轉載地址：http://blog.csdn.net/wangxiaotongfan/article/details/51346063
這一章，我們對WeakHashMap進行學習。
我們先對WeakHashMap有個整體認識，然後再學習它的源碼，最後再通過實例來學會使用WeakHashMap。
第1部分 WeakHashMap介紹
第2部分 WeakHashMap數據結構
第3部分 WeakHashMap源碼解析(基於JDK1.6.0_45)
第4部分 WeakHashMap遍歷方式
第5部分 WeakHashMap示例
WeakHashMap 繼承於AbstractMap，實現了Map接口。
和HashMap一樣，WeakHashMap 也是一個散列表，它存儲的內容也是鍵值對(key-value)映射，而且鍵和值都可以是null。
不過WeakHashMap的鍵是“弱鍵”。在 WeakHashMap 中，當某個鍵不再正常使用時，會被從WeakHashMap中被自動移除。更精確地說，對於一個給定的鍵，其映射的存在並不阻止垃圾回收器對該鍵的丟棄，這就使該鍵成爲可終止的，被終止，然後被回收。某個鍵被終止時，它對應的鍵值對也就從映射中有效地移除了。
這個“弱鍵”的原理呢？大致上就是，通過WeakReference和ReferenceQueue實現的。 WeakHashMap的key是“弱鍵”，即是WeakReference類型的；ReferenceQueue是一個隊列，它會保存被GC回收的“弱鍵”。實現步驟是：
(01) 新建WeakHashMap，將“鍵值對”添加到WeakHashMap中。實際上，WeakHashMap是通過數組table保存Entry(鍵值對)；每一個Entry實際上是一個單向鏈表，即Entry是鍵值對鏈表。
(02) 當某“弱鍵”不再被其它對象引用，並被GC回收時。在GC回收該“弱鍵”時，這個“弱鍵”也同時會被添加到ReferenceQueue(queue)隊列中。
(03) 當下一次我們需要操作WeakHashMap時，會先同步table和queue。table中保存了全部的鍵值對，而queue中保存被GC回收的鍵值對；同步它們，就是刪除table中被GC回收的鍵值對。
這就是“弱鍵”如何被自動從WeakHashMap中刪除的步驟了。
和HashMap一樣，WeakHashMap是不同步的。可以使用 Collections.synchronizedMap 方法來構造同步的 WeakHashMap。
WeakHashMap共有4個構造函數,如下：
WeakHashMap()
WeakHashMap(int capacity)
WeakHashMap(int capacity, float loadFactor)
WeakHashMap(Map<? extends K, ? extends V> map)
void clear()
Object clone()
boolean containsKey(Object key)
boolean containsValue(Object value)
Set<Entry<K, V>> entrySet()
V get(Object key)
boolean isEmpty()
Set<K> keySet()
V put(K key, V value)
void putAll(Map<? extends K, ? extends V> map)
V remove(Object key)
int size()
Collection<V> values()
WeakHashMap的繼承關係如下
java.lang.Object
↳ java.util.AbstractMap<K, V>
↳ java.util.WeakHashMap<K, V>
public class WeakHashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V> {}
WeakHashMap與Map關係如下圖：
![]()
從圖中可以看出：
(01) WeakHashMap繼承於AbstractMap，並且實現了Map接口。
(02) WeakHashMap是哈希表，但是它的鍵是”弱鍵”。WeakHashMap中保護幾個重要的成員變量：table, size, threshold, loadFactor, modCount, queue。
table是一個Entry[]數組類型，而Entry實際上就是一個單向鏈表。哈希表的”key-value鍵值對”都是存儲在Entry數組中的。
size是Hashtable的大小，它是Hashtable保存的鍵值對的數量。
threshold是Hashtable的閾值，用於判斷是否需要調整Hashtable的容量。threshold的值=”容量*加載因子”。
loadFactor就是加載因子。
modCount是用來實現fail-fast機制的
queue保存的是“已被GC清除”的“弱引用的鍵”。
|
第3部分 WeakHashMap源碼解析(基於JDK1.6.0_45) |
下面對WeakHashMap的源碼進行說明
1 package java.util;
2 import java.lang.ref.WeakReference;
3 import java.lang.ref.ReferenceQueue;
4
5 public class WeakHashMap<K,V>
6 extends AbstractMap<K,V>
7 implements Map<K,V> {
8
9
10 private static final int DEFAULT_INITIAL_CAPACITY = 16;
11
12
13 private static final int MAXIMUM_CAPACITY = 1 << 30;
14
15
16 private static final float DEFAULT_LOAD_FACTOR = 0.75f;
17
18
19
20 private Entry[] table;
21
22
23 private int size;
24
25
26 private int threshold;
27
28
29 private final float loadFactor;
30
31
32
33 private final ReferenceQueue<K> queue = new ReferenceQueue<K>();
34
35
36 private volatile int modCount;
37
38
39 public WeakHashMap(int initialCapacity, float loadFactor) {
40 if (initialCapacity < 0)
41 throw new IllegalArgumentException("Illegal Initial Capacity: "+
42 initialCapacity);
43
44 if (initialCapacity > MAXIMUM_CAPACITY)
45 initialCapacity = MAXIMUM_CAPACITY;
46
47 if (loadFactor <= 0 || Float.isNaN(loadFactor))
48 throw new IllegalArgumentException("Illegal Load factor: "+
49 loadFactor);
50
51 int capacity = 1;
52 while (capacity < initialCapacity)
53 capacity <<= 1;
54
55 table = new Entry[capacity];
56
57 this.loadFactor = loadFactor;
