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1   /*
2    * Copyright (c) 1998, 2013, 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
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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).
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17   * You should have received a copy of the GNU General Public License version
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20   *
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25  
26  package java.util;
27  
28  import java.lang.ref.WeakReference;
29  import java.lang.ref.ReferenceQueue;
30  import java.util.concurrent.ThreadLocalRandom;
31  import java.util.function.BiConsumer;
32  import java.util.function.BiFunction;
33  import java.util.function.Consumer;
34  
35  
36  /**
37   * Hash table based implementation of the <tt>Map</tt> interface, with
38   * <em>weak keys</em>.
39   * An entry in a <tt>WeakHashMap</tt> will automatically be removed when
40   * its key is no longer in ordinary use.  More precisely, the presence of a
41   * mapping for a given key will not prevent the key from being discarded by the
42   * garbage collector, that is, made finalizable, finalized, and then reclaimed.
43   * When a key has been discarded its entry is effectively removed from the map,
44   * so this class behaves somewhat differently from other <tt>Map</tt>
45   * implementations.
46   *
47   * <p> Both null values and the null key are supported. This class has
48   * performance characteristics similar to those of the <tt>HashMap</tt>
49   * class, and has the same efficiency parameters of <em>initial capacity</em>
50   * and <em>load factor</em>.
51   *
52   * <p> Like most collection classes, this class is not synchronized.
53   * A synchronized <tt>WeakHashMap</tt> may be constructed using the
54   * {@link Collections#synchronizedMap Collections.synchronizedMap}
55   * method.
56   *
57   * <p> This class is intended primarily for use with key objects whose
58   * <tt>equals</tt> methods test for object identity using the
59   * <tt>==</tt> operator.  Once such a key is discarded it can never be
60   * recreated, so it is impossible to do a lookup of that key in a
61   * <tt>WeakHashMap</tt> at some later time and be surprised that its entry
62   * has been removed.  This class will work perfectly well with key objects
63   * whose <tt>equals</tt> methods are not based upon object identity, such
64   * as <tt>String</tt> instances.  With such recreatable key objects,
65   * however, the automatic removal of <tt>WeakHashMap</tt> entries whose
66   * keys have been discarded may prove to be confusing.
67   *
68   * <p> The behavior of the <tt>WeakHashMap</tt> class depends in part upon
69   * the actions of the garbage collector, so several familiar (though not
70   * required) <tt>Map</tt> invariants do not hold for this class.  Because
71   * the garbage collector may discard keys at any time, a
72   * <tt>WeakHashMap</tt> may behave as though an unknown thread is silently
73   * removing entries.  In particular, even if you synchronize on a
74   * <tt>WeakHashMap</tt> instance and invoke none of its mutator methods, it
75   * is possible for the <tt>size</tt> method to return smaller values over
76   * time, for the <tt>isEmpty</tt> method to return <tt>false</tt> and
77   * then <tt>true</tt>, for the <tt>containsKey</tt> method to return
78   * <tt>true</tt> and later <tt>false</tt> for a given key, for the
79   * <tt>get</tt> method to return a value for a given key but later return
80   * <tt>null</tt>, for the <tt>put</tt> method to return
81   * <tt>null</tt> and the <tt>remove</tt> method to return
82   * <tt>false</tt> for a key that previously appeared to be in the map, and
83   * for successive examinations of the key set, the value collection, and
84   * the entry set to yield successively smaller numbers of elements.
85   *
86   * <p> Each key object in a <tt>WeakHashMap</tt> is stored indirectly as
87   * the referent of a weak reference.  Therefore a key will automatically be
88   * removed only after the weak references to it, both inside and outside of the
89   * map, have been cleared by the garbage collector.
90   *
91   * <p> <strong>Implementation note:</strong> The value objects in a
92   * <tt>WeakHashMap</tt> are held by ordinary strong references.  Thus care
93   * should be taken to ensure that value objects do not strongly refer to their
94   * own keys, either directly or indirectly, since that will prevent the keys
95   * from being discarded.  Note that a value object may refer indirectly to its
96   * key via the <tt>WeakHashMap</tt> itself; that is, a value object may
97   * strongly refer to some other key object whose associated value object, in
98   * turn, strongly refers to the key of the first value object.  If the values
99   * in the map do not rely on the map holding strong references to them, one way
100  * to deal with this is to wrap values themselves within
101  * <tt>WeakReferences</tt> before
102  * inserting, as in: <tt>m.put(key, new WeakReference(value))</tt>,
103  * and then unwrapping upon each <tt>get</tt>.
104  *
105  * <p>The iterators returned by the <tt>iterator</tt> method of the collections
106  * returned by all of this class's "collection view methods" are
107  * <i>fail-fast</i>: if the map is structurally modified at any time after the
108  * iterator is created, in any way except through the iterator's own
109  * <tt>remove</tt> method, the iterator will throw a {@link
110  * ConcurrentModificationException}.  Thus, in the face of concurrent
111  * modification, the iterator fails quickly and cleanly, rather than risking
112  * arbitrary, non-deterministic behavior at an undetermined time in the future.
113  *
114  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
115  * as it is, generally speaking, impossible to make any hard guarantees in the
116  * presence of unsynchronized concurrent modification.  Fail-fast iterators
117  * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
118  * Therefore, it would be wrong to write a program that depended on this
119  * exception for its correctness:  <i>the fail-fast behavior of iterators
120  * should be used only to detect bugs.</i>
121  *
122  * <p>This class is a member of the
123  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
124  * Java Collections Framework</a>.
