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 /**
188 * A randomizing value associated with this instance that is applied to
189 * hash code of keys to make hash collisions harder to find.
190 */
191 transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);
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. Note: Null keys always map to hash 0, thus index 0.
296 */
297 int hash(Object k) {
298 if (null == k) {
299 return 0;
300 }
301
302 int h = hashSeed;
303 if (k instanceof String) {
304 h ^= ((String) k).hash32();
305 } else {
306 h ^= k.hashCode();
307 }
308
309 // This function ensures that hashCodes that differ only by
310 // constant multiples at each bit position have a bounded
311 // number of collisions (approximately 8 at default load factor).
312 h ^= (h >>> 20) ^ (h >>> 12);
313 return h ^ (h >>> 7) ^ (h >>> 4);
314 }
315
316 /**
317 * Returns index for hash code h.
318 */
319 private static int indexFor(int h, int length) {
320 return h & (length-1);
321 }
322
323 /**
324 * Expunges stale entries from the table.
325 */
326 private void expungeStaleEntries() {
327 for (Object x; (x = queue.poll()) != null; ) {
328 synchronized (queue) {
329 @SuppressWarnings("unchecked")
330 Entry<K,V> e = (Entry<K,V>) x;
331 int i = indexFor(e.hash, table.length);
332
333 Entry<K,V> prev = table[i];
334 Entry<K,V> p = prev;
335 while (p != null) {
336 Entry<K,V> next = p.next;
337 if (p == e) {
338 if (prev == e)
339 table[i] = next;
340 else
341 prev.next = next;
342 // Must not null out e.next;
343 // stale entries may be in use by a HashIterator
344 e.value = null; // Help GC
345 size--;
346 break;
347 }
348 prev = p;
349 p = next;
350 }
351 }
352 }
353 }
354
355 /**
356 * Returns the table after first expunging stale entries.
357 */
358 private Entry<K,V>[] getTable() {
359 expungeStaleEntries();
360 return table;
361 }
362
363 /**
364 * Returns the number of key-value mappings in this map.
365 * This result is a snapshot, and may not reflect unprocessed
366 * entries that will be removed before next attempted access
367 * because they are no longer referenced.
368 */
369 public int size() {
370 if (size == 0)
371 return 0;
372 expungeStaleEntries();
373 return size;
374 }
375
376 /**
377 * Returns <tt>true</tt> if this map contains no key-value mappings.
378 * This result is a snapshot, and may not reflect unprocessed
379 * entries that will be removed before next attempted access
380 * because they are no longer referenced.
381 */
382 public boolean isEmpty() {
383 return size() == 0;
384 }
385
386 /**
387 * Returns the value to which the specified key is mapped,
388 * or {@code null} if this map contains no mapping for the key.
389 *
390 * <p>More formally, if this map contains a mapping from a key
391 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
392 * key.equals(k))}, then this method returns {@code v}; otherwise
393 * it returns {@code null}. (There can be at most one such mapping.)
394 *
395 * <p>A return value of {@code null} does not <i>necessarily</i>
396 * indicate that the map contains no mapping for the key; it's also
397 * possible that the map explicitly maps the key to {@code null}.
398 * The {@link #containsKey containsKey} operation may be used to
399 * distinguish these two cases.
400 *
401 * @see #put(Object, Object)
402 */
403 public V get(Object key) {
404 Object k = maskNull(key);
405 int h = hash(k);
406 Entry<K,V>[] tab = getTable();
407 int index = indexFor(h, tab.length);
408 Entry<K,V> e = tab[index];
409 while (e != null) {
410 if (e.hash == h && eq(k, e.get()))
411 return e.value;
412 e = e.next;
413 }
414 return null;
415 }
416
417 /**
418 * Returns <tt>true</tt> if this map contains a mapping for the
419 * specified key.
420 *
421 * @param key The key whose presence in this map is to be tested
422 * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;
423 * <tt>false</tt> otherwise
424 */
425 public boolean containsKey(Object key) {
426 return getEntry(key) != null;
427 }
428
429 /**
430 * Returns the entry associated with the specified key in this map.
431 * Returns null if the map contains no mapping for this key.
