1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

/**

* Value representing null keys inside tables.

*/

private static final Object NULL_KEY = new Object();

/**

* Use NULL_KEY for key if it is null.

*/

private static Object maskNull(Object key) {

return (key == null) ? NULL_KEY : key;

}

/**

* Returns index for hash code h.

*/

private static int indexFor(int h, int length) {

return h & (length-1);

}

public V get(Object key) {

// WeakHashMap允許null key與null value

// null key會被替換為一個虛擬值

Object k = maskNull(key);

int h = hash(k);

Entry<K,V>[] tab = getTable();

int index = indexFor(h, tab.length);

Entry<K,V> e = tab[index];

// 遍歷連結串列

while (e != null) {

if (e.hash == h && eq(k, e.get()))

return e.value;

e = e.next;

}

return null;

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

/**

* The table, resized as necessary. Length MUST Always be a power of two.

*/

Entry<K,V>[] table;

/**

* Reference queue for cleared WeakEntries

*/

private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

/**

* Expunges stale entries from the table.

*/

private void expungeStaleEntries() {

// 遍歷ReferenceQueue，然後清理table中無效的Entry

for (Object x; (x = queue.poll()) != null; ) {

synchronized (queue) {

@SuppressWarnings("unchecked")

Entry<K,V> e = (Entry<K,V>) x;

int i = indexFor(e.hash, table.length);

Entry<K,V> prev = table[i];

Entry<K,V> p = prev;

while (p != null) {

Entry<K,V> next = p.next;

if (p == e) {

if (prev == e)

table[i] = next;

else

prev.next = next;

// Must not null out e.next;

// stale entries may be in use by a HashIterator

e.value = null; // Help GC

size--;

break;

}

prev = p;

p = next;

}

}

}

}

/**

* Returns the table after first expunging stale entries.

*/

private Entry<K,V>[] getTable() {

expungeStaleEntries();

return table;

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

public V put(K key, V value) {

Object k = maskNull(key);

int h = hash(k);

Entry<K,V>[] tab = getTable();

int i = indexFor(h, tab.length);

for (Entry<K,V> e = tab[i]; e != null; e = e.next) {

if (h == e.hash && eq(k, e.get())) {

V oldValue = e.value;

if (value != oldValue)

e.value = value;

return oldValue;

}

}

modCount++;

Entry<K,V> e = tab[i];

// e被連線在new Entry的後面

tab[i] = new Entry<>(k, value, queue, h, e);

if (++size >= threshold)

resize(tab.length * 2);

return null;

}

public V remove(Object key) {

Object k = maskNull(key);

int h = hash(k);

Entry<K,V>[] tab = getTable();

int i = indexFor(h, tab.length);

Entry<K,V> prev = tab[i];

Entry<K,V> e = prev;

while (e != null) {

Entry<K,V> next = e.next;

if (h == e.hash && eq(k, e.get())) {

modCount++;

size--;

if (prev == e)

tab[i] = next;

else

prev.next = next;

return e.value;

}

prev = e;

e = next;

}

return null;

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

/**

* The entries in this hash table extend WeakReference, using its main ref

* field as the key.

*/

private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {

V value;

final int hash;

Entry<K,V> next;

/**

* Creates new entry.

*/

Entry(Object key, V value,

ReferenceQueue<Object> queue,

int hash, Entry<K,V> next) {

super(key, queue);

this.value = value;

this.hash  = hash;

this.next  = next;

}

@SuppressWarnings("unchecked")

public K getKey() {

return (K) WeakHashMap.unmaskNull(get());

}

public V getValue() {

return value;

}

public V setValue(V newValue) {

V oldValue = value;

value = newValue;

return oldValue;

}

public boolean equals(Object o) {

if (!(o instanceof Map.Entry))

return false;

Map.Entry<?,?> e = (Map.Entry<?,?>)o;

K k1 = getKey();

Object k2 = e.getKey();

if (k1 == k2 || (k1 != null && k1.equals(k2))) {

V v1 = getValue();

Object v2 = e.getValue();

if (v1 == v2 || (v1 != null && v1.equals(v2)))

return true;

}

return false;

}

public int hashCode() {

K k = getKey();

V v = getValue();

return Objects.hashCode(k) ^ Objects.hashCode(v);

}

public String toString() {

return getKey() + "=" + getValue();

}

}