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 random mask value that is used for hashcode values associated with this
243 * instance to make hash collisions harder to find.
244 */
245 transient final int hashMask = sun.misc.Hashing.makeHashMask(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 = hashMask;
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 h ^= (h >>> 7) ^ (h >>> 4);
375
376 return h;
377 }
378
379 /**
380 * Returns index for hash code h.
381 */
382 private static int indexFor(int h, int length) {
383 return h & (length-1);
384 }
385
386 /**
387 * Expunges stale entries from the table.
388 */
389 private void expungeStaleEntries() {
390 for (Object x; (x = queue.poll()) != null; ) {
391 synchronized (queue) {
392 @SuppressWarnings("unchecked")
393 Entry<K,V> e = (Entry<K,V>) x;
394 int i = indexFor(e.hash, table.length);
395
396 Entry<K,V> prev = table[i];
397 Entry<K,V> p = prev;
398 while (p != null) {
399 Entry<K,V> next = p.next;
400 if (p == e) {
401 if (prev == e)
402 table[i] = next;
403 else
404 prev.next = next;
405 // Must not null out e.next;
406 // stale entries may be in use by a HashIterator
407 e.value = null; // Help GC
408 size--;
409 break;
410 }
411 prev = p;
412 p = next;
413 }
414 }
415 }
416 }
417
418 /**
419 * Returns the table after first expunging stale entries.
420 */
421 private Entry<K,V>[] getTable() {
422 expungeStaleEntries();
423 return table;
424 }
425
426 /**
427 * Returns the number of key-value mappings in this map.
428 * This result is a snapshot, and may not reflect unprocessed
429 * entries that will be removed before next attempted access
430 * because they are no longer referenced.
431 */
432 public int size() {
433 if (size == 0)
434 return 0;
435 expungeStaleEntries();
436 return size;
437 }
438
439 /**
440 * Returns <tt>true</tt> if this map contains no key-value mappings.
441 * This result is a snapshot, and may not reflect unprocessed
442 * entries that will be removed before next attempted access
443 * because they are no longer referenced.
444 */
445 public boolean isEmpty() {
446 return size() == 0;
447 }
448
449 /**
450 * Returns the value to which the specified key is mapped,
451 * or {@code null} if this map contains no mapping for the key.
452 *
453 * <p>More formally, if this map contains a mapping from a key
454 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
455 * key.equals(k))}, then this method returns {@code v}; otherwise
456 * it returns {@code null}. (There can be at most one such mapping.)
457 *
458 * <p>A return value of {@code null} does not <i>necessarily</i>
459 * indicate that the map contains no mapping for the key; it's also
460 * possible that the map explicitly maps the key to {@code null}.
461 * The {@link #containsKey containsKey} operation may be used to
462 * distinguish these two cases.
463 *
464 * @see #put(Object, Object)
465 */
466 public V get(Object key) {
467 Object k = maskNull(key);
468 int h = hash(k);
469 Entry<K,V>[] tab = getTable();
470 int index = indexFor(h, tab.length);
471 Entry<K,V> e = tab[index];
472 while (e != null) {
473 if (e.hash == h && eq(k, e.get()))
474 return e.value;
475 e = e.next;
476 }
477 return null;
478 }
479
480 /**
481 * Returns <tt>true</tt> if this map contains a mapping for the
482 * specified key.
483 *
484 * @param key The key whose presence in this map is to be tested
485 * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;
486 * <tt>false</tt> otherwise
487 */
488 public boolean containsKey(Object key) {
489 return getEntry(key) != null;
490 }
491
492 /**
493 * Returns the entry associated with the specified key in this map.
494 * Returns null if the map contains no mapping for this key.
495 */
496 Entry<K,V> getEntry(Object key) {
497 Object k = maskNull(key);
498 int h = hash(k);
499 Entry<K,V>[] tab = getTable();
500 int index = indexFor(h, tab.length);
501 Entry<K,V> e = tab[index];
502 while (e != null && !(e.hash == h && eq(k, e.get())))
503 e = e.next;
504 return e;
505 }
506
507 /**
508 * Associates the specified value with the specified key in this map.
509 * If the map previously contained a mapping for this key, the old
510 * value is replaced.
511 *
512 * @param key key with which the specified value is to be associated.
