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 * The default threshold of capacity above which alternate hashing is
189 * used. Alternative hashing reduces the incidence of collisions due to
190 * weak hash code calculation.
191 * <p/>
192 * This value may be overridden by defining the system property
193 * {@code java.util.althashing.threshold} to an integer value. A property
194 * value of {@code 1} forces alternative hashing to be used at all times
195 * whereas {@code 2147483648 } ({@code Integer.MAX_VALUE}) value ensures
196 * that alternative hashing is never used.
197 */
198 static final int ALTERNATE_HASHING_THRESHOLD_DEFAULT = 0;
199
200 /**
201 * holds values which can't be initialized until after VM is booted.
202 */
203 private static class Holder {
204
205 /**
206 * Table capacity above which to switch to use alternate hashing.
207 */
208 static final int ALTERNATE_HASHING_THRESHOLD;
209
210 static {
211 String altThreshold = java.security.AccessController.doPrivileged(
212 new sun.security.action.GetPropertyAction(
213 "jdk.map.althashing.threshold"));
214
215 int threshold;
216 try {
217 threshold = (null != altThreshold)
218 ? Integer.parseInt(altThreshold)
219 : ALTERNATE_HASHING_THRESHOLD_DEFAULT;
220
221 if(threshold == -1) {
222 threshold = Integer.MAX_VALUE;
223 }
224
225 if(threshold < 0) {
226 throw new IllegalArgumentException("value must be positive integer.");
227 }
228 } catch(IllegalArgumentException failed) {
229 throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
230 }
231 ALTERNATE_HASHING_THRESHOLD = threshold;
232 }
233 }
234
235 /**
236 * If {@code true} then perform alternate hashing to reduce the incidence of
237 * collisions due to weak hash code calculation.
238 */
239 transient boolean useAltHashing;
240
241 /**
242 * A randomizing value associated with this instance that is applied to
243 * hash code of keys to make hash collisions harder to find.
244 */
245 transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);
246
247 @SuppressWarnings("unchecked")
248 private Entry<K,V>[] newTable(int n) {
249 return (Entry<K,V>[]) new Entry[n];
250 }
251
252 /**
253 * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
254 * capacity and the given load factor.
255 *
256 * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
257 * @param loadFactor The load factor of the <tt>WeakHashMap</tt>
258 * @throws IllegalArgumentException if the initial capacity is negative,
259 * or if the load factor is nonpositive.
260 */
261 public WeakHashMap(int initialCapacity, float loadFactor) {
262 if (initialCapacity < 0)
263 throw new IllegalArgumentException("Illegal Initial Capacity: "+
264 initialCapacity);
265 if (initialCapacity > MAXIMUM_CAPACITY)
266 initialCapacity = MAXIMUM_CAPACITY;
267
268 if (loadFactor <= 0 || Float.isNaN(loadFactor))
269 throw new IllegalArgumentException("Illegal Load factor: "+
270 loadFactor);
271 int capacity = 1;
272 while (capacity < initialCapacity)
273 capacity <<= 1;
274 table = newTable(capacity);
275 this.loadFactor = loadFactor;
276 threshold = (int)(capacity * loadFactor);
277 useAltHashing = sun.misc.VM.isBooted() &&
278 (capacity >= Holder.ALTERNATE_HASHING_THRESHOLD);
279 }
280
281 /**
282 * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
283 * capacity and the default load factor (0.75).
284 *
285 * @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
286 * @throws IllegalArgumentException if the initial capacity is negative
287 */
288 public WeakHashMap(int initialCapacity) {
289 this(initialCapacity, DEFAULT_LOAD_FACTOR);
290 }
291
292 /**
293 * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial
294 * capacity (16) and load factor (0.75).
295 */
296 public WeakHashMap() {
297 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
298 }
299
300 /**
301 * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the
302 * specified map. The <tt>WeakHashMap</tt> is created with the default
303 * load factor (0.75) and an initial capacity sufficient to hold the
304 * mappings in the specified map.
305 *
306 * @param m the map whose mappings are to be placed in this map
307 * @throws NullPointerException if the specified map is null
308 * @since 1.3
309 */
310 public WeakHashMap(Map<? extends K, ? extends V> m) {
311 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
312 DEFAULT_INITIAL_CAPACITY),
313 DEFAULT_LOAD_FACTOR);
314 putAll(m);
315 }
316
317 // internal utilities
318
319 /**
320 * Value representing null keys inside tables.