125  *
126  * @param <K> the type of keys maintained by this map
127  * @param <V> the type of mapped values
128  *
129  * @author      Doug Lea
130  * @author      Josh Bloch
131  * @author      Mark Reinhold
132  * @since       1.2
133  * @see         java.util.HashMap
134  * @see         java.lang.ref.WeakReference
135  */
136 public class WeakHashMap<K,V>
137     extends AbstractMap<K,V>
138     implements Map<K,V> {
139 
140     /**
141      * The default initial capacity -- MUST be a power of two.
142      */
143     private static final int DEFAULT_INITIAL_CAPACITY = 16;
144 
145     /**
146      * The maximum capacity, used if a higher value is implicitly specified
147      * by either of the constructors with arguments.
148      * MUST be a power of two <= 1<<30.
149      */
150     private static final int MAXIMUM_CAPACITY = 1 << 30;
151 
152     /**
153      * The load factor used when none specified in constructor.
154      */
155     private static final float DEFAULT_LOAD_FACTOR = 0.75f;
156 
157     /**
158      * The table, resized as necessary. Length MUST Always be a power of two.
159      */
160     Entry<K,V>[] table;
161 
162     /**
163      * The number of key-value mappings contained in this weak hash map.
164      */
165     private int size;
166 
167     /**
168      * The next size value at which to resize (capacity * load factor).
169      */
170     private int threshold;
171 
172     /**
173      * The load factor for the hash table.
174      */
175     private final float loadFactor;
176 
177     /**
178      * Reference queue for cleared WeakEntries
179      */
180     private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
181 
182     /**
183      * The number of times this WeakHashMap has been structurally modified.
184      * Structural modifications are those that change the number of
185      * mappings in the map or otherwise modify its internal structure
186      * (e.g., rehash).  This field is used to make iterators on
187      * Collection-views of the map fail-fast.
188      *
189      * @see ConcurrentModificationException
190      */
191     int modCount;
192 
193     @SuppressWarnings("unchecked")
194     private Entry<K,V>[] newTable(int n) {
195         return (Entry<K,V>[]) new Entry<?,?>[n];
196     }
197 
198     /**
199      * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
200      * capacity and the given load factor.
201      *
202      * @param  initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
203      * @param  loadFactor      The load factor of the <tt>WeakHashMap</tt>
204      * @throws IllegalArgumentException if the initial capacity is negative,
205      *         or if the load factor is nonpositive.
206      */
207     public WeakHashMap(int initialCapacity, float loadFactor) {
208         if (initialCapacity < 0)
209             throw new IllegalArgumentException("Illegal Initial Capacity: "+
210                                                initialCapacity);
211         if (initialCapacity > MAXIMUM_CAPACITY)
212             initialCapacity = MAXIMUM_CAPACITY;
213 
214         if (loadFactor <= 0 || Float.isNaN(loadFactor))
215             throw new IllegalArgumentException("Illegal Load factor: "+
216                                                loadFactor);
217         int capacity = 1;
218         while (capacity < initialCapacity)
219             capacity <<= 1;
220         table = newTable(capacity);
221         this.loadFactor = loadFactor;
222         threshold = (int)(capacity * loadFactor);
223     }
224 
225     /**
226      * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
227      * capacity and the default load factor (0.75).
228      *
229      * @param  initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
230      * @throws IllegalArgumentException if the initial capacity is negative
231      */
232     public WeakHashMap(int initialCapacity) {
233         this(initialCapacity, DEFAULT_LOAD_FACTOR);
234     }
235 
236     /**
237      * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial
238      * capacity (16) and load factor (0.75).
239      */
240     public WeakHashMap() {
241         this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
242     }
243 
244     /**
245      * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the
246      * specified map.  The <tt>WeakHashMap</tt> is created with the default
247      * load factor (0.75) and an initial capacity sufficient to hold the
248      * mappings in the specified map.
249      *
250      * @param   m the map whose mappings are to be placed in this map
251      * @throws  NullPointerException if the specified map is null
252      * @since   1.3
253      */
254     public WeakHashMap(Map<? extends K, ? extends V> m) {
255         this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
256                 DEFAULT_INITIAL_CAPACITY),
257              DEFAULT_LOAD_FACTOR);
258         putAll(m);
259     }
260 
261     // internal utilities
262 
263     /**
264      * Value representing null keys inside tables.
265      */
266     private static final Object NULL_KEY = new Object();
267 
268     /**
269      * Use NULL_KEY for key if it is null.
270      */
271     private static Object maskNull(Object key) {
272         return (key == null) ? NULL_KEY : key;
273     }
274 
275     /**
276      * Returns internal representation of null key back to caller as null.
277      */
278     static Object unmaskNull(Object key) {
279         return (key == NULL_KEY) ? null : key;
280     }
281 
282     /**
283      * Checks for equality of non-null reference x and possibly-null y.  By
284      * default uses Object.equals.
285      */
286     private static boolean eq(Object x, Object y) {
287         return x == y || x.equals(y);
288     }
289 
290     /**
291      * Retrieve object hash code and applies a supplemental hash function to the
292      * result hash, which defends against poor quality hash functions.  This is
293      * critical because HashMap uses power-of-two length hash tables, that
294      * otherwise encounter collisions for hashCodes that do not differ
295      * in lower bits.