513 * @param value value to be associated with the specified key.
514 * @return the previous value associated with <tt>key</tt>, or
515 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
516 * (A <tt>null</tt> return can also indicate that the map
517 * previously associated <tt>null</tt> with <tt>key</tt>.)
518 */
519 public V put(K key, V value) {
520 Object k = maskNull(key);
521 int h = hash(k);
522 Entry<K,V>[] tab = getTable();
523 int i = indexFor(h, tab.length);
524
525 for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
526 if (h == e.hash && eq(k, e.get())) {
527 V oldValue = e.value;
528 if (value != oldValue)
529 e.value = value;
530 return oldValue;
531 }
532 }
533
534 modCount++;
535 Entry<K,V> e = tab[i];
536 tab[i] = new Entry<>(k, value, queue, h, e);
537 if (++size >= threshold)
538 resize(tab.length * 2);
539 return null;
540 }
541
542 /**
543 * Rehashes the contents of this map into a new array with a
544 * larger capacity. This method is called automatically when the
545 * number of keys in this map reaches its threshold.
546 *
547 * If current capacity is MAXIMUM_CAPACITY, this method does not
548 * resize the map, but sets threshold to Integer.MAX_VALUE.
549 * This has the effect of preventing future calls.
550 *
551 * @param newCapacity the new capacity, MUST be a power of two;
552 * must be greater than current capacity unless current
553 * capacity is MAXIMUM_CAPACITY (in which case value
554 * is irrelevant).
555 */
556 void resize(int newCapacity) {
557 Entry<K,V>[] oldTable = getTable();
558 int oldCapacity = oldTable.length;
559 if (oldCapacity == MAXIMUM_CAPACITY) {
560 threshold = Integer.MAX_VALUE;
561 return;
562 }
563
564 Entry<K,V>[] newTable = newTable(newCapacity);
565 boolean oldAltHashing = useAltHashing;
566 useAltHashing |= sun.misc.VM.isBooted() &&
567 (newCapacity >= Holder.ALTERNATE_HASHING_THRESHOLD);
568 boolean rehash = oldAltHashing ^ useAltHashing;
569 transfer(oldTable, newTable, rehash);
570 table = newTable;
571
572 /*
573 * If ignoring null elements and processing ref queue caused massive
574 * shrinkage, then restore old table. This should be rare, but avoids
575 * unbounded expansion of garbage-filled tables.
576 */
577 if (size >= threshold / 2) {
578 threshold = (int)(newCapacity * loadFactor);
579 } else {
580 expungeStaleEntries();
581 transfer(newTable, oldTable, false);
582 table = oldTable;
583 }
584 }
585
586 /** Transfers all entries from src to dest tables */
587 private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest, boolean rehash) {
588 for (int j = 0; j < src.length; ++j) {
589 Entry<K,V> e = src[j];
590 src[j] = null;
591 while (e != null) {
592 Entry<K,V> next = e.next;
593 Object key = e.get();
594 if (key == null) {
595 e.next = null; // Help GC
596 e.value = null; // " "
597 size--;
598 } else {
599 if(rehash) {
600 e.hash = hash(key);
601 }
602 int i = indexFor(e.hash, dest.length);
603 e.next = dest[i];
604 dest[i] = e;
605 }
606 e = next;
607 }
608 }
609 }
610
611 /**
612 * Copies all of the mappings from the specified map to this map.
613 * These mappings will replace any mappings that this map had for any
614 * of the keys currently in the specified map.
615 *
616 * @param m mappings to be stored in this map.
617 * @throws NullPointerException if the specified map is null.
618 */
619 public void putAll(Map<? extends K, ? extends V> m) {
620 int numKeysToBeAdded = m.size();
621 if (numKeysToBeAdded == 0)
622 return;
623
624 /*
625 * Expand the map if the map if the number of mappings to be added
626 * is greater than or equal to threshold. This is conservative; the
627 * obvious condition is (m.size() + size) >= threshold, but this
628 * condition could result in a map with twice the appropriate capacity,
629 * if the keys to be added overlap with the keys already in this map.
630 * By using the conservative calculation, we subject ourself
631 * to at most one extra resize.
