java.util
Class WeakHashMap<K,V>

java.lang.Object
  extended by java.util.AbstractMap<K,V>
      extended by java.util.WeakHashMap<K,V>
All Implemented Interfaces:
Map<K,V>

public class WeakHashMap<K,V>
extends AbstractMap<K,V>

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:
1.2
See Also:
HashMap, WeakReference, LinkedHashMap

Nested Class Summary
 
Nested classes/interfaces inherited from class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>
 
Nested classes/interfaces inherited from interface java.util.Map
Map.Entry<K,V>
 
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(Map<? extends K,? extends V> m)
          Construct a new WeakHashMap with the same mappings as the given map.
 
Method Summary
 void clear()
          Clears all entries from this map.
 boolean containsKey(Object key)
          Tells if the map contains the given key.
 boolean containsValue(Object value)
          Returns true if the map contains at least one key which points to the specified object as a value.
 Set<Map.Entry<K,V>> entrySet()
          Returns a set representation of the entries in this map.
 V get(Object key)
          Gets the value the key is mapped to.
 boolean isEmpty()
          Tells if the map is empty.
 Set<K> keySet()
          Returns a set representation of the keys in this map.
 V put(K key, V value)
          Adds a new key/value mapping to this map.
 void putAll(Map<? extends K,? extends V> m)
          Puts all of the mappings from the given map into this one.
 V remove(Object key)
          Removes the key and the corresponding value from this map.
 int size()
          Returns the size of this hash map.
 Collection<V> values()
          Returns a collection representation of the values in this map.
 
Methods inherited from class java.util.AbstractMap
clone, equals, hashCode, toString
 
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
 

Constructor Detail

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:
initialCapacity - the initial capacity
Throws:
IllegalArgumentException - if initialCapacity is negative

WeakHashMap

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

Parameters:
initialCapacity - the initial capacity.
loadFactor - the load factor (see class description of HashMap).
Throws:
IllegalArgumentException - if initialCapacity is negative, or loadFactor is non-positive

WeakHashMap

public WeakHashMap(Map<? extends K,? extends V> m)
Construct a new WeakHashMap with the same mappings as the given map. The WeakHashMap has a default load factor of 0.75.

Parameters:
m - the map to copy
Throws:
NullPointerException - if m is null
Since:
1.3
Method Detail

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.

Specified by:
size in interface Map<K,V>
Overrides:
size in class AbstractMap<K,V>
Returns:
the number of entries in