321 */
322 private static final Object NULL_KEY = new Object();
323
324 /**
325 * Use NULL_KEY for key if it is null.
326 */
327 private static Object maskNull(Object key) {
328 return (key == null) ? NULL_KEY : key;
329 }
330
331 /**
332 * Returns internal representation of null key back to caller as null.
333 */
334 static Object unmaskNull(Object key) {
335 return (key == NULL_KEY) ? null : key;
336 }
337
338 /**
339 * Checks for equality of non-null reference x and possibly-null y. By
340 * default uses Object.equals.
341 */
342 private static boolean eq(Object x, Object y) {
343 return x == y || x.equals(y);
344 }
345
346 /**
347 * Retrieve object hash code and applies a supplemental hash function to the
348 * result hash, which defends against poor quality hash functions. This is
349 * critical because HashMap uses power-of-two length hash tables, that
350 * otherwise encounter collisions for hashCodes that do not differ
351 * in lower bits. Note: Null keys always map to hash 0, thus index 0.
352 */
353 int hash(Object k) {
354 if (null == k) {
355 return 0;
356 }
357
358 int h;
359 if (useAltHashing) {
360 h = hashSeed;
361 if (k instanceof String) {
362 return h ^ sun.misc.Hashing.stringHash32((String) k);
363 } else {
364 h ^= k.hashCode();
365 }
366 } else {
367 h = k.hashCode();
368 }
369
370 // This function ensures that hashCodes that differ only by
371 // constant multiples at each bit position have a bounded
372 // number of collisions (approximately 8 at default load factor).
373 h ^= (h >>> 20) ^ (h >>> 12);
374 return h ^ (h >>> 7) ^ (h >>> 4);
375 }
376
377 /**
378 * Returns index for hash code h.
379 */
380 private static int indexFor(int h, int length) {
381 return h & (length-1);
382 }
383
384 /**
385 * Expunges stale entries from the table.
386 */
387 private void expungeStaleEntries() {
388 for (Object x; (x = queue.poll()) != null; ) {
389 synchronized (queue) {
390 @SuppressWarnings("unchecked")
391 Entry<K,V> e = (Entry<K,V>) x;
392 int i = indexFor(e.hash, table.length);
393
394 Entry<K,V> prev = table[i];
395 Entry<K,V> p = prev;
396 while (p != null) {
397 Entry<K,V> next = p.next;
398 if (p == e) {
399 if (prev == e)
400 table[i] = next;
401 else
402 prev.next = next;
403 // Must not null out e.next;
404 // stale entries may be in use by a HashIterator
405 e.value = null; // Help GC
406 size--;
407 break;
408 }
409 prev = p;
410 p = next;
411 }
412 }
413 }
414 }
415
416 /**
417 * Returns the table after first expunging stale entries.
418 */
419 private Entry<K,V>[] getTable() {
420 expungeStaleEntries();
421 return table;
422 }
423
424 /**
425 * Returns the number of key-value mappings in this map.
426 * This result is a snapshot, and may not reflect unprocessed
427 * entries that will be removed before next attempted access
428 * because they are no longer referenced.
429 */
430 public int size() {
431 if (size == 0)
432 return 0;
433 expungeStaleEntries();
434 return size;
435 }
436
437 /**
438 * Returns <tt>true</tt> if this map contains no key-value mappings.
439 * This result is a snapshot, and may not reflect unprocessed
440 * entries that will be removed before next attempted access
441 * because they are no longer referenced.
442 */
443 public boolean isEmpty() {
444 return size() == 0;
445 }
446
447 /**
448 * Returns the value to which the specified key is mapped,
449 * or {@code null} if this map contains no mapping for the key.
450 *
451 * <p>More formally, if this map contains a mapping from a key
452 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
453 * key.equals(k))}, then this method returns {@code v}; otherwise
454 * it returns {@code null}. (There can be at most one such mapping.)
455 *
456 * <p>A return value of {@code null} does not <i>necessarily</i>
457 * indicate that the map contains no mapping for the key; it's also
458 * possible that the map explicitly maps the key to {@code null}.
459 * The {@link #containsKey containsKey} operation may be used to
460 * distinguish these two cases.
461 *
462 * @see #put(Object, Object)
463 */
464 public V get(Object key) {
465 Object k = maskNull(key);
466 int h = hash(k);
467 Entry<K,V>[] tab = getTable();
468 int index = indexFor(h, tab.length);
469 Entry<K,V> e = tab[index];
470 while (e != null) {
471 if (e.hash == h && eq(k, e.get()))
472 return e.value;
473 e = e.next;
474 }
475 return null;
476 }
477
478 /**
479 * Returns <tt>true</tt> if this map contains a mapping for the
480 * specified key.