296      */
297     final int hash(Object k) {
298         int h = k.hashCode();
299 
300         // This function ensures that hashCodes that differ only by
301         // constant multiples at each bit position have a bounded
302         // number of collisions (approximately 8 at default load factor).
303         h ^= (h >>> 20) ^ (h >>> 12);
304         return h ^ (h >>> 7) ^ (h >>> 4);
305     }
306 
307     /**
308      * Returns index for hash code h.
309      */
310     private static int indexFor(int h, int length) {
311         return h & (length-1);
312     }
313 
314     /**
315      * Expunges stale entries from the table.
316      */
317     private void expungeStaleEntries() {
318         for (Object x; (x = queue.poll()) != null; ) {
319             synchronized (queue) {
320                 @SuppressWarnings("unchecked")
321                     Entry<K,V> e = (Entry<K,V>) x;
322                 int i = indexFor(e.hash, table.length);
323 
324                 Entry<K,V> prev = table[i];
325                 Entry<K,V> p = prev;
326                 while (p != null) {
327                     Entry<K,V> next = p.next;
328                     if (p == e) {
329                         if (prev == e)
330                             table[i] = next;
331                         else
332                             prev.next = next;
333                         // Must not null out e.next;
334                         // stale entries may be in use by a HashIterator
335                         e.value = null; // Help GC
336                         size--;
337                         break;
338                     }
339                     prev = p;
340                     p = next;
341                 }
342             }
343         }
344     }
345 
346     /**
347      * Returns the table after first expunging stale entries.
348      */
349     private Entry<K,V>[] getTable() {
350         expungeStaleEntries();
351         return table;
352     }
353 
354     /**
355      * Returns the number of key-value mappings in this map.
356      * This result is a snapshot, and may not reflect unprocessed
357      * entries that will be removed before next attempted access
358      * because they are no longer referenced.
359      */
360     public int size() {
361         if (size == 0)
362             return 0;
363         expungeStaleEntries();
364         return size;
365     }
366 
367     /**
368      * Returns <tt>true</tt> if this map contains no key-value mappings.
369      * This result is a snapshot, and may not reflect unprocessed
370      * entries that will be removed before next attempted access
371      * because they are no longer referenced.
372      */
373     public boolean isEmpty() {
374         return size() == 0;
375     }
376 
377     /**
378      * Returns the value to which the specified key is mapped,
379      * or {@code null} if this map contains no mapping for the key.
380      *
381      * <p>More formally, if this map contains a mapping from a key
382      * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
383      * key.equals(k))}, then this method returns {@code v}; otherwise
384      * it returns {@code null}.  (There can be at most one such mapping.)
385      *
386      * <p>A return value of {@code null} does not <i>necessarily</i>
387      * indicate that the map contains no mapping for the key; it's also
388      * possible that the map explicitly maps the key to {@code null}.
389      * The {@link #containsKey containsKey} operation may be used to
390      * distinguish these two cases.
391      *
392      * @see #put(Object, Object)
393      */
394     public V get(Object key) {
395         Object k = maskNull(key);
396         int h = hash(k);
397         Entry<K,V>[] tab = getTable();
398         int index = indexFor(h, tab.length);
399         Entry<K,V> e = tab[index];
400         while (e != null) {
401             if (e.hash == h && eq(k, e.get()))
402                 return e.value;
403             e = e.next;
404         }
405         return null;
406     }
407 
408     /**
409      * Returns <tt>true</tt> if this map contains a mapping for the
410      * specified key.
411      *
412      * @param  key   The key whose presence in this map is to be tested
413      * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;
414      *         <tt>false</tt> otherwise
415      */
416     public boolean containsKey(Object key) {
417         return getEntry(key) != null;
418     }
419 
420     /**
421      * Returns the entry associated with the specified key in this map.
422      * Returns null if the map contains no mapping for this key.
423      */
424     Entry<K,V> getEntry(Object key) {
425         Object k = maskNull(key);
426         int h = hash(k);
427         Entry<K,V>[] tab = getTable();
428         int index = indexFor(h, tab.length);
429         Entry<K,V> e = tab[index];
430         while (e != null && !(e.hash == h && eq(k, e.get())))
431             e = e.next;
432         return e;
433     }
434 
435     /**
436      * Associates the specified value with the specified key in this map.
437      * If the map previously contained a mapping for this key, the old
438      * value is replaced.
439      *
440      * @param key key with which the specified value is to be associated.
441      * @param value value to be associated with the specified key.
442      * @return the previous value associated with <tt>key</tt>, or
443      *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
444      *         (A <tt>null</tt> return can also indicate that the map
445      *         previously associated <tt>null</tt> with <tt>key</tt>.)
446      */
447     public V put(K key, V value) {
448         Object k = maskNull(key);
449         int h = hash(k);
450         Entry<K,V>[] tab = getTable();
451         int i = indexFor(h, tab.length);
452 
453         for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
454             if (h == e.hash && eq(k, e.get())) {
455                 V oldValue = e.value;
456                 if (value != oldValue)
457                     e.value = value;
458                 return oldValue;
459             }
460         }
461 
462         modCount++;
463         Entry<K,V> e = tab[i];
464         tab[i] = new Entry<>(k, value, queue, h, e);
465         if (++size >= threshold)
466             resize(tab.length * 2);
467         return null;
468     }
469 
470     /**
471      * Rehashes the contents of this map into a new array with a
472      * larger capacity.  This method is called automatically when the
473      * number of keys in this map reaches its threshold.