58
59 threshold = (int)(capacity * loadFactor);
60 }
61
62
63 public WeakHashMap(int initialCapacity) {
64 this(initialCapacity, DEFAULT_LOAD_FACTOR);
65 }
66
67
68 public WeakHashMap() {
69 this.loadFactor = DEFAULT_LOAD_FACTOR;
70 threshold = (int)(DEFAULT_INITIAL_CAPACITY);
71 table = new Entry[DEFAULT_INITIAL_CAPACITY];
72 }
73
74
75 public WeakHashMap(Map<? extends K, ? extends V> m) {
76 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, 16),
77 DEFAULT_LOAD_FACTOR);
78
79 putAll(m);
80 }
81
82
83
84
85 private static final Object NULL_KEY = new Object();
86
87
88 private static Object maskNull(Object key) {
89 return (key == null ? NULL_KEY : key);
90 }
91
92
93 private static <K> K unmaskNull(Object key) {
94 return (K) (key == NULL_KEY ? null : key);
95 }
96
97
98 static boolean eq(Object x, Object y) {
99 return x == y || x.equals(y);
100 }
101
102
103
104 static int indexFor(int h, int length) {
105 return h & (length-1);
106 }
107
108
109
110
111
112
113
114 private void expungeStaleEntries() {
115 Entry<K,V> e;
116 while ( (e = (Entry<K,V>) queue.poll()) != null) {
117 int h = e.hash;
118 int i = indexFor(h, table.length);
119
120 Entry<K,V> prev = table[i];
121 Entry<K,V> p = prev;
122 while (p != null) {
123 Entry<K,V> next = p.next;
124 if (p == e) {
125 if (prev == e)
126 table[i] = next;
127 else
128 prev.next = next;
129 e.next = null;
130 e.value = null;
131 size--;
132 break;
133 }
134 prev = p;
135 p = next;
136 }
137 }
138 }
139
140
141 private Entry[] getTable() {
142
143 expungeStaleEntries();
144 return table;
145 }
146
147
148 public int size() {
149 if (size == 0)
150 return 0;
151
152 expungeStaleEntries();
153 return size;
154 }
155
156 public boolean isEmpty() {
157 return size() == 0;
158 }
159
160
161 public V get(Object key) {
162 Object k = maskNull(key);
163
164 int h = HashMap.hash(k.hashCode());
165 Entry[] tab = getTable();
166 int index = indexFor(h, tab.length);
167 Entry<K,V> e = tab[index];
168
169 while (e != null) {
170 if (e.hash == h && eq(k, e.get()))
171 return e.value;
172 e = e.next;
173 }
174 return null;
175 }
176
177
178 public boolean containsKey(Object key) {
179 return getEntry(key) != null;
180 }
181
182
183 Entry<K,V> getEntry(Object key) {
184 Object k = maskNull(key);
185 int h = HashMap.hash(k.hashCode());
186 Entry[] tab = getTable();
187 int index = indexFor(h, tab.length);
188 Entry<K,V> e = tab[index];
189 while (e != null && !(e.hash == h && eq(k, e.get())))
190 e = e.next;
191 return e;
192 }
193
194
195 public V put(K key, V value) {
196 K k = (K) maskNull(key);
197 int h = HashMap.hash(k.hashCode());
198 Entry[] tab = getTable();
199 int i = indexFor(h, tab.length);
200
201 for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
202
203 if (h == e.hash && eq(k, e.get())) {
204 V oldValue = e.value;
205 if (value != oldValue)
206 e.value = value;
207 return oldValue;
208 }
209 }
210
211
212 modCount++;
213 Entry<K,V> e = tab[i];
214 tab[i] = new Entry<K,V>(k, value, queue, h, e);
215 if (++size >= threshold)
216 resize(tab.length * 2);
217 return null;
218 }
219
220
221 void resize(int newCapacity) {
222 Entry[] oldTable = getTable();
223 int oldCapacity = oldTable.length;
224 if (oldCapacity == MAXIMUM_CAPACITY) {
225 threshold = Integer.MAX_VALUE;
226 return;
227 }
228
229
230
231 Entry[] newTable = new Entry[newCapacity];
232 transfer(oldTable, newTable);
