public class WeakHashMap extends AbstractMap implements Map
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 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 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 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 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.
HashMap,
WeakReference| Constructor and Description |
|---|
WeakHashMap()
Constructs a new, empty WeakHashMap with the default initial
capacity (16) and the default load factor (0.75).
|
WeakHashMap(int initialCapacity)
Constructs a new, empty WeakHashMap with the given initial
capacity and the default load factor, which is 0.75.
|
WeakHashMap(int initialCapacity,
float loadFactor)
Constructs a new, empty WeakHashMap with the given initial
capacity and the given load factor.
|
WeakHashMap(Map t)
Constructs a new WeakHashMap with the same mappings as the
specified Map.
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| Modifier and Type | Method and Description |
|---|---|
void |
clear()
Removes all 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 |
entrySet()
Returns a collection view of the mappings contained in this map.
|
Object |
get(Object key)
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.
|
boolean |
isEmpty()
Returns true if this map contains no key-value mappings.
|
Set |
keySet()
Returns a set view of the keys contained in this map.
|
Object |
put(Object key,
Object value)
Associates the specified value with the specified key in this map.
|
void |
putAll(Map m)
Copies all of the mappings from the specified map to this map These
mappings will replace any mappings that this map had for any of the
keys currently in the specified map.
|
Object |
remove(Object key)
Removes the mapping for this key from this map if present.
|
int |
size()
Returns the number of key-value mappings in this map.
|
Collection |
values()
Returns a collection view of the values contained in this map.
|
clone, equals, hashCode, toStringpublic WeakHashMap(int initialCapacity,
float loadFactor)
initialCapacity - The initial capacity of the WeakHashMaploadFactor - The load factor of the WeakHashMapIllegalArgumentException - If the initial capacity is negative,
or if the load factor is nonpositive.public WeakHashMap(int initialCapacity)
initialCapacity - The initial capacity of the WeakHashMapIllegalArgumentException - If the initial capacity is negative.public WeakHashMap()
public WeakHashMap(Map t)
t - the map whose mappings are to be placed in this map.NullPointerException - if the specified map is null.public int size()
size in interface Mapsize in class AbstractMappublic boolean isEmpty()
isEmpty in interface MapisEmpty in class AbstractMappublic Object get(Object key)
get in interface Mapget in class AbstractMapkey - the key whose associated value is to be returned.put(Object, Object)public boolean containsKey(Object key)
containsKey in interface MapcontainsKey in class AbstractMapkey - The key whose presence in this map is to be testedpublic Object put(Object key, Object value)
put in interface Mapput in class AbstractMapkey - key with which the specified value is to be associated.value - value to be associated with the specified key.public void putAll(Map m)
putAll in interface MapputAll in class AbstractMapm - mappings to be stored in this map.NullPointerException - if the specified map is null.public Object remove(Object key)
remove in interface Mapremove in class AbstractMapkey - key whose mapping is to be removed from the map.public void clear()
clear in interface Mapclear in class AbstractMappublic boolean containsValue(Object value)
containsValue in interface MapcontainsValue in class AbstractMapvalue - value whose presence in this map is to be tested.public Set keySet()
keySet in interface MapkeySet in class AbstractMappublic Collection values()
values in interface Mapvalues in class AbstractMappublic Set entrySet()
entrySet in interface MapentrySet in class AbstractMapMap.EntryCopyright © 2012. All Rights Reserved.