432 */
433 Entry<K,V> getEntry(Object key) {
434 Object k = maskNull(key);
435 int h = hash(k);
436 Entry<K,V>[] tab = getTable();
437 int index = indexFor(h, tab.length);
438 Entry<K,V> e = tab[index];
439 while (e != null && !(e.hash == h && eq(k, e.get())))
440 e = e.next;
441 return e;
442 }
443
444 /**
445 * Associates the specified value with the specified key in this map.
446 * If the map previously contained a mapping for this key, the old
447 * value is replaced.
448 *
449 * @param key key with which the specified value is to be associated.
450 * @param value value to be associated with the specified key.
451 * @return the previous value associated with <tt>key</tt>, or
452 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
453 * (A <tt>null</tt> return can also indicate that the map
454 * previously associated <tt>null</tt> with <tt>key</tt>.)
455 */
456 public V put(K key, V value) {
457 Object k = maskNull(key);
458 int h = hash(k);
459 Entry<K,V>[] tab = getTable();
460 int i = indexFor(h, tab.length);
461
462 for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
463 if (h == e.hash && eq(k, e.get())) {
464 V oldValue = e.value;
465 if (value != oldValue)
466 e.value = value;
467 return oldValue;
468 }
469 }
470
471 modCount++;
472 Entry<K,V> e = tab[i];
473 tab[i] = new Entry<>(k, value, queue, h, e);
474 if (++size >= threshold)
475 resize(tab.length * 2);
476 return null;
477 }
478
479 /**
480 * Rehashes the contents of this map into a new array with a
481 * larger capacity. This method is called automatically when the
482 * number of keys in this map reaches its threshold.
483 *
484 * If current capacity is MAXIMUM_CAPACITY, this method does not
485 * resize the map, but sets threshold to Integer.MAX_VALUE.
486 * This has the effect of preventing future calls.
487 *
488 * @param newCapacity the new capacity, MUST be a power of two;
489 * must be greater than current capacity unless current
490 * capacity is MAXIMUM_CAPACITY (in which case value
491 * is irrelevant).
492 */
493 void resize(int newCapacity) {
494 Entry<K,V>[] oldTable = getTable();
495 int oldCapacity = oldTable.length;
496 if (oldCapacity == MAXIMUM_CAPACITY) {
497 threshold = Integer.MAX_VALUE;
498 return;
499 }
500
501 Entry<K,V>[] newTable = newTable(newCapacity);
502 transfer(oldTable, newTable);
503 table = newTable;
504
505 /*
506 * If ignoring null elements and processing ref queue caused massive
507 * shrinkage, then restore old table. This should be rare, but avoids
508 * unbounded expansion of garbage-filled tables.
509 */
510 if (size >= threshold / 2) {
511 threshold = (int)(newCapacity * loadFactor);
512 } else {
513 expungeStaleEntries();
514 transfer(newTable, oldTable);
515 table = oldTable;
516 }
517 }
518
519 /** Transfers all entries from src to dest tables */
520 private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest) {
521 for (int j = 0; j < src.length; ++j) {
522 Entry<K,V> e = src[j];
523 src[j] = null;
524 while (e != null) {
525 Entry<K,V> next = e.next;
526 Object key = e.get();
527 if (key == null) {
528 e.next = null; // Help GC
529 e.value = null; // " "
530 size--;
531 } else {
532 int i = indexFor(e.hash, dest.length);
533 e.next = dest[i];
534 dest[i] = e;
535 }
536 e = next;
537 }
538 }
539 }
540
541 /**
542 * Copies all of the mappings from the specified map to this map.
543 * These mappings will replace any mappings that this map had for any
544 * of the keys currently in the specified map.
545 *
546 * @param m mappings to be stored in this map.
547 * @throws NullPointerException if the specified map is null.
548 */
549 public void putAll(Map<? extends K, ? extends V> m) {
550 int numKeysToBeAdded = m.size();
551 if (numKeysToBeAdded == 0)
552 return;
553
554 /*
555 * Expand the map if the map if the number of mappings to be added
556 * is greater than or equal to threshold. This is conservative; the
557 * obvious condition is (m.size() + size) >= threshold, but this
558 * condition could result in a map with twice the appropriate capacity,
559 * if the keys to be added overlap with the keys already in this map.
