java.lang.Object java.util.AbstractMap<K,V> java.util.WeakHashMap<K,V>
public class WeakHashMap<K,V>
A hashtable-based
Map
implementation with weak keys. An entry in a
WeakHashMap
will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently
from
than
other
Map
implementations.
Both null values and the null key are supported. This class has performance characteristics similar to those of the HashMap class, and has the same efficiency parameters of initial capacity and load factor.
Like most collection classes, this class is not synchronized. A synchronized
WeakHashMap
may be constructed using the
Collections.synchronizedMap
Collections.synchronizedMap
method.
This class is intended primarily for use with key objects whose equals methods test for object identity using the == operator. Once such a key is discarded it can never be recreated, so it is impossible to do a lookup of that key in a WeakHashMap at some later time and be surprised that its entry has been removed. This class will work perfectly well with key objects whose equals methods are not based upon object identity, such as String instances. With such recreatable key objects, however, the automatic removal of WeakHashMap entries whose keys have been discarded may prove to be confusing.
The behavior of the
WeakHashMap
class depends in part upon the actions of the garbage collector, so several familiar (though not required)
Map
invariants do not hold for this class. Because the garbage collector may discard keys at any time, a
WeakHashMap
may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a
WeakHashMap
instance and invoke none of its mutator methods, it is possible for the
size
method to return smaller values over time, for the
isEmpty
method to return
false
and then
true
, for the
containsKey
method to return
true
and later
false
for a given key, for the
get
method to return a value for a given key but later return
null
, for the
put
method to return
null
and the
remove
method to return
false
for a key that previously appeared to be in the map, and for successive examinations of the key set, the value
collection,
set,
and the entry set to yield successively smaller numbers of elements.
Each key object in a WeakHashMap is stored indirectly as the referent of a weak reference. Therefore a key will automatically be removed only after the weak references to it, both inside and outside of the map, have been cleared by the garbage collector.
Implementation note: The value objects in a WeakHashMap are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded. Note that a value object may refer indirectly to its key via the WeakHashMap itself; that is, a value object may strongly refer to some other key object whose associated value object, in turn, strongly refers to the key of the first value object. One way to deal with this is to wrap values themselves within WeakReferences before inserting, as in: m.put(key, new WeakReference(value)) , and then unwrapping upon each get .
The iterators returned by
the
iterator
method of the collections returned by
all of this class's "collection view methods" are
fail-fast
: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own
remove
method, the iterator will throw a
ConcurrentModificationException
or
add
methods, the iterator will throw a
ConcurrentModificationException
. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
This class is a member of the Java Collections Framework .
Nested Class Summary |
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Nested classes/interfaces inherited from class java.util. AbstractMap |
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AbstractMap.SimpleEntry < K , V >, AbstractMap.SimpleImmutableEntry < K , V |
Constructor Summary | |
---|---|
WeakHashMap
() Constructs a new, empty WeakHashMap with the default initial capacity (16) and |
|
WeakHashMap
(int initialCapacity) Constructs a new, empty WeakHashMap with the given initial capacity and the default load factor (0.75). |
|
WeakHashMap
(int initialCapacity, float loadFactor) Constructs a new, empty WeakHashMap with the given initial capacity and the given load factor. |
|
WeakHashMap
(
Map
<? extends
K
,? extends
V
Constructs a new WeakHashMap with the same mappings as the specified map. |
Method Summary | |
---|---|
void |
clear
() Removes all of the mappings from this map. |
boolean |
containsKey
(
Object
key) Returns true if this map contains a mapping for the specified key. |
boolean |
containsValue
(
Object
value) Returns true if this map maps one or more keys to the specified value. |
Set < Map.Entry < K , V |
entrySet
() Returns a Set |
V |
get
(
Object
Returns the value to which the specified key is mapped, |
boolean |
isEmpty
() Returns true if this map contains no key-value mappings. |
Set < K |
keySet
() Returns a Set |
V |
put
(
K
key,
V
value) Associates the specified value with the specified key in this map. |
void |
putAll
(
Map
<? extends
K
,? extends
V
Copies all of the mappings from the specified map to this |
V |
remove
(
Object
Removes the mapping for a |
int |
size
() Returns the number of key-value mappings in this map. |
Collection < V |
values
() Returns a Collection |
Methods inherited from class java.util. AbstractMap |
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clone , equals , hashCode , toString |
Methods inherited from class java.lang. Object |
---|
finalize , getClass , notify , notifyAll , wait , wait , wait |
Methods inherited from interface java.util. Map |
---|
equals , hashCode |
Constructor Detail |
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public WeakHashMap(int initialCapacity, float loadFactor)
public WeakHashMap(int initialCapacity)
public WeakHashMap()
public WeakHashMap(Map<? extends K,? extends V> m)> t)
Method Detail |
---|
public int size()
public boolean isEmpty()
public V get(Object key)
More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)
A return value of null does not
Returns the value to which the specified key is mapped in this weak hash map, or
null
if the map contains no mapping for this key. A return value of
null
does not
necessarily
indicate that the map contains no mapping for the key;
it's
it is
also possible that the map explicitly maps the key to
null.
null
.
The
containsKey
operation
containsKey
method
may be used to distinguish these two cases.
public boolean containsKey(Object key)
public V put(K key, V value)
public void putAll(Map<? extends K,? extends V> m)
public V remove(Object key)
Returns the value to which this map previously associated the key, or null if the map contained no mapping for the key. A return value of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null .
The map will not contain a mapping for the specified key once the call returns.
public void clear()
public boolean containsValue(Object value)
public Set<K> keySet()
public Collection<V> values()
public Set<Map.Entry<K,V>> entrySet()