632 */
633 if (numKeysToBeAdded > threshold) {
634 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
635 if (targetCapacity > MAXIMUM_CAPACITY)
636 targetCapacity = MAXIMUM_CAPACITY;
637 int newCapacity = table.length;
638 while (newCapacity < targetCapacity)
639 newCapacity <<= 1;
640 if (newCapacity > table.length)
641 resize(newCapacity);
642 }
643
644 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
645 put(e.getKey(), e.getValue());
646 }
647
648 /**
649 * Removes the mapping for a key from this weak hash map if it is present.
650 * More formally, if this map contains a mapping from key <tt>k</tt> to
651 * value <tt>v</tt> such that <code>(key==null ? k==null :
652 * key.equals(k))</code>, that mapping is removed. (The map can contain
653 * at most one such mapping.)
654 *
655 * <p>Returns the value to which this map previously associated the key,
656 * or <tt>null</tt> if the map contained no mapping for the key. A
657 * return value of <tt>null</tt> does not <i>necessarily</i> indicate
658 * that the map contained no mapping for the key; it's also possible
659 * that the map explicitly mapped the key to <tt>null</tt>.
660 *
661 * <p>The map will not contain a mapping for the specified key once the
662 * call returns.
663 *
664 * @param key key whose mapping is to be removed from the map
665 * @return the previous value associated with <tt>key</tt>, or
666 * <tt>null</tt> if there was no mapping for <tt>key</tt>
667 */
668 public V remove(Object key) {
669 Object k = maskNull(key);
670 int h = hash(k);
671 Entry<K,V>[] tab = getTable();
672 int i = indexFor(h, tab.length);
673 Entry<K,V> prev = tab[i];
674 Entry<K,V> e = prev;
675
676 while (e != null) {
677 Entry<K,V> next = e.next;
678 if (h == e.hash && eq(k, e.get())) {
679 modCount++;
680 size--;
681 if (prev == e)
682 tab[i] = next;
683 else
684 prev.next = next;
685 return e.value;
686 }
687 prev = e;
688 e = next;
689 }
690
691 return null;
692 }
693
694 /** Special version of remove needed by Entry set */
695 boolean removeMapping(Object o) {
696 if (!(o instanceof Map.Entry))
697 return false;
698 Entry<K,V>[] tab = getTable();
699 Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
700 Object k = maskNull(entry.getKey());
701 int h = hash(k);
702 int i = indexFor(h, tab.length);
703 Entry<K,V> prev = tab[i];
704 Entry<K,V> e = prev;
705
706 while (e != null) {
707 Entry<K,V> next = e.next;
708 if (h == e.hash && e.equals(entry)) {
709 modCount++;
710 size--;
711 if (prev == e)
712 tab[i] = next;
713 else
714 prev.next = next;
715 return true;
716 }
717 prev = e;
718 e = next;
719 }
720
721 return false;
722 }
723
724 /**
725 * Removes all of the mappings from this map.
726 * The map will be empty after this call returns.
727 */
728 public void clear() {
729 // clear out ref queue. We don't need to expunge entries
730 // since table is getting cleared.
731 while (queue.poll() != null)
732 ;
733
734 modCount++;
735 Arrays.fill(table, null);
736 size = 0;
737
738 // Allocation of array may have caused GC, which may have caused
739 // additional entries to go stale. Removing these entries from the
740 // reference queue will make them eligible for reclamation.
741 while (queue.poll() != null)
742 ;
743 }
744
745 /**
746 * Returns <tt>true</tt> if this map maps one or more keys to the
747 * specified value.
748 *
749 * @param value value whose presence in this map is to be tested
750 * @return <tt>true</tt> if this map maps one or more keys to the
751 * specified value
752 */
753 public boolean containsValue(Object value) {
754 if (value==null)
755 return containsNullValue();
756
757 Entry<K,V>[] tab = getTable();
758 for (int i = tab.length; i-- > 0;)
759 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
760 if (value.equals(e.value))
761 return true;
762 return false;
763 }
764
765 /**
766 * Special-case code for containsValue with null argument
767 */
768 private boolean containsNullValue() {
769 Entry<K,V>[] tab = getTable();
770 for (int i = tab.length; i-- > 0;)
771 for (Entry<K,V> e = tab[i]; e != null; e = e.next)
772 if (e.value==null)
773 return true;
774 return false;
775 }
776
777 /**
778 * The entries in this hash table extend WeakReference, using its main ref
779 * field as the key.
780 */
781 private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
782 V value;
783 int hash;
784 Entry<K,V> next;
785
786 /**
787 * Creates new entry.
788 */
789 Entry(Object key, V value,
790 ReferenceQueue<Object> queue,
791 int hash, Entry<K,V> next) {
792 super(key, queue);
793 this.value = value;
794 this.hash = hash;
795 this.next = next;
796 }
797
798 @SuppressWarnings("unchecked")
799 public K getKey() {
800 return (K) WeakHashMap.unmaskNull(get());
801 }
802
803 public V getValue() {
804 return value;
805 }
806
807 public V setValue(V newValue) {
808 V oldValue = value;
809 value = newValue;
810 return oldValue;
811 }
812
813 public boolean equals(Object o) {
814 if (!(o instanceof Map.Entry))
815 return false;
816 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
817 K k1 = getKey();
818 Object k2 = e.getKey();
819 if (k1 == k2 || (k1 != null && k1.equals(k2))) {
820 V v1 = getValue();
821 Object v2 = e.getValue();
822 if (v1 == v2 || (v1 != null && v1.equals(v2)))
823 return true;
824 }
825 return false;
826 }
827
828 public int hashCode() {
829 K k = getKey();
830 V v = getValue();
831 return ((k==null ? 0 : k.hashCode()) ^
832 (v==null ? 0 : v.hashCode()));
833 }
834
835 public String toString() {
836 return getKey() + "=" + getValue();
837 }
838 }
839
840 private abstract class HashIterator<T> implements Iterator<T> {
841 private int index;
842 private Entry<K,V> entry = null;
843 private Entry<K,V> lastReturned = null;
844 private int expectedModCount = modCount;
845
846 /**
847 * Strong reference needed to avoid disappearance of key
848 * between hasNext and next
849 */
850 private Object nextKey = null;
851
852 /**
853 * Strong reference needed to avoid disappearance of key
854 * between nextEntry() and any use of the entry
855 */
856 private Object currentKey = null;
857
858 HashIterator() {
859 index = isEmpty() ? 0 : table.length;
860 }
861
862 public boolean hasNext() {
863 Entry<K,V>[] t = table;
864
865 while (nextKey == null) {
866 Entry<K,V> e = entry;
867 int i = index;
868 while (e == null && i > 0)
869 e = t[--i];
870 entry = e;
871 index = i;
872 if (e == null) {
873 currentKey = null;
874 return false;
875 }
876 nextKey = e.get(); // hold on to key in strong ref
877 if (nextKey == null)
878 entry = entry.next;
879 }
880 return true;
881 }
882
883 /** The common parts of next() across different types of iterators */
884 protected Entry<K,V> nextEntry() {
885 if (modCount != expectedModCount)
886 throw new ConcurrentModificationException();
887 if (nextKey == null && !hasNext())
888 throw new NoSuchElementException();
889
890 lastReturned = entry;
891 entry = entry.next;
892 currentKey = nextKey;
893 nextKey = null;
894 return lastReturned;
895 }
896
897 public void remove() {
898 if (lastReturned == null)
899 throw new IllegalStateException();
900 if (modCount != expectedModCount)
901 throw new ConcurrentModificationException();
902
903 WeakHashMap.this.remove(currentKey);
904 expectedModCount = modCount;
905 lastReturned = null;
906 currentKey = null;
907 }
908
909 }
910
911 private class ValueIterator extends HashIterator<V> {
912 public V next() {
913 return nextEntry().value;
914 }
915 }
916
917 private class KeyIterator extends HashIterator<K> {
918 public K next() {
919 return nextEntry().getKey();
920 }
921 }
922
923 private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
924 public Map.Entry<K,V> next() {
925 return nextEntry();
926 }
927 }
928
929 // Views
930
931 private transient Set<Map.Entry<K,V>> entrySet = null;
932
933 /**
934 * Returns a {@link Set} view of the keys contained in this map.
935 * The set is backed by the map, so changes to the map are
936 * reflected in the set, and vice-versa. If the map is modified
937 * while an iteration over the set is in progress (except through
938 * the iterator's own <tt>remove</tt> operation), the results of
939 * the iteration are undefined. The set supports element removal,
940 * which removes the corresponding mapping from the map, via the
941 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
942 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
943 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
944 * operations.
945 */
946 public Set<K> keySet() {
947 Set<K> ks = keySet;
948 return (ks != null ? ks : (keySet = new KeySet()));
949 }
950
951 private class KeySet extends AbstractSet<K> {
952 public Iterator<K> iterator() {
953 return new KeyIterator();
954 }
955
956 public int size() {
957 return WeakHashMap.this.size();
958 }
959
960 public boolean contains(Object o) {
961 return containsKey(o);
962 }
963
964 public boolean remove(Object o) {
965 if (containsKey(o)) {
966 WeakHashMap.this.remove(o);
967 return true;
968 }
969 else
970 return false;
971 }
972
973 public void clear() {
974 WeakHashMap.this.clear();
975 }
976 }
977
978 /**
979 * Returns a {@link Collection} view of the values contained in this map.
980 * The collection is backed by the map, so changes to the map are
981 * reflected in the collection, and vice-versa. If the map is
982 * modified while an iteration over the collection is in progress
983 * (except through the iterator's own <tt>remove</tt> operation),
984 * the results of the iteration are undefined. The collection
985 * supports element removal, which removes the corresponding
986 * mapping from the map, via the <tt>Iterator.remove</tt>,
987 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
988 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
989 * support the <tt>add</tt> or <tt>addAll</tt> operations.
990 */
991 public Collection<V> values() {
992 Collection<V> vs = values;
993 return (vs != null) ? vs : (values = new Values());
994 }
995
996 private class Values extends AbstractCollection<V> {
997 public Iterator<V> iterator() {
998 return new ValueIterator();
999 }
1000
1001 public int size() {
1002 return WeakHashMap.this.size();
1003 }
1004
1005 public boolean contains(Object o) {
1006 return containsValue(o);
1007 }
1008
1009 public void clear() {
1010 WeakHashMap.this.clear();
1011 }
1012 }
1013
1014 /**
1015 * Returns a {@link Set} view of the mappings contained in this map.
1016 * The set is backed by the map, so changes to the map are
1017 * reflected in the set, and vice-versa. If the map is modified
1018 * while an iteration over the set is in progress (except through
1019 * the iterator's own <tt>remove</tt> operation, or through the
1020 * <tt>setValue</tt> operation on a map entry returned by the
1021 * iterator) the results of the iteration are undefined. The set
1022 * supports element removal, which removes the corresponding
1023 * mapping from the map, via the <tt>Iterator.remove</tt>,
1024 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
1025 * <tt>clear</tt> operations. It does not support the
1026 * <tt>add</tt> or <tt>addAll</tt> operations.
1027 */
1028 public Set<Map.Entry<K,V>> entrySet() {
1029 Set<Map.Entry<K,V>> es = entrySet;
1030 return es != null ? es : (entrySet = new EntrySet());
1031 }
1032
1033 private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
1034 public Iterator<Map.Entry<K,V>> iterator() {
1035 return new EntryIterator();
1036 }
1037
1038 public boolean contains(Object o) {
1039 if (!(o instanceof Map.Entry))
1040 return false;
1041 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
1042 Entry<K,V> candidate = getEntry(e.getKey());
1043 return candidate != null && candidate.equals(e);
1044 }
1045
1046 public boolean remove(Object o) {
1047 return removeMapping(o);
1048 }
1049
1050 public int size() {
1051 return WeakHashMap.this.size();
1052 }
1053
1054 public void clear() {
1055 WeakHashMap.this.clear();
1056 }
1057
1058 private List<Map.Entry<K,V>> deepCopy() {
1059 List<Map.Entry<K,V>> list = new ArrayList<>(size());
1060 for (Map.Entry<K,V> e : this)
1061 list.add(new AbstractMap.SimpleEntry<>(e));
1062 return list;
1063 }
1064
1065 public Object[] toArray() {
1066 return deepCopy().toArray();
1067 }
1068
1069 public <T> T[] toArray(T[] a) {
1070 return deepCopy().toArray(a);
1071 }
1072 }
1073 }