481 *
482 * @param key The key whose presence in this map is to be tested
483 * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;
484 * <tt>false</tt> otherwise
485 */
486 public boolean containsKey(Object key) {
487 return getEntry(key) != null;
488 }
489
490 /**
491 * Returns the entry associated with the specified key in this map.
492 * Returns null if the map contains no mapping for this key.
493 */
494 Entry<K,V> getEntry(Object key) {
495 Object k = maskNull(key);
496 int h = hash(k);
497 Entry<K,V>[] tab = getTable();
498 int index = indexFor(h, tab.length);
499 Entry<K,V> e = tab[index];
500 while (e != null && !(e.hash == h && eq(k, e.get())))
501 e = e.next;
502 return e;
503 }
504
505 /**
506 * Associates the specified value with the specified key in this map.
507 * If the map previously contained a mapping for this key, the old
508 * value is replaced.
509 *
510 * @param key key with which the specified value is to be associated.
511 * @param value value to be associated with the specified key.
512 * @return the previous value associated with <tt>key</tt>, or
513 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
514 * (A <tt>null</tt> return can also indicate that the map
515 * previously associated <tt>null</tt> with <tt>key</tt>.)
516 */
517 public V put(K key, V value) {
518 Object k = maskNull(key);
519 int h = hash(k);
520 Entry<K,V>[] tab = getTable();
521 int i = indexFor(h, tab.length);
522
523 for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
524 if (h == e.hash && eq(k, e.get())) {
525 V oldValue = e.value;
526 if (value != oldValue)
527 e.value = value;
528 return oldValue;
529 }
530 }
531
532 modCount++;
533 Entry<K,V> e = tab[i];
534 tab[i] = new Entry<>(k, value, queue, h, e);
535 if (++size >= threshold)
536 resize(tab.length * 2);
537 return null;
538 }
539
540 /**
541 * Rehashes the contents of this map into a new array with a
542 * larger capacity. This method is called automatically when the
543 * number of keys in this map reaches its threshold.
544 *
545 * If current capacity is MAXIMUM_CAPACITY, this method does not
546 * resize the map, but sets threshold to Integer.MAX_VALUE.
547 * This has the effect of preventing future calls.
548 *
549 * @param newCapacity the new capacity, MUST be a power of two;
550 * must be greater than current capacity unless current
551 * capacity is MAXIMUM_CAPACITY (in which case value
552 * is irrelevant).
553 */
554 void resize(int newCapacity) {
555 Entry<K,V>[] oldTable = getTable();
556 int oldCapacity = oldTable.length;
557 if (oldCapacity == MAXIMUM_CAPACITY) {
558 threshold = Integer.MAX_VALUE;
559 return;
560 }
561
562 Entry<K,V>[] newTable = newTable(newCapacity);
563 boolean oldAltHashing = useAltHashing;
564 useAltHashing |= sun.misc.VM.isBooted() &&
565 (newCapacity >= Holder.ALTERNATE_HASHING_THRESHOLD);
566 boolean rehash = oldAltHashing ^ useAltHashing;
567 transfer(oldTable, newTable, rehash);
568 table = newTable;
569
570 /*
571 * If ignoring null elements and processing ref queue caused massive
572 * shrinkage, then restore old table. This should be rare, but avoids
573 * unbounded expansion of garbage-filled tables.
574 */
575 if (size >= threshold / 2) {
576 threshold = (int)(newCapacity * loadFactor);
577 } else {
578 expungeStaleEntries();
579 transfer(newTable, oldTable, false);
580 table = oldTable;
581 }
582 }
583
584 /** Transfers all entries from src to dest tables */
585 private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest, boolean rehash) {
586 for (int j = 0; j < src.length; ++j) {
587 Entry<K,V> e = src[j];
588 src[j] = null;
589 while (e != null) {
590 Entry<K,V> next = e.next;
591 Object key = e.get();
592 if (key == null) {
593 e.next = null; // Help GC
594 e.value = null; // " "
595 size--;
596 } else {
597 if(rehash) {
598 e.hash = hash(key);
599 }
600 int i = indexFor(e.hash, dest.length);
601 e.next = dest[i];
602 dest[i] = e;
603 }
604 e = next;
605 }
606 }
607 }
608
609 /**
610 * Copies all of the mappings from the specified map to this map.
