Index (Frames) | Index (No Frames) | Package | Package Tree | Tree
java.util

Class WeakHashMap

java.lang.Object
|
+--java.util.AbstractMap
   |
   +--java.util.WeakHashMap

All Implemented Interfaces:

Map, Map


public class WeakHashMap

extends AbstractMap

implements Map

A weak hash map has only weak references to the key. This means that it allows the key to be garbage collected if it is not used otherwise. If this happens, the entry will eventually disappear from the map, asynchronously.

A weak hash map makes most sense when the keys doesn't override the equals method: If there is no other reference to the key nobody can ever look up the key in this table and so the entry can be removed. This table also works when the equals method is overloaded, such as String keys, but you should be prepared to deal with some entries disappearing spontaneously.

Other strange behaviors to be aware of: The size of this map may spontaneously shrink (even if you use a synchronized map and synchronize it); it behaves as if another thread removes entries from this table without synchronization. The entry set returned by entrySet has similar phenomenons: The size may spontaneously shrink, or an entry, that was in the set before, suddenly disappears.

A weak hash map is not meant for caches; use a normal map, with soft references as values instead, or try LinkedHashMap.

The weak hash map supports null values and null keys. The null key is never deleted from the map (except explictly of course). The performance of the methods are similar to that of a hash map.

The value objects are strongly referenced by this table. So if a value object maintains a strong reference to the key (either direct or indirect) the key will never be removed from this map. According to Sun, this problem may be fixed in a future release. It is not possible to do it with the jdk 1.2 reference model, though.

Since:Authors:See Also:

Constructor Summary

WeakHashMap()

Creates a new weak hash map with default load factor and default capacity.
WeakHashMap(int initialCapacity)

Creates a new weak hash map with default load factor and the given capacity.
WeakHashMap(int initialCapacity, float loadFactor)

Creates a new weak hash map with the given initial capacity and load factor.
WeakHashMap(java.util.Map m)

Construct a new WeakHashMap with the same mappings as the given map.

Method Summary

voidclear()

Clears all entries from this map.
booleancontainsKey(java.lang.Object key)

Tells if the map contains the given key.
booleancontainsValue(java.lang.Object value)

Returns true if the map contains at least one key which points to the specified object as a value.
java.util.SetentrySet()

Returns a set representation of the entries in this map.
java.lang.Objectget(java.lang.Object key)

Gets the value the key is mapped to.
booleanisEmpty()

Tells if the map is empty.
java.util.SetkeySet()

Returns a set representation of the keys in this map.
java.lang.Objectput(java.lang.Object key, java.lang.Object value)

Adds a new key/value mapping to this map.
voidputAll(java.util.Map m)

Puts all of the mappings from the given map into this one.
java.lang.Objectremove(java.lang.Object key)

Removes the key and the corresponding value from this map.
intsize()

Returns the size of this hash map.
java.util.Collectionvalues()

Returns a collection representation of the values in this map.

Constructor Details

WeakHashMap

public WeakHashMap()

Creates a new weak hash map with default load factor and default capacity.


WeakHashMap

public WeakHashMap(int initialCapacity)

Creates a new weak hash map with default load factor and the given capacity.

Parameters:

Throws:


WeakHashMap

public WeakHashMap(int initialCapacity, float loadFactor)

Creates a new weak hash map with the given initial capacity and load factor.

Parameters:

Throws:


WeakHashMap

public WeakHashMap(java.util.Map m)

Construct a new WeakHashMap with the same mappings as the given map. The WeakHashMap has a default load factor of 0.75.

Since:Parameters:

Throws:


Method Details

clear

public void clear()

Clears all entries from this map.


containsKey

public boolean containsKey(java.lang.Object key)

Tells if the map contains the given key. Note that the result may change spontanously, if the key was only weakly reachable.

Parameters:

Returns:


containsValue

public boolean containsValue(java.lang.Object value)

Returns true if the map contains at least one key which points to the specified object as a value. Note that the result may change spontanously, if its key was only weakly reachable.

Parameters:

Returns:


entrySet

public Set entrySet()

Returns a set representation of the entries in this map. This set will not have strong references to the keys, so they can be silently removed. The returned set has therefore the same strange behaviour (shrinking size(), disappearing entries) as this weak hash map.

Returns:


get

public Object get(java.lang.Object key)

Gets the value the key is mapped to.

Parameters:

Returns:


isEmpty

public boolean isEmpty()

Tells if the map is empty. Note that the result may change spontanously, if all of the keys were only weakly reachable.

Returns:


keySet

public Set keySet()

Returns a set representation of the keys in this map. This set will not have strong references to the keys, so they can be silently removed. The returned set has therefore the same strange behaviour (shrinking size(), disappearing entries) as this weak hash map.

Returns:


put

public Object put(java.lang.Object key, java.lang.Object value)

Adds a new key/value mapping to this map.

Parameters:

Returns:


putAll

public void putAll(java.util.Map m)

Puts all of the mappings from the given map into this one. If the key already exists in this map, its value is replaced.

Parameters:


remove

public Object remove(java.lang.Object key)

Removes the key and the corresponding value from this map.

Parameters:

Returns:


size

public int size()

Returns the size of this hash map. Note that the size() may shrink spontaneously, if the some of the keys were only weakly reachable.

Returns:


values

public Collection values()

Returns a collection representation of the values in this map. This collection will not have strong references to the keys, so mappings can be silently removed. The returned collection has therefore the same strange behaviour (shrinking size(), disappearing entries) as this weak hash map.

Returns: