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