233 table = newTable;
234
235 if (size >= threshold / 2) {
236 threshold = (int)(newCapacity * loadFactor);
237 } else {
238
239 expungeStaleEntries();
240 transfer(newTable, oldTable);
241 table = oldTable;
242 }
243 }
244
245
246 private void transfer(Entry[] src, Entry[] dest) {
247 for (int j = 0; j < src.length; ++j) {
248 Entry<K,V> e = src[j];
249 src[j] = null;
250 while (e != null) {
251 Entry<K,V> next = e.next;
252 Object key = e.get();
253 if (key == null) {
254 e.next = null;
255 e.value = null;
256 size--;
257 } else {
258 int i = indexFor(e.hash, dest.length);
259 e.next = dest[i];
260 dest[i] = e;
261 }
262 e = next;
263 }
264 }
265 }
266
267
268 public void putAll(Map<? extends K, ? extends V> m) {
269 int numKeysToBeAdded = m.size();
270 if (numKeysToBeAdded == 0)
271 return;
272
273
274
275 if (numKeysToBeAdded > threshold) {
276 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
277 if (targetCapacity > MAXIMUM_CAPACITY)
278 targetCapacity = MAXIMUM_CAPACITY;
279 int newCapacity = table.length;
280 while (newCapacity < targetCapacity)
281 newCapacity <<= 1;
282 if (newCapacity > table.length)
283 resize(newCapacity);
284 }
285
286
287 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
288 put(e.getKey(), e.getValue());
289 }
290
291
292 public V remove(Object key) {
293 Object k = maskNull(key);
294
295 int h = HashMap.hash(k.hashCode());
296 Entry[] tab = getTable();
297 int i = indexFor(h, tab.length);
298 Entry<K,V> prev = tab[i];
299 Entry<K,V> e = prev;
300
301
302
303 while (e != null) {
304 Entry<K,V> next = e.next;
305 if (h == e.hash && eq(k, e.get())) {
306 modCount++;
307 size--;
308 if (prev == e)
309 tab[i] = next;
310 else
311 prev.next = next;
312 return e.value;
313 }
314 prev = e;
315 e = next;
316 }
317
318 return null;
319 }
320
321
322 Entry<K,V> removeMapping(Object o) {
323 if (!(o instanceof Map.Entry))
324 return null;
325 Entry[] tab = getTable();
326 Map.Entry entry = (Map.Entry)o;
327 Object k = maskNull(entry.getKey());
328 int h = HashMap.hash(k.hashCode());
329 int i = indexFor(h, tab.length);
330 Entry<K,V> prev = tab[i];
331 Entry<K,V> e = prev;
332
333
334
335 while (e != null) {
336 Entry<K,V> next = e.next;
337 if (h == e.hash && e.equals(entry)) {
338 modCount++;
339 size--;
340 if (prev == e)
341 tab[i] = next;
342 else
343 prev.next = next;
344 return e;
345 }
346 prev = e;
347 e = next;
348 }
349
350 return null;
351 }
352
353
354 public void clear() {
355 while (queue.poll() != null)
356 ;
357
358 modCount++;
359 Entry[] tab = table;
360 for (int i = 0; i < tab.length; ++i)
361 tab[i] = null;
362 size = 0;
363
364 while (queue.poll() != null)
365 ;
366 }
367
368
369 public boolean containsValue(Object value) {
370
371 if (value==null)
372 return containsNullValue();
373
374
375 Entry[] tab = getTable();
376 for (int i = tab.length ; i-- > 0 ;)
377 for (Entry e = tab[i] ; e != null ; e = e.next)
378 if (value.equals(e.value))
379 return true;
380 return false;
381 }
382
383
384 private boolean containsNullValue() {
385 Entry[] tab = getTable();
386 for (int i = tab.length ; i-- > 0 ;)
387 for (Entry e = tab[i] ; e != null ; e = e.next)
388 if (e.value==null)
389 return true;
390 return false;
391 }
392
393
394
395
396 private static class Entry<K,V> extends WeakReference<K> implements Map.Entry<K,V> {
397 private V value;
398 private final int hash;