560 * By using the conservative calculation, we subject ourself
561 * to at most one extra resize.
562 */
563 if (numKeysToBeAdded > threshold) {
564 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
565 if (targetCapacity > MAXIMUM_CAPACITY)
566 targetCapacity = MAXIMUM_CAPACITY;
567 int newCapacity = table.length;
568 while (newCapacity < targetCapacity)
569 newCapacity <<= 1;
570 if (newCapacity > table.length)
571 resize(newCapacity);
572 }
573
574 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
575 put(e.getKey(), e.getValue());
576 }
577
578 /**
579 * Removes the mapping for a key from this weak hash map if it is present.
580 * More formally, if this map contains a mapping from key <tt>k</tt> to
581 * value <tt>v</tt> such that <code>(key==null ? k==null :
582 * key.equals(k))</code>, that mapping is removed. (The map can contain
583 * at most one such mapping.)
584 *
585 * <p>Returns the value to which this map previously associated the key,
586 * or <tt>null</tt> if the map contained no mapping for the key. A
587 * return value of <tt>null</tt> does not <i>necessarily</i> indicate
588 * that the map contained no mapping for the key; it's also possible
589 * that the map explicitly mapped the key to <tt>null</tt>.
590 *
591 * <p>The map will not contain a mapping for the specified key once the
592 * call returns.
593 *
594 * @param key key whose mapping is to be removed from the map
595 * @return the previous value associated with <tt>key</tt>, or
596 * <tt>null</tt> if there was no mapping for <tt>key</tt>
597 */
598 public V remove(Object key) {
599 Object k = maskNull(key);
600 int h = hash(k);
601 Entry<K,V>[] tab = getTable();
602 int i = indexFor(h, tab.length);
603 Entry<K,V> prev = tab[i];
604 Entry<K,V> e = prev;
605
606 while (e != null) {
607 Entry<K,V> next = e.next;
608 if (h == e.hash && eq(k, e.get())) {
609 modCount++;
610 size--;
611 if (prev == e)
612 tab[i] = next;
613 else
614 prev.next = next;
615 return e.value;
616 }
617 prev = e;
618 e = next;
619 }
620
621 return null;
622 }
623
624 /** Special version of remove needed by Entry set */
625 boolean removeMapping(Object o) {
626 if (!(o instanceof Map.Entry))
627 return false;
628 Entry<K,V>[] tab = getTable();
629 Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
630 Object k = maskNull(entry.getKey());
631 int h = hash(k);
632 int i = indexFor(h, tab.length);
633 Entry<K,V> prev = tab[i];
634 Entry<K,V> e = prev;
635
636 while (e != null) {
637 Entry<K,V> next = e.next;
638 if (h == e.hash && e.equals(entry)) {
639 modCount++;
640 size--;
641 if (prev == e)
642 tab[i] = next;
643 else
644 prev.next = next;
645 return true;
646 }
647 prev = e;
648 e = next;
649 }
650
651 return false;
652 }
653
654 /**
655 * Removes all of the mappings from this map.
656 * The map will be empty after this call returns.
657 */
658 public void clear() {
659 // clear out ref queue. We don't need to expunge entries
660 // since table is getting cleared.
661 while (queue.poll() != null)
662 ;
663
664 modCount++;
665 Arrays.fill(table, null);
666 size = 0;
667
668 // Allocation of array may have caused GC, which may have caused
669 // additional entries to go stale. Removing these entries from the
670 // reference queue will make them eligible for reclamation.
671 while (queue.poll() != null)
672 ;
673 }
674
675 /**
676 * Returns <tt>true</tt> if this map maps one or more keys to the
677 * specified value.
678 *
679 * @param value value whose presence in this map is to be tested
680 * @return <tt>true</tt> if this map maps one or more keys to the
681 * specified value
682 */
683 public boolean containsValue(Object value) {
684 if (value==null)
685 return containsNullValue();
686
687 Entry<K,V>[] tab = getTable();
688 for (int i = tab.length; i-- > 0;)
689 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
690 if (value.equals(e.value))
691 return true;
692 return false;
693 }
694
695 /**
696 * Special-case code for containsValue with null argument
697 */
698 private boolean containsNullValue() {
699 Entry<K,V>[] tab = getTable();
700 for (int i = tab.length; i-- > 0;)
701 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
702 if (e.value==null)
703 return true;
704 return false;
705 }
706
707 /**
708 * The entries in this hash table extend WeakReference, using its main ref
709 * field as the key.