474      *
475      * If current capacity is MAXIMUM_CAPACITY, this method does not
476      * resize the map, but sets threshold to Integer.MAX_VALUE.
477      * This has the effect of preventing future calls.
478      *
479      * @param newCapacity the new capacity, MUST be a power of two;
480      *        must be greater than current capacity unless current
481      *        capacity is MAXIMUM_CAPACITY (in which case value
482      *        is irrelevant).
483      */
484     void resize(int newCapacity) {
485         Entry<K,V>[] oldTable = getTable();
486         int oldCapacity = oldTable.length;
487         if (oldCapacity == MAXIMUM_CAPACITY) {
488             threshold = Integer.MAX_VALUE;
489             return;
490         }
491 
492         Entry<K,V>[] newTable = newTable(newCapacity);
493         transfer(oldTable, newTable);
494         table = newTable;
495 
496         /*
497          * If ignoring null elements and processing ref queue caused massive
498          * shrinkage, then restore old table.  This should be rare, but avoids
499          * unbounded expansion of garbage-filled tables.
500          */
501         if (size >= threshold / 2) {
502             threshold = (int)(newCapacity * loadFactor);
503         } else {
504             expungeStaleEntries();
505             transfer(newTable, oldTable);
506             table = oldTable;
507         }
508     }
509 
510     /** Transfers all entries from src to dest tables */
511     private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest) {
512         for (int j = 0; j < src.length; ++j) {
513             Entry<K,V> e = src[j];
514             src[j] = null;
515             while (e != null) {
516                 Entry<K,V> next = e.next;
517                 Object key = e.get();
518                 if (key == null) {
519                     e.next = null;  // Help GC
520                     e.value = null; //  "   "
521                     size--;
522                 } else {
523                     int i = indexFor(e.hash, dest.length);
524                     e.next = dest[i];
525                     dest[i] = e;
526                 }
527                 e = next;
528             }
529         }
530     }
531 
532     /**
533      * Copies all of the mappings from the specified map to this map.
534      * These mappings will replace any mappings that this map had for any
535      * of the keys currently in the specified map.
536      *
537      * @param m mappings to be stored in this map.
538      * @throws  NullPointerException if the specified map is null.
539      */
540     public void putAll(Map<? extends K, ? extends V> m) {
541         int numKeysToBeAdded = m.size();
542         if (numKeysToBeAdded == 0)
543             return;
544 
545         /*
546          * Expand the map if the map if the number of mappings to be added
547          * is greater than or equal to threshold.  This is conservative; the
548          * obvious condition is (m.size() + size) >= threshold, but this
549          * condition could result in a map with twice the appropriate capacity,
550          * if the keys to be added overlap with the keys already in this map.
551          * By using the conservative calculation, we subject ourself
552          * to at most one extra resize.
553          */
554         if (numKeysToBeAdded > threshold) {
555             int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
556             if (targetCapacity > MAXIMUM_CAPACITY)
557                 targetCapacity = MAXIMUM_CAPACITY;
558             int newCapacity = table.length;
559             while (newCapacity < targetCapacity)
560                 newCapacity <<= 1;
561             if (newCapacity > table.length)
562                 resize(newCapacity);
563         }
564 
565         for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
566             put(e.getKey(), e.getValue());
567     }
568 
569     /**
570      * Removes the mapping for a key from this weak hash map if it is present.
571      * More formally, if this map contains a mapping from key <tt>k</tt> to
572      * value <tt>v</tt> such that <code>(key==null ?  k==null :
573      * key.equals(k))</code>, that mapping is removed.  (The map can contain
574      * at most one such mapping.)
575      *
576      * <p>Returns the value to which this map previously associated the key,
577      * or <tt>null</tt> if the map contained no mapping for the key.  A
578      * return value of <tt>null</tt> does not <i>necessarily</i> indicate
579      * that the map contained no mapping for the key; it's also possible
580      * that the map explicitly mapped the key to <tt>null</tt>.
581      *
582      * <p>The map will not contain a mapping for the specified key once the
583      * call returns.
584      *
585      * @param key key whose mapping is to be removed from the map
586      * @return the previous value associated with <tt>key</tt>, or
587      *         <tt>null</tt> if there was no mapping for <tt>key</tt>
588      */
589     public V remove(Object key) {
590         Object k = maskNull(key);
591         int h = hash(k);
592         Entry<K,V>[] tab = getTable();
593         int i = indexFor(h, tab.length);
594         Entry<K,V> prev = tab[i];
595         Entry<K,V> e = prev;
596 
597         while (e != null) {
598             Entry<K,V> next = e.next;
599             if (h == e.hash && eq(k, e.get())) {
600                 modCount++;
601                 size--;
602                 if (prev == e)
603                     tab[i] = next;
604                 else
605                     prev.next = next;
606                 return e.value;
607             }
608             prev = e;
609             e = next;
610         }
611 
612         return null;
613     }
614 
615     /** Special version of remove needed by Entry set */
616     boolean removeMapping(Object o) {
617         if (!(o instanceof Map.Entry))
618             return false;
619         Entry<K,V>[] tab = getTable();
620         Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
621         Object k = maskNull(entry.getKey());
622         int h = hash(k);
623         int i = indexFor(h, tab.length);
624         Entry<K,V> prev = tab[i];
625         Entry<K,V> e = prev;
626 
627         while (e != null) {
628             Entry<K,V> next = e.next;
629             if (h == e.hash && e.equals(entry)) {
630                 modCount++;
631                 size--;
632                 if (prev == e)
633                     tab[i] = next;
634                 else
635                     prev.next = next;
636                 return true;
637             }
638             prev = e;
639             e = next;
640         }
641 
642         return false;
643     }
644 
645     /**
646      * Removes all of the mappings from this map.