399
400 private Entry<K,V> next;
401
402
403 Entry(K key, V value,
404 ReferenceQueue<K> queue,
405 int hash, Entry<K,V> next) {
406 super(key, queue);
407 this.value = value;
408 this.hash = hash;
409 this.next = next;
410 }
411
412 public K getKey() {
413 return WeakHashMap.<K>unmaskNull(get());
414 }
415
416 public V getValue() {
417 return value;
418 }
419
420 public V setValue(V newValue) {
421 V oldValue = value;
422 value = newValue;
423 return oldValue;
424 }
425
426
427
428
429 public boolean equals(Object o) {
430 if (!(o instanceof Map.Entry))
431 return false;
432 Map.Entry e = (Map.Entry)o;
433 Object k1 = getKey();
434 Object k2 = e.getKey();
435 if (k1 == k2 || (k1 != null && k1.equals(k2))) {
436 Object v1 = getValue();
437 Object v2 = e.getValue();
438 if (v1 == v2 || (v1 != null && v1.equals(v2)))
439 return true;
440 }
441 return false;
442 }
443
444
445 public int hashCode() {
446 Object k = getKey();
447 Object v = getValue();
448 return ((k==null ? 0 : k.hashCode()) ^
449 (v==null ? 0 : v.hashCode()));
450 }
451
452 public String toString() {
453 return getKey() + "=" + getValue();
454 }
455 }
456
457
458
459 private abstract class HashIterator<T> implements Iterator<T> {
460
461 int index;
462
463 Entry<K,V> entry = null;
464
465 Entry<K,V> lastReturned = null;
466
467 int expectedModCount = modCount;
468
469
470 Object nextKey = null;
471
472
473 Object currentKey = null;
474
475
476 HashIterator() {
477 index = (size() != 0 ? table.length : 0);
478 }
479
480
481 public boolean hasNext() {
482 Entry[] t = table;
483
484
485
486
487 while (nextKey == null) {
488 Entry<K,V> e = entry;
489 int i = index;
490 while (e == null && i > 0)
491 e = t[--i];
492 entry = e;
493 index = i;
494 if (e == null) {
495 currentKey = null;
496 return false;
497 }
498 nextKey = e.get();
499 if (nextKey == null)
500 entry = entry.next;
501 }
502 return true;
503 }
504
505
506 protected Entry<K,V> nextEntry() {
507 if (modCount != expectedModCount)
508 throw new ConcurrentModificationException();
509 if (nextKey == null && !hasNext())
510 throw new NoSuchElementException();
511
512 lastReturned = entry;
513 entry = entry.next;
514 currentKey = nextKey;
515 nextKey = null;
516 return lastReturned;
517 }
518
519
520 public void remove() {
521 if (lastReturned == null)
522 throw new IllegalStateException();
523 if (modCount != expectedModCount)
524 throw new ConcurrentModificationException();
525
526 WeakHashMap.this.remove(currentKey);
527 expectedModCount = modCount;
528 lastReturned = null;
529 currentKey = null;
530 }
531
532 }
533
534
535 private class ValueIterator extends HashIterator<V> {
536 public V next() {
537 return nextEntry().value;
538 }
539 }
540
541
542 private class KeyIterator extends HashIterator<K> {
543 public K next() {
544 return nextEntry().getKey();
545 }
546 }
547
548
549 private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
550 public Map.Entry<K,V> next() {
551 return nextEntry();
552 }
553 }
554
555
556 private transient Set<Map.Entry<K,V>> entrySet = null;
557
558
559 public Set<K> keySet() {
560 Set<K> ks = keySet;
561 return (ks != null ? ks : (keySet = new KeySet()));
562 }
563
564
565
566 private class KeySet extends AbstractSet<K> {
567 public Iterator<K> iterator() {
568 return new KeyIterator();
569 }
570
571 public int size() {
572 return WeakHashMap.this.size();
573 }
574