611 * These mappings will replace any mappings that this map had for any
612 * of the keys currently in the specified map.
613 *
614 * @param m mappings to be stored in this map.
615 * @throws NullPointerException if the specified map is null.
616 */
617 public void putAll(Map<? extends K, ? extends V> m) {
618 int numKeysToBeAdded = m.size();
619 if (numKeysToBeAdded == 0)
620 return;
621
622 /*
623 * Expand the map if the map if the number of mappings to be added
624 * is greater than or equal to threshold. This is conservative; the
625 * obvious condition is (m.size() + size) >= threshold, but this
626 * condition could result in a map with twice the appropriate capacity,
627 * if the keys to be added overlap with the keys already in this map.
628 * By using the conservative calculation, we subject ourself
629 * to at most one extra resize.
630 */
631 if (numKeysToBeAdded > threshold) {
632 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
633 if (targetCapacity > MAXIMUM_CAPACITY)
634 targetCapacity = MAXIMUM_CAPACITY;
635 int newCapacity = table.length;
636 while (newCapacity < targetCapacity)
637 newCapacity <<= 1;
638 if (newCapacity > table.length)
639 resize(newCapacity);
640 }
641
642 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
643 put(e.getKey(), e.getValue());
644 }
645
646 /**
647 * Removes the mapping for a key from this weak hash map if it is present.
648 * More formally, if this map contains a mapping from key <tt>k</tt> to
649 * value <tt>v</tt> such that <code>(key==null ? k==null :
650 * key.equals(k))</code>, that mapping is removed. (The map can contain
651 * at most one such mapping.)
652 *
653 * <p>Returns the value to which this map previously associated the key,
654 * or <tt>null</tt> if the map contained no mapping for the key. A
655 * return value of <tt>null</tt> does not <i>necessarily</i> indicate
656 * that the map contained no mapping for the key; it's also possible
657 * that the map explicitly mapped the key to <tt>null</tt>.
658 *
659 * <p>The map will not contain a mapping for the specified key once the
660 * call returns.
661 *
662 * @param key key whose mapping is to be removed from the map
663 * @return the previous value associated with <tt>key</tt>, or
664 * <tt>null</tt> if there was no mapping for <tt>key</tt>
665 */
666 public V remove(Object key) {
667 Object k = maskNull(key);
668 int h = hash(k);
669 Entry<K,V>[] tab = getTable();
670 int i = indexFor(h, tab.length);
671 Entry<K,V> prev = tab[i];
672 Entry<K,V> e = prev;
673
674 while (e != null) {
675 Entry<K,V> next = e.next;
676 if (h == e.hash && eq(k, e.get())) {
677 modCount++;
678 size--;
679 if (prev == e)
680 tab[i] = next;
681 else
682 prev.next = next;
683 return e.value;
684 }
685 prev = e;
686 e = next;
687 }
688
689 return null;
690 }
691
692 /** Special version of remove needed by Entry set */
693 boolean removeMapping(Object o) {
694 if (!(o instanceof Map.Entry))
695 return false;
696 Entry<K,V>[] tab = getTable();
697 Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
698 Object k = maskNull(entry.getKey());
699 int h = hash(k);
700 int i = indexFor(h, tab.length);
701 Entry<K,V> prev = tab[i];
702 Entry<K,V> e = prev;
703
704 while (e != null) {
705 Entry<K,V> next = e.next;
706 if (h == e.hash && e.equals(entry)) {
707 modCount++;
708 size--;
709 if (prev == e)
710 tab[i] = next;
711 else
712 prev.next = next;
713 return true;
714 }
715 prev = e;
716 e = next;
717 }
718
719 return false;
720 }
721
722 /**
723 * Removes all of the mappings from this map.
724 * The map will be empty after this call returns.
725 */
726 public void clear() {
727 // clear out ref queue. We don't need to expunge entries
728 // since table is getting cleared.
729 while (queue.poll() != null)
730 ;
731
732 modCount++;
733 Arrays.fill(table, null);
734 size = 0;
735
736 // Allocation of array may have caused GC, which may have caused
737 // additional entries to go stale. Removing these entries from the
738 // reference queue will make them eligible for reclamation.
739 while (queue.poll() != null)
740 ;
741 }
742
743 /**
744 * Returns <tt>true</tt> if this map maps one or more keys to the
745 * specified value.