647      * The map will be empty after this call returns.
648      */
649     public void clear() {
650         // clear out ref queue. We don't need to expunge entries
651         // since table is getting cleared.
652         while (queue.poll() != null)
653             ;
654 
655         modCount++;
656         Arrays.fill(table, null);
657         size = 0;
658 
659         // Allocation of array may have caused GC, which may have caused
660         // additional entries to go stale.  Removing these entries from the
661         // reference queue will make them eligible for reclamation.
662         while (queue.poll() != null)
663             ;
664     }
665 
666     /**
667      * Returns <tt>true</tt> if this map maps one or more keys to the
668      * specified value.
669      *
670      * @param value value whose presence in this map is to be tested
671      * @return <tt>true</tt> if this map maps one or more keys to the
672      *         specified value
673      */
674     public boolean containsValue(Object value) {
675         if (value==null)
676             return containsNullValue();
677 
678         Entry<K,V>[] tab = getTable();
679         for (int i = tab.length; i-- > 0;)
680             for (Entry<K,V> e = tab[i]; e != null; e = e.next)
681                 if (value.equals(e.value))
682                     return true;
683         return false;
684     }
685 
686     /**
687      * Special-case code for containsValue with null argument
688      */
689     private boolean containsNullValue() {
690         Entry<K,V>[] tab = getTable();
691         for (int i = tab.length; i-- > 0;)
692             for (Entry<K,V> e = tab[i]; e != null; e = e.next)
693                 if (e.value==null)
694                     return true;
695         return false;
696     }
697 
698     /**
699      * The entries in this hash table extend WeakReference, using its main ref
700      * field as the key.
701      */
702     private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
703         V value;
704         final int hash;
705         Entry<K,V> next;
706 
707         /**
708          * Creates new entry.
709          */
710         Entry(Object key, V value,
711               ReferenceQueue<Object> queue,
712               int hash, Entry<K,V> next) {
713             super(key, queue);
714             this.value = value;
715             this.hash  = hash;
716             this.next  = next;
717         }
718 
719         @SuppressWarnings("unchecked")
720         public K getKey() {
721             return (K) WeakHashMap.unmaskNull(get());
722         }
723 
724         public V getValue() {
725             return value;
726         }
727 
728         public V setValue(V newValue) {
729             V oldValue = value;
730             value = newValue;
731             return oldValue;
732         }
733 
734         public boolean equals(Object o) {
735             if (!(o instanceof Map.Entry))
736                 return false;
737             Map.Entry<?,?> e = (Map.Entry<?,?>)o;
738             K k1 = getKey();
739             Object k2 = e.getKey();
740             if (k1 == k2 || (k1 != null && k1.equals(k2))) {
741                 V v1 = getValue();
742                 Object v2 = e.getValue();
743                 if (v1 == v2 || (v1 != null && v1.equals(v2)))
744                     return true;
745             }
746             return false;
747         }
748 
749         public int hashCode() {
750             K k = getKey();
751             V v = getValue();
752             return Objects.hashCode(k) ^ Objects.hashCode(v);
753         }
754 
755         public String toString() {
756             return getKey() + "=" + getValue();
757         }
758     }
759 
760     private abstract class HashIterator<T> implements Iterator<T> {
761         private int index;
762         private Entry<K,V> entry = null;
763         private Entry<K,V> lastReturned = null;
764         private int expectedModCount = modCount;
765 
766         /**
767          * Strong reference needed to avoid disappearance of key
768          * between hasNext and next
769          */
770         private Object nextKey = null;
771 
772         /**
773          * Strong reference needed to avoid disappearance of key
774          * between nextEntry() and any use of the entry
775          */
776         private Object currentKey = null;
777 
778         HashIterator() {
779             index = isEmpty() ? 0 : table.length;
780         }
781 
782         public boolean hasNext() {
783             Entry<K,V>[] t = table;
784 
785             while (nextKey == null) {
786                 Entry<K,V> e = entry;
787                 int i = index;
788                 while (e == null && i > 0)
789                     e = t[--i];
790                 entry = e;
791                 index = i;
792                 if (e == null) {
793                     currentKey = null;
794                     return false;
795                 }
796                 nextKey = e.get(); // hold on to key in strong ref
797                 if (nextKey == null)
798                     entry = entry.next;
799             }
800             return true;
801         }
802 
803         /** The common parts of next() across different types of iterators */
804         protected Entry<K,V> nextEntry() {
805             if (modCount != expectedModCount)
806                 throw new ConcurrentModificationException();
807             if (nextKey == null && !hasNext())
808                 throw new NoSuchElementException();
809 
810             lastReturned = entry;
811             entry = entry.next;
812             currentKey = nextKey;
813             nextKey = null;
814             return lastReturned;
815         }
816 
817         public void remove() {
818             if (lastReturned == null)
819                 throw new IllegalStateException();
820             if (modCount != expectedModCount)
821                 throw new ConcurrentModificationException();
822 
823             WeakHashMap.this.remove(currentKey);
824             expectedModCount = modCount;
825             lastReturned = null;
826             currentKey = null;
827         }
828 
829     }
830 
831     private class ValueIterator extends HashIterator<V> {
832         public V next() {
833             return nextEntry().value;
834         }
835     }
836 
837     private class KeyIterator extends HashIterator<K> {
838         public K next() {
839             return nextEntry().getKey();
840         }
841     }
842 
843     private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
844         public Map.Entry<K,V> next() {
845             return nextEntry();
846         }
847     }
848 
849     // Views
850 
851     private transient Set<Map.Entry<K,V>> entrySet = null;
852 
853     /**
854      * Returns a {@link Set} view of the keys contained in this map.