575 public boolean contains(Object o) {
576 return containsKey(o);
577 }
578
579 public boolean remove(Object o) {
580 if (containsKey(o)) {
581 WeakHashMap.this.remove(o);
582 return true;
583 }
584 else
585 return false;
586 }
587
588 public void clear() {
589 WeakHashMap.this.clear();
590 }
591 }
592
593
594 public Collection<V> values() {
595 Collection<V> vs = values;
596 return (vs != null ? vs : (values = new Values()));
597 }
598
599
600
601
602 private class Values extends AbstractCollection<V> {
603 public Iterator<V> iterator() {
604 return new ValueIterator();
605 }
606
607 public int size() {
608 return WeakHashMap.this.size();
609 }
610
611 public boolean contains(Object o) {
612 return containsValue(o);
613 }
614
615 public void clear() {
616 WeakHashMap.this.clear();
617 }
618 }
619
620
621
622 public Set<Map.Entry<K,V>> entrySet() {
623 Set<Map.Entry<K,V>> es = entrySet;
624 return es != null ? es : (entrySet = new EntrySet());
625 }
626
627
628
629 private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
630 public Iterator<Map.Entry<K,V>> iterator() {
631 return new EntryIterator();
632 }
633
634
635 public boolean contains(Object o) {
636 if (!(o instanceof Map.Entry))
637 return false;
638 Map.Entry e = (Map.Entry)o;
639 Object k = e.getKey();
640 Entry candidate = getEntry(e.getKey());
641 return candidate != null && candidate.equals(e);
642 }
643
644
645 public boolean remove(Object o) {
646 return removeMapping(o) != null;
647 }
648
649
650 public int size() {
651 return WeakHashMap.this.size();
652 }
653
654
655 public void clear() {
656 WeakHashMap.this.clear();
657 }
658
659
660 private List<Map.Entry<K,V>> deepCopy() {
661 List<Map.Entry<K,V>> list = new ArrayList<Map.Entry<K,V>>(size());
662 for (Map.Entry<K,V> e : this)
663 list.add(new AbstractMap.SimpleEntry<K,V>(e));
664 return list;
665 }
666
667
668 public Object[] toArray() {
669 return deepCopy().toArray();
670 }
671
672
673 public <T> T[] toArray(T[] a) {
674 return deepCopy().toArray(a);
675 }
676 }
677 }
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說明：WeakHashMap和HashMap都是通過”拉鍊法”實現的散列表。它們的源碼絕大部分內容都一樣，這裏就只是對它們不同的部分就是說明。
WeakReference是“弱鍵”實現的哈希表。它這個“弱鍵”的目的就是：實現對“鍵值對”的動態回收。當“弱鍵”不再被使用到時，GC會回收它，WeakReference也會將“弱鍵”對應的鍵值對刪除。
“弱鍵”是一個“弱引用(WeakReference)”，在Java中，WeakReference和ReferenceQueue 是聯合使用的。在WeakHashMap中亦是如此：如果弱引用所引用的對象被垃圾回收，Java虛擬機就會把這個弱引用加入到與之關聯的引用隊列中。 接着，WeakHashMap會根據“引用隊列”，來刪除“WeakHashMap中已被GC回收的‘弱鍵’對應的鍵值對”。
另外，理解上面思想的重點是通過 expungeStaleEntries() 函數去理解。
第一步：根據entrySet()獲取WeakHashMap的“鍵值對”的Set集合。
第二步：通過Iterator迭代器遍歷“第一步”得到的集合。
Integer integ = null;
Iterator iter = map.entrySet().iterator();
while(iter.hasNext()) {
Map.Entry entry = (Map.Entry)iter.next();
key = (String)entry.getKey();
integ = (Integer)entry.getValue();
}
第一步：根據keySet()獲取WeakHashMap的“鍵”的Set集合。
第二步：通過Iterator迭代器遍歷“第一步”得到的集合。
String key = null;
Integer integ = null;
Iterator iter = map.keySet().iterator();
while (iter.hasNext()) {
key = (String)iter.next();
integ = (Integer)map.get(key);
}
第一步：根據value()獲取WeakHashMap的“值”的集合。
第二步：通過Iterator迭代器遍歷“第一步”得到的集合。
Integer value = null;
Collection c = map.values();
Iterator iter= c.iterator();
while (iter.hasNext()) {
value = (Integer)iter.next();
}
WeakHashMap遍歷測試程序如下：
1 import java.util.Map;
2 import java.util.Random;
3 import java.util.Iterator;
4 import java.util.WeakHashMap;
5 import java.util.HashSet;
6 import java.util.Map.Entry;