746 *
747 * @param value value whose presence in this map is to be tested
748 * @return <tt>true</tt> if this map maps one or more keys to the
749 * specified value
750 */
751 public boolean containsValue(Object value) {
752 if (value==null)
753 return containsNullValue();
754
755 Entry<K,V>[] tab = getTable();
756 for (int i = tab.length; i-- > 0;)
757 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
758 if (value.equals(e.value))
759 return true;
760 return false;
761 }
762
763 /**
764 * Special-case code for containsValue with null argument
765 */
766 private boolean containsNullValue() {
767 Entry<K,V>[] tab = getTable();
768 for (int i = tab.length; i-- > 0;)
769 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
770 if (e.value==null)
771 return true;
772 return false;
773 }
774
775 /**
776 * The entries in this hash table extend WeakReference, using its main ref
777 * field as the key.
778 */
779 private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
780 V value;
781 int hash;
782 Entry<K,V> next;
783
784 /**
785 * Creates new entry.
786 */
787 Entry(Object key, V value,
788 ReferenceQueue<Object> queue,
789 int hash, Entry<K,V> next) {
790 super(key, queue);
791 this.value = value;
792 this.hash = hash;
793 this.next = next;
794 }
795
796 @SuppressWarnings("unchecked")
797 public K getKey() {
798 return (K) WeakHashMap.unmaskNull(get());
799 }
800
801 public V getValue() {
802 return value;
803 }
804
805 public V setValue(V newValue) {
806 V oldValue = value;
807 value = newValue;
808 return oldValue;
809 }
810
811 public boolean equals(Object o) {
812 if (!(o instanceof Map.Entry))
813 return false;
814 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
815 K k1 = getKey();
816 Object k2 = e.getKey();
817 if (k1 == k2 || (k1 != null && k1.equals(k2))) {
818 V v1 = getValue();
819 Object v2 = e.getValue();
820 if (v1 == v2 || (v1 != null && v1.equals(v2)))
821 return true;
822 }
823 return false;
824 }
825
826 public int hashCode() {
827 K k = getKey();
828 V v = getValue();
829 return ((k==null ? 0 : k.hashCode()) ^
830 (v==null ? 0 : v.hashCode()));
831 }
832
833 public String toString() {
834 return getKey() + "=" + getValue();
835 }
836 }
837
838 private abstract class HashIterator<T> implements Iterator<T> {
839 private int index;
840 private Entry<K,V> entry = null;
841 private Entry<K,V> lastReturned = null;
842 private int expectedModCount = modCount;
843
844 /**
845 * Strong reference needed to avoid disappearance of key
846 * between hasNext and next
847 */
848 private Object nextKey = null;
849
850 /**
851 * Strong reference needed to avoid disappearance of key
852 * between nextEntry() and any use of the entry
853 */
854 private Object currentKey = null;
855
856 HashIterator() {
857 index = isEmpty() ? 0 : table.length;
858 }
859
860 public boolean hasNext() {
861 Entry<K,V>[] t = table;
862
863 while (nextKey == null) {
864 Entry<K,V> e = entry;
865 int i = index;
866 while (e == null && i > 0)
867 e = t[--i];
868 entry = e;
869 index = i;
870 if (e == null) {
871 currentKey = null;
872 return false;
873 }
874 nextKey = e.get(); // hold on to key in strong ref
875 if (nextKey == null)
876 entry = entry.next;
877 }
878 return true;
879 }
880
881 /** The common parts of next() across different types of iterators */
882 protected Entry<K,V> nextEntry() {
883 if (modCount != expectedModCount)
884 throw new ConcurrentModificationException();
885 if (nextKey == null && !hasNext())
886 throw new NoSuchElementException();
887
888 lastReturned = entry;
889 entry = entry.next;
890 currentKey = nextKey;
891 nextKey = null;
892 return lastReturned;
893 }
894
895 public void remove() {
896 if (lastReturned == null)
897 throw new IllegalStateException();
898 if (modCount != expectedModCount)
899 throw new ConcurrentModificationException();
900
901 WeakHashMap.this.remove(currentKey);
902 expectedModCount = modCount;
903 lastReturned = null;
904 currentKey = null;
905 }
906
907 }
908
909 private class ValueIterator extends HashIterator<V> {
910 public V next() {
911 return nextEntry().value;
912 }
913 }
914
915 private class KeyIterator extends HashIterator<K> {
916 public K next() {
917 return nextEntry().getKey();
918 }
919 }
920
921 private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
922 public Map.Entry<K,V> next() {
923 return nextEntry();
924 }
925 }
926
927 // Views
928
929 private transient Set<Map.Entry<K,V>> entrySet = null;
930
931 /**
932 * Returns a {@link Set} view of the keys contained in this map.