710 */
711 private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
712 V value;
713 final int hash;
714 Entry<K,V> next;
715
716 /**
717 * Creates new entry.
718 */
719 Entry(Object key, V value,
720 ReferenceQueue<Object> queue,
721 int hash, Entry<K,V> next) {
722 super(key, queue);
723 this.value = value;
724 this.hash = hash;
725 this.next = next;
726 }
727
728 @SuppressWarnings("unchecked")
729 public K getKey() {
730 return (K) WeakHashMap.unmaskNull(get());
731 }
732
733 public V getValue() {
734 return value;
735 }
736
737 public V setValue(V newValue) {
738 V oldValue = value;
739 value = newValue;
740 return oldValue;
741 }
742
743 public boolean equals(Object o) {
744 if (!(o instanceof Map.Entry))
745 return false;
746 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
747 K k1 = getKey();
748 Object k2 = e.getKey();
749 if (k1 == k2 || (k1 != null && k1.equals(k2))) {
750 V v1 = getValue();
751 Object v2 = e.getValue();
752 if (v1 == v2 || (v1 != null && v1.equals(v2)))
753 return true;
754 }
755 return false;
756 }
757
758 public int hashCode() {
759 K k = getKey();
760 V v = getValue();
761 return ((k==null ? 0 : k.hashCode()) ^
762 (v==null ? 0 : v.hashCode()));
763 }
764
765 public String toString() {
766 return getKey() + "=" + getValue();
767 }
768 }
769
770 private abstract class HashIterator<T> implements Iterator<T> {
771 private int index;
772 private Entry<K,V> entry = null;
773 private Entry<K,V> lastReturned = null;
774 private int expectedModCount = modCount;
775
776 /**
777 * Strong reference needed to avoid disappearance of key
778 * between hasNext and next
779 */
780 private Object nextKey = null;
781
782 /**
783 * Strong reference needed to avoid disappearance of key
784 * between nextEntry() and any use of the entry
785 */
786 private Object currentKey = null;
787
788 HashIterator() {
789 index = isEmpty() ? 0 : table.length;
790 }
791
792 public boolean hasNext() {
793 Entry<K,V>[] t = table;
794
795 while (nextKey == null) {
796 Entry<K,V> e = entry;
797 int i = index;
798 while (e == null && i > 0)
799 e = t[--i];
800 entry = e;
801 index = i;
802 if (e == null) {
803 currentKey = null;
804 return false;
805 }
806 nextKey = e.get(); // hold on to key in strong ref
807 if (nextKey == null)
808 entry = entry.next;
809 }
810 return true;
811 }
812
813 /** The common parts of next() across different types of iterators */
814 protected Entry<K,V> nextEntry() {
815 if (modCount != expectedModCount)
816 throw new ConcurrentModificationException();
817 if (nextKey == null && !hasNext())
818 throw new NoSuchElementException();
819
820 lastReturned = entry;
821 entry = entry.next;
822 currentKey = nextKey;
823 nextKey = null;
824 return lastReturned;
825 }
826
827 public void remove() {
828 if (lastReturned == null)
829 throw new IllegalStateException();
830 if (modCount != expectedModCount)
831 throw new ConcurrentModificationException();
832
833 WeakHashMap.this.remove(currentKey);
834 expectedModCount = modCount;
835 lastReturned = null;
836 currentKey = null;
837 }
838
839 }
840
841 private class ValueIterator extends HashIterator<V> {
842 public V next() {
843 return nextEntry().value;
844 }
845 }
846
847 private class KeyIterator extends HashIterator<K> {
848 public K next() {
849 return nextEntry().getKey();
850 }
851 }
852
853 private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
854 public Map.Entry<K,V> next() {
855 return nextEntry();
856 }
857 }
858
859 // Views
860
861 private transient Set<Map.Entry<K,V>> entrySet = null;
862
863 /**
864 * Returns a {@link Set} view of the keys contained in this map.