855      * The set is backed by the map, so changes to the map are
856      * reflected in the set, and vice-versa.  If the map is modified
857      * while an iteration over the set is in progress (except through
858      * the iterator's own <tt>remove</tt> operation), the results of
859      * the iteration are undefined.  The set supports element removal,
860      * which removes the corresponding mapping from the map, via the
861      * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
862      * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
863      * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
864      * operations.
865      */
866     public Set<K> keySet() {
867         Set<K> ks = keySet;
868         return (ks != null ? ks : (keySet = new KeySet()));
869     }
870 
871     private class KeySet extends AbstractSet<K> {
872         public Iterator<K> iterator() {
873             return new KeyIterator();
874         }
875 
876         public int size() {
877             return WeakHashMap.this.size();
878         }
879 
880         public boolean contains(Object o) {
881             return containsKey(o);
882         }
883 
884         public boolean remove(Object o) {
885             if (containsKey(o)) {
886                 WeakHashMap.this.remove(o);
887                 return true;
888             }
889             else
890                 return false;
891         }
892 
893         public void clear() {
894             WeakHashMap.this.clear();
895         }
896 
897         public Spliterator<K> spliterator() {
898             return new KeySpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
899         }
900     }
901 
902     /**
903      * Returns a {@link Collection} view of the values contained in this map.
904      * The collection is backed by the map, so changes to the map are
905      * reflected in the collection, and vice-versa.  If the map is
906      * modified while an iteration over the collection is in progress
907      * (except through the iterator's own <tt>remove</tt> operation),
908      * the results of the iteration are undefined.  The collection
909      * supports element removal, which removes the corresponding
910      * mapping from the map, via the <tt>Iterator.remove</tt>,
911      * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
912      * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
913      * support the <tt>add</tt> or <tt>addAll</tt> operations.
914      */
915     public Collection<V> values() {
916         Collection<V> vs = values;
917         return (vs != null) ? vs : (values = new Values());
918     }
919 
920     private class Values extends AbstractCollection<V> {
921         public Iterator<V> iterator() {
922             return new ValueIterator();
923         }
924 
925         public int size() {
926             return WeakHashMap.this.size();
927         }
928 
929         public boolean contains(Object o) {
930             return containsValue(o);
931         }
932 
933         public void clear() {
934             WeakHashMap.this.clear();
935         }
936 
937         public Spliterator<V> spliterator() {
938             return new ValueSpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
939         }
940     }
941 
942     /**
943      * Returns a {@link Set} view of the mappings contained in this map.
944      * The set is backed by the map, so changes to the map are
945      * reflected in the set, and vice-versa.  If the map is modified
946      * while an iteration over the set is in progress (except through
947      * the iterator's own <tt>remove</tt> operation, or through the
948      * <tt>setValue</tt> operation on a map entry returned by the
949      * iterator) the results of the iteration are undefined.  The set
950      * supports element removal, which removes the corresponding
951      * mapping from the map, via the <tt>Iterator.remove</tt>,
952      * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
953      * <tt>clear</tt> operations.  It does not support the
954      * <tt>add</tt> or <tt>addAll</tt> operations.
955      */
956     public Set<Map.Entry<K,V>> entrySet() {
957         Set<Map.Entry<K,V>> es = entrySet;
958         return es != null ? es : (entrySet = new EntrySet());
959     }
960 
961     private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
962         public Iterator<Map.Entry<K,V>> iterator() {
963             return new EntryIterator();
964         }
965 
966         public boolean contains(Object o) {
967             if (!(o instanceof Map.Entry))
968                 return false;
969             Map.Entry<?,?> e = (Map.Entry<?,?>)o;
970             Entry<K,V> candidate = getEntry(e.getKey());
971             return candidate != null && candidate.equals(e);
972         }
973 
974         public boolean remove(Object o) {
975             return removeMapping(o);
976         }
977 
978         public int size() {
979             return WeakHashMap.this.size();
980         }
981 
982         public void clear() {
983             WeakHashMap.this.clear();
984         }
985 
986         private List<Map.Entry<K,V>> deepCopy() {
987             List<Map.Entry<K,V>> list = new ArrayList<>(size());
988             for (Map.Entry<K,V> e : this)
989                 list.add(new AbstractMap.SimpleEntry<>(e));
990             return list;
991         }
992 
993         public Object[] toArray() {
994             return deepCopy().toArray();
995         }
996 
997         public <T> T[] toArray(T[] a) {
998             return deepCopy().toArray(a);
999         }
1000 
1001         public Spliterator<Map.Entry<K,V>> spliterator() {
1002             return new EntrySpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
1003         }
1004     }
1005 
1006     @SuppressWarnings("unchecked")
1007     @Override
1008     public void forEach(BiConsumer<? super K, ? super V> action) {
1009         Objects.requireNonNull(action);
1010         int expectedModCount = modCount;
1011 
1012         Entry<K, V>[] tab = getTable();
1013         for (Entry<K, V> entry : tab) {
1014             while (entry != null) {
1015                 Object key = entry.get();
1016                 if (key != null) {
1017                     action.accept((K)WeakHashMap.unmaskNull(key), entry.value);
1018                 }
1019                 entry = entry.next;
1020 
1021                 if (expectedModCount != modCount) {
1022                     throw new ConcurrentModificationException();
1023                 }
1024             }
1025         }
1026     }
1027 
1028     @SuppressWarnings("unchecked")
1029     @Override
1030     public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
1031         Objects.requireNonNull(function);
1032         int expectedModCount = modCount;
1033 
1034         Entry<K, V>[] tab = getTable();;
1035         for (Entry<K, V> entry : tab) {
1036             while (entry != null) {
1037                 Object key = entry.get();
1038                 if (key != null) {
1039                     entry.value = function.apply((K)WeakHashMap.unmaskNull(key), entry.value);
1040                 }
1041                 entry = entry.next;
1042 
1043                 if (expectedModCount != modCount) {
1044                     throw new ConcurrentModificationException();
1045                 }
1046             }
1047         }
1048     }
1049 
1050     /**
1051      * Similar form as other hash Spliterators, but skips dead
1052      * elements.