933 * The set is backed by the map, so changes to the map are
934 * reflected in the set, and vice-versa. If the map is modified
935 * while an iteration over the set is in progress (except through
936 * the iterator's own <tt>remove</tt> operation), the results of
937 * the iteration are undefined. The set supports element removal,
938 * which removes the corresponding mapping from the map, via the
939 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
940 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
941 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
942 * operations.
943 */
944 public Set<K> keySet() {
945 Set<K> ks = keySet;
946 return (ks != null ? ks : (keySet = new KeySet()));
947 }
948
949 private class KeySet extends AbstractSet<K> {
950 public Iterator<K> iterator() {
951 return new KeyIterator();
952 }
953
954 public int size() {
955 return WeakHashMap.this.size();
956 }
957
958 public boolean contains(Object o) {
959 return containsKey(o);
960 }
961
962 public boolean remove(Object o) {
963 if (containsKey(o)) {
964 WeakHashMap.this.remove(o);
965 return true;
966 }
967 else
968 return false;
969 }
970
971 public void clear() {
972 WeakHashMap.this.clear();
973 }
974 }
975
976 /**
977 * Returns a {@link Collection} view of the values contained in this map.
978 * The collection is backed by the map, so changes to the map are
979 * reflected in the collection, and vice-versa. If the map is
980 * modified while an iteration over the collection is in progress
981 * (except through the iterator's own <tt>remove</tt> operation),
982 * the results of the iteration are undefined. The collection
983 * supports element removal, which removes the corresponding
984 * mapping from the map, via the <tt>Iterator.remove</tt>,
985 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
986 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
987 * support the <tt>add</tt> or <tt>addAll</tt> operations.
988 */
989 public Collection<V> values() {
990 Collection<V> vs = values;
991 return (vs != null) ? vs : (values = new Values());
992 }
993
994 private class Values extends AbstractCollection<V> {
995 public Iterator<V> iterator() {
996 return new ValueIterator();
997 }
998
999 public int size() {
1000 return WeakHashMap.this.size();
1001 }
1002
1003 public boolean contains(Object o) {
1004 return containsValue(o);
1005 }
1006
1007 public void clear() {
1008 WeakHashMap.this.clear();
1009 }
1010 }
1011
1012 /**
1013 * Returns a {@link Set} view of the mappings contained in this map.
1014 * The set is backed by the map, so changes to the map are
1015 * reflected in the set, and vice-versa. If the map is modified
1016 * while an iteration over the set is in progress (except through
1017 * the iterator's own <tt>remove</tt> operation, or through the
1018 * <tt>setValue</tt> operation on a map entry returned by the
1019 * iterator) the results of the iteration are undefined. The set
1020 * supports element removal, which removes the corresponding
1021 * mapping from the map, via the <tt>Iterator.remove</tt>,
1022 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
1023 * <tt>clear</tt> operations. It does not support the
1024 * <tt>add</tt> or <tt>addAll</tt> operations.
1025 */
1026 public Set<Map.Entry<K,V>> entrySet() {
1027 Set<Map.Entry<K,V>> es = entrySet;
1028 return es != null ? es : (entrySet = new EntrySet());
1029 }
1030
1031 private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
1032 public Iterator<Map.Entry<K,V>> iterator() {
1033 return new EntryIterator();
1034 }
1035
1036 public boolean contains(Object o) {
1037 if (!(o instanceof Map.Entry))
1038 return false;
1039 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
1040 Entry<K,V> candidate = getEntry(e.getKey());
1041 return candidate != null && candidate.equals(e);
1042 }
1043
1044 public boolean remove(Object o) {
1045 return removeMapping(o);
1046 }
1047
1048 public int size() {
1049 return WeakHashMap.this.size();
1050 }
1051
1052 public void clear() {
1053 WeakHashMap.this.clear();
1054 }
1055
1056 private List<Map.Entry<K,V>> deepCopy() {
1057 List<Map.Entry<K,V>> list = new ArrayList<>(size());
1058 for (Map.Entry<K,V> e : this)
1059 list.add(new AbstractMap.SimpleEntry<>(e));
1060 return list;
1061 }
1062
1063 public Object[] toArray() {
1064 return deepCopy().toArray();
1065 }
1066
1067 public <T> T[] toArray(T[] a) {
1068 return deepCopy().toArray(a);
1069 }
1070 }
1071 }