865 * The set is backed by the map, so changes to the map are
866 * reflected in the set, and vice-versa. If the map is modified
867 * while an iteration over the set is in progress (except through
868 * the iterator's own <tt>remove</tt> operation), the results of
869 * the iteration are undefined. The set supports element removal,
870 * which removes the corresponding mapping from the map, via the
871 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
872 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
873 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
874 * operations.
875 */
876 public Set<K> keySet() {
877 Set<K> ks = keySet;
878 return (ks != null ? ks : (keySet = new KeySet()));
879 }
880
881 private class KeySet extends AbstractSet<K> {
882 public Iterator<K> iterator() {
883 return new KeyIterator();
884 }
885
886 public int size() {
887 return WeakHashMap.this.size();
888 }
889
890 public boolean contains(Object o) {
891 return containsKey(o);
892 }
893
894 public boolean remove(Object o) {
895 if (containsKey(o)) {
896 WeakHashMap.this.remove(o);
897 return true;
898 }
899 else
900 return false;
901 }
902
903 public void clear() {
904 WeakHashMap.this.clear();
905 }
906 }
907
908 /**
909 * Returns a {@link Collection} view of the values contained in this map.
910 * The collection is backed by the map, so changes to the map are
911 * reflected in the collection, and vice-versa. If the map is
912 * modified while an iteration over the collection is in progress
913 * (except through the iterator's own <tt>remove</tt> operation),
914 * the results of the iteration are undefined. The collection
915 * supports element removal, which removes the corresponding
916 * mapping from the map, via the <tt>Iterator.remove</tt>,
917 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
918 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
919 * support the <tt>add</tt> or <tt>addAll</tt> operations.
920 */
921 public Collection<V> values() {
922 Collection<V> vs = values;
923 return (vs != null) ? vs : (values = new Values());
924 }
925
926 private class Values extends AbstractCollection<V> {
927 public Iterator<V> iterator() {
928 return new ValueIterator();
929 }
930
931 public int size() {
932 return WeakHashMap.this.size();
933 }
934
935 public boolean contains(Object o) {
936 return containsValue(o);
937 }
938
939 public void clear() {
940 WeakHashMap.this.clear();
941 }
942 }
943
944 /**
945 * Returns a {@link Set} view of the mappings contained in this map.
946 * The set is backed by the map, so changes to the map are
947 * reflected in the set, and vice-versa. If the map is modified
948 * while an iteration over the set is in progress (except through
949 * the iterator's own <tt>remove</tt> operation, or through the
950 * <tt>setValue</tt> operation on a map entry returned by the
951 * iterator) the results of the iteration are undefined. The set
952 * supports element removal, which removes the corresponding
953 * mapping from the map, via the <tt>Iterator.remove</tt>,
954 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
955 * <tt>clear</tt> operations. It does not support the
956 * <tt>add</tt> or <tt>addAll</tt> operations.
957 */
958 public Set<Map.Entry<K,V>> entrySet() {
959 Set<Map.Entry<K,V>> es = entrySet;
960 return es != null ? es : (entrySet = new EntrySet());
961 }
962
963 private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
964 public Iterator<Map.Entry<K,V>> iterator() {
965 return new EntryIterator();
966 }
967
968 public boolean contains(Object o) {
969 if (!(o instanceof Map.Entry))
970 return false;
971 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
972 Entry<K,V> candidate = getEntry(e.getKey());
973 return candidate != null && candidate.equals(e);
974 }
975
976 public boolean remove(Object o) {
977 return removeMapping(o);
978 }
979
980 public int size() {
981 return WeakHashMap.this.size();
982 }
983
984 public void clear() {
985 WeakHashMap.this.clear();
986 }
987
988 private List<Map.Entry<K,V>> deepCopy() {
989 List<Map.Entry<K,V>> list = new ArrayList<>(size());
990 for (Map.Entry<K,V> e : this)
991 list.add(new AbstractMap.SimpleEntry<>(e));
992 return list;
993 }
994
995 public Object[] toArray() {
996 return deepCopy().toArray();
997 }
998
999 public <T> T[] toArray(T[] a) {
1000 return deepCopy().toArray(a);
1001 }
1002 }
1003 }