1053      */
1054     static class WeakHashMapSpliterator<K,V> {
1055         final WeakHashMap<K,V> map;
1056         WeakHashMap.Entry<K,V> current; // current node
1057         int index;             // current index, modified on advance/split
1058         int fence;             // -1 until first use; then one past last index
1059         int est;               // size estimate
1060         int expectedModCount;  // for comodification checks
1061 
1062         WeakHashMapSpliterator(WeakHashMap<K,V> m, int origin,
1063                                int fence, int est,
1064                                int expectedModCount) {
1065             this.map = m;
1066             this.index = origin;
1067             this.fence = fence;
1068             this.est = est;
1069             this.expectedModCount = expectedModCount;
1070         }
1071 
1072         final int getFence() { // initialize fence and size on first use
1073             int hi;
1074             if ((hi = fence) < 0) {
1075                 WeakHashMap<K,V> m = map;
1076                 est = m.size();
1077                 expectedModCount = m.modCount;
1078                 hi = fence = m.table.length;
1079             }
1080             return hi;
1081         }
1082 
1083         public final long estimateSize() {
1084             getFence(); // force init
1085             return (long) est;
1086         }
1087     }
1088 
1089     static final class KeySpliterator<K,V>
1090         extends WeakHashMapSpliterator<K,V>
1091         implements Spliterator<K> {
1092         KeySpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
1093                        int expectedModCount) {
1094             super(m, origin, fence, est, expectedModCount);
1095         }
1096 
1097         public KeySpliterator<K,V> trySplit() {
1098             int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
1099             return (lo >= mid) ? null :
1100                 new KeySpliterator<K,V>(map, lo, index = mid, est >>>= 1,
1101                                         expectedModCount);
1102         }
1103 
1104         public void forEachRemaining(Consumer<? super K> action) {
1105             int i, hi, mc;
1106             if (action == null)
1107                 throw new NullPointerException();
1108             WeakHashMap<K,V> m = map;
1109             WeakHashMap.Entry<K,V>[] tab = m.table;
1110             if ((hi = fence) < 0) {
1111                 mc = expectedModCount = m.modCount;
1112                 hi = fence = tab.length;
1113             }
1114             else
1115                 mc = expectedModCount;
1116             if (tab.length >= hi && (i = index) >= 0 &&
1117                 (i < (index = hi) || current != null)) {
1118                 WeakHashMap.Entry<K,V> p = current;
1119                 current = null; // exhaust
1120                 do {
1121                     if (p == null)
1122                         p = tab[i++];
1123                     else {
1124                         Object x = p.get();
1125                         p = p.next;
1126                         if (x != null) {
1127                             @SuppressWarnings("unchecked") K k =
1128                                 (K) WeakHashMap.unmaskNull(x);
1129                             action.accept(k);
1130                         }
1131                     }
1132                 } while (p != null || i < hi);
1133             }
1134             if (m.modCount != mc)
1135                 throw new ConcurrentModificationException();
1136         }
1137 
1138         public boolean tryAdvance(Consumer<? super K> action) {
1139             int hi;
1140             if (action == null)
1141                 throw new NullPointerException();
1142             WeakHashMap.Entry<K,V>[] tab = map.table;
1143             if (tab.length >= (hi = getFence()) && index >= 0) {
1144                 while (current != null || index < hi) {
1145                     if (current == null)
1146                         current = tab[index++];
1147                     else {
1148                         Object x = current.get();
1149                         current = current.next;
1150                         if (x != null) {
1151                             @SuppressWarnings("unchecked") K k =
1152                                 (K) WeakHashMap.unmaskNull(x);
1153                             action.accept(k);
1154                             if (map.modCount != expectedModCount)
1155                                 throw new ConcurrentModificationException();
1156                             return true;
1157                         }
1158                     }
1159                 }
1160             }
1161             return false;
1162         }
1163 
1164         public int characteristics() {
1165             return Spliterator.DISTINCT;
1166         }
1167     }
1168 
1169     static final class ValueSpliterator<K,V>
1170         extends WeakHashMapSpliterator<K,V>
1171         implements Spliterator<V> {
1172         ValueSpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
1173                          int expectedModCount) {
1174             super(m, origin, fence, est, expectedModCount);
1175         }
1176 
1177         public ValueSpliterator<K,V> trySplit() {
1178             int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
1179             return (lo >= mid) ? null :
1180                 new ValueSpliterator<K,V>(map, lo, index = mid, est >>>= 1,
1181                                           expectedModCount);
1182         }
1183 
1184         public void forEachRemaining(Consumer<? super V> action) {
1185             int i, hi, mc;
1186             if (action == null)
1187                 throw new NullPointerException();
1188             WeakHashMap<K,V> m = map;
1189             WeakHashMap.Entry<K,V>[] tab = m.table;
1190             if ((hi = fence) < 0) {
1191                 mc = expectedModCount = m.modCount;
