the some of the keys were only weakly reachable.
721   * @return the number of entries in this hash map.
722   */
723  public int size()
724  {
725    cleanQueue();
726    return size;
727  }
728
729  /**
730   * Tells if the map is empty.  Note that the result may change
731   * spontanously, if all of the keys were only weakly reachable.
732   * @return true, iff the map is empty.
733   */
734  public boolean isEmpty()
735  {
736    cleanQueue();
737    return size == 0;
738  }
739
740  /**
741   * Tells if the map contains the given key.  Note that the result
742   * may change spontanously, if the key was only weakly
743   * reachable.
744   * @param key the key to look for
745   * @return true, iff the map contains an entry for the given key.
746   */
747  public boolean containsKey(Object key)
748  {
749    cleanQueue();
750    return internalGet(key) != null;
751  }
752
753  /**
754   * Gets the value the key is mapped to.
755   * @return the value the key was mapped to.  It returns null if
756   *         the key wasn't in this map, or if the mapped value was
757   *         explicitly set to null.
758   */
759  public V get(Object key)
760  {
761    cleanQueue();
762    WeakBucket<K, V>.WeakEntry entry = internalGet(key);
763    return entry == null ? null : entry.getValue();
764  }
765
766  /**
767   * Adds a new key/value mapping to this map.
768   * @param key the key, may be null
769   * @param value the value, may be null
770   * @return the value the key was mapped to previously.  It returns
771   *         null if the key wasn't in this map, or if the mapped value
772   *         was explicitly set to null.
773   */
774  public V put(K key, V value)
775  {
776    cleanQueue();
777    WeakBucket<K, V>.WeakEntry entry = internalGet(key);
778    if (entry != null)
779      return entry.setValue(value);
780
781    modCount++;
782    if (size >= threshold)
783      rehash();
784
785    internalAdd(key, value);
786    return null;
787  }
788
789  /**
790   * Removes the key and the corresponding value from this map.
791   * @param key the key. This may be null.
792   * @return the value the key was mapped to previously.  It returns
793   *         null if the key wasn't in this map, or if the mapped value was
794   *         explicitly set to null.
795   */
796  public V remove(Object key)
797  {
798    cleanQueue();
799    WeakBucket<K, V>.WeakEntry entry = internalGet(key);
800    if (entry == null)
801      return null;
802
803    modCount++;
804    internalRemove(entry.getBucket());
805    return entry.getValue();
806  }
807
808  /**
809   * Returns a set representation of the entries in this map.  This
810   * set will not have strong references to the keys, so they can be
811   * silently removed.  The returned set has therefore the same
812   * strange behaviour (shrinking size(), disappearing entries) as
813   * this weak hash map.
814   * @return a set representation of the entries.
815   */
816  public Set<Map.Entry<K,V>> entrySet()
817  {
818    cleanQueue();
819    return theEntrySet;
820  }
821
822  /**
823   * Clears all entries from this map.
824   */
825  public void clear()
826  {
827    super.clear();
828  }
829
830  /**
831   * Returns true if the map contains at least one key which points to
832   * the specified object as a value.  Note that the result
833   * may change spontanously, if its key was only weakly reachable.
834   * @param value the value to search for
835   * @return true if it is found in the set.
836   */
837  public boolean containsValue(Object value)
838  {
839    cleanQueue();
840    return super.containsValue(value);
841  }
842
843  /**
844   * Returns a set representation of the keys in this map.  This
845   * set will not have strong references to the keys, so they can be
846   * silently removed.  The returned set has therefore the same
847   * strange behaviour (shrinking size(), disappearing entries) as
848   * this weak hash map.
849   * @return a set representation of the keys.
850   */
851  public Set<K> keySet()
852  {
853    cleanQueue();
854    return super.keySet();
855  }
856
857  /**
858   * Puts all of the mappings from the given map into this one. If the
859   * key already exists in this map, its value is replaced.
860   * @param m the map to copy in
861   */
862  public void putAll(Map<? extends K, ? extends V> m)
863  {
864    super.putAll(m);
865  }
866
867  /**
868   * Returns a collection representation of the values in this map.  This
869   * collection will not have strong references to the keys, so mappings
870   * can be silently removed.  The returned collection has therefore the same
871   * strange behaviour (shrinking size(), disappearing entries) as
872   * this weak hash map.
873   * @return a collection representation of the values.
874   */
875  public Collection<V> values()
876  {
877    cleanQueue();
878    return super.values();
879  }
880} // class WeakHashMap