7 import java.util.Collection;
8
9
20 public class WeakHashMapIteratorTest {
21
22 public static void main(String[] args) {
23 int val = 0;
24 String key = null;
25 Integer value = null;
26 Random r = new Random();
27 WeakHashMap map = new WeakHashMap();
28
29 for (int i=0; i<12; i++) {
30
31 val = r.nextInt(100);
32
33 key = String.valueOf(val);
34 value = r.nextInt(5);
35
36 map.put(key, value);
37 System.out.println(" key:"+key+" value:"+value);
38 }
39
40 iteratorHashMapByEntryset(map) ;
41
42
43 iteratorHashMapByKeyset(map) ;
44
45
46 iteratorHashMapJustValues(map);
47 }
48
49
53 private static void iteratorHashMapByEntryset(WeakHashMap map) {
54 if (map == null)
55 return ;
56
57 System.out.println("\niterator WeakHashMap By entryset");
58 String key = null;
59 Integer integ = null;
60 Iterator iter = map.entrySet().iterator();
61 while(iter.hasNext()) {
62 Map.Entry entry = (Map.Entry)iter.next();
63
64 key = (String)entry.getKey();
65 integ = (Integer)entry.getValue();
66 System.out.println(key+" -- "+integ.intValue());
67 }
68 }
69
70
74 private static void iteratorHashMapByKeyset(WeakHashMap map) {
75 if (map == null)
76 return ;
77
78 System.out.println("\niterator WeakHashMap By keyset");
79 String key = null;
80 Integer integ = null;
81 Iterator iter = map.keySet().iterator();
82 while (iter.hasNext()) {
83 key = (String)iter.next();
84 integ = (Integer)map.get(key);
85 System.out.println(key+" -- "+integ.intValue());
86 }
87 }
88
89
90
93 private static void iteratorHashMapJustValues(WeakHashMap map) {
94 if (map == null)
95 return ;
96
97 Collection c = map.values();
98 Iterator iter= c.iterator();
99 while (iter.hasNext()) {
100 System.out.println(iter.next());
101 }
102 }
103 }
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下面通過實例來學習如何使用WeakHashMap
1 import java.util.Iterator;
2 import java.util.Map;
3 import java.util.WeakHashMap;
4 import java.util.Date;
5 import java.lang.ref.WeakReference;
6
7 /**
8 * @desc WeakHashMap測試程序
9 *
10 * @author skywang
11 * @email [email protected]
12 */
13 public class WeakHashMapTest {
14
15 public static void main(String[] args) throws Exception {
16 testWeakHashMapAPIs();
17 }
18
19 private static void testWeakHashMapAPIs() {
20
21 String w1 = new String("one");
22 String w2 = new String("two");
23 String w3 = new String("three");
24
25 Map wmap = new WeakHashMap();
26
27
28 wmap.put(w1, "w1");
29 wmap.put(w2, "w2");
30 wmap.put(w3, "w3");
31
32
33 System.out.printf("\nwmap:%s\n",wmap );
34
35
36 System.out.printf("contains key two : %s\n",wmap.containsKey("two"));
37 System.out.printf("contains key five : %s\n",wmap.containsKey("five"));
38
39
40 System.out.printf("contains value 0 : %s\n",wmap.containsValue(new Integer(0)));
41
42
43 wmap.remove("three");
44
45 System.out.printf("wmap: %s\n",wmap );
46
47
48
49
50
51
52
53 w1 = null;
54
55 System.gc();
56
57
58 Iterator iter = wmap.entrySet().iterator();
59 while (iter.hasNext()) {
60 Map.Entry en = (Map.Entry)iter.next();
61 System.out.printf("next : %s - %s\n",en.getKey(),en.getValue());
62 }
63
64 System.out.printf(" after gc WeakHashMap size:%s\n", wmap.size());
65 }
66 }
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運行結果：
wmap:{three=w3, one=w1, two=w2}
contains key two : true
contains key five : false
contains value 0 : false
wmap: {one=w1, two=w2}
next : two - w2
after gc WeakHashMap size:1