1192                 hi = fence = tab.length;
1193             }
1194             else
1195                 mc = expectedModCount;
1196             if (tab.length >= hi && (i = index) >= 0 &&
1197                 (i < (index = hi) || current != null)) {
1198                 WeakHashMap.Entry<K,V> p = current;
1199                 current = null; // exhaust
1200                 do {
1201                     if (p == null)
1202                         p = tab[i++];
1203                     else {
1204                         Object x = p.get();
1205                         V v = p.value;
1206                         p = p.next;
1207                         if (x != null)
1208                             action.accept(v);
1209                     }
1210                 } while (p != null || i < hi);
1211             }
1212             if (m.modCount != mc)
1213                 throw new ConcurrentModificationException();
1214         }
1215 
1216         public boolean tryAdvance(Consumer<? super V> action) {
1217             int hi;
1218             if (action == null)
1219                 throw new NullPointerException();
1220             WeakHashMap.Entry<K,V>[] tab = map.table;
1221             if (tab.length >= (hi = getFence()) && index >= 0) {
1222                 while (current != null || index < hi) {
1223                     if (current == null)
1224                         current = tab[index++];
1225                     else {
1226                         Object x = current.get();
1227                         V v = current.value;
1228                         current = current.next;
1229                         if (x != null) {
1230                             action.accept(v);
1231                             if (map.modCount != expectedModCount)
1232                                 throw new ConcurrentModificationException();
1233                             return true;
1234                         }
1235                     }
1236                 }
1237             }
1238             return false;
1239         }
1240 
1241         public int characteristics() {
1242             return 0;
1243         }
1244     }
1245 
1246     static final class EntrySpliterator<K,V>
1247         extends WeakHashMapSpliterator<K,V>
1248         implements Spliterator<Map.Entry<K,V>> {
1249         EntrySpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
1250                        int expectedModCount) {
1251             super(m, origin, fence, est, expectedModCount);
1252         }
1253 
1254         public EntrySpliterator<K,V> trySplit() {
1255             int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
1256             return (lo >= mid) ? null :
1257                 new EntrySpliterator<K,V>(map, lo, index = mid, est >>>= 1,
1258                                           expectedModCount);
1259         }
1260 
1261 
1262         public void forEachRemaining(Consumer<? super Map.Entry<K, V>> action) {
1263             int i, hi, mc;
1264             if (action == null)
1265                 throw new NullPointerException();
1266             WeakHashMap<K,V> m = map;
1267             WeakHashMap.Entry<K,V>[] tab = m.table;
1268             if ((hi = fence) < 0) {
1269                 mc = expectedModCount = m.modCount;
1270                 hi = fence = tab.length;
1271             }
1272             else
1273                 mc = expectedModCount;
1274             if (tab.length >= hi && (i = index) >= 0 &&
1275                 (i < (index = hi) || current != null)) {
1276                 WeakHashMap.Entry<K,V> p = current;
1277                 current = null; // exhaust
1278                 do {
1279                     if (p == null)
1280                         p = tab[i++];
1281                     else {
1282                         Object x = p.get();
1283                         V v = p.value;
1284                         p = p.next;
1285                         if (x != null) {
1286                             @SuppressWarnings("unchecked") K k =
1287                                 (K) WeakHashMap.unmaskNull(x);
1288                             action.accept
1289                                 (new AbstractMap.SimpleImmutableEntry<K,V>(k, v));
1290                         }
1291                     }
1292                 } while (p != null || i < hi);
1293             }
1294             if (m.modCount != mc)
1295                 throw new ConcurrentModificationException();
1296         }
1297 
1298         public boolean tryAdvance(Consumer<? super Map.Entry<K,V>> action) {
1299             int hi;
1300             if (action == null)
1301                 throw new NullPointerException();
1302             WeakHashMap.Entry<K,V>[] tab = map.table;
1303             if (tab.length >= (hi = getFence()) && index >= 0) {
1304                 while (current != null || index < hi) {
1305                     if (current == null)
1306                         current = tab[index++];
1307                     else {
1308                         Object x = current.get();
1309                         V v = current.value;
1310                         current = current.next;
1311                         if (x != null) {
1312                             @SuppressWarnings("unchecked") K k =
1313                                 (K) WeakHashMap.unmaskNull(x);
1314                             action.accept
1315                                 (new AbstractMap.SimpleImmutableEntry<K,V>(k, v));
1316                             if (map.modCount != expectedModCount)
1317                                 throw new ConcurrentModificationException();
1318                             return true;
1319                         }
1320                     }
1321                 }
1322             }
1323             return false;
1324         }
1325 
1326         public int characteristics() {
1327             return Spliterator.DISTINCT;
1328         }
1329     }
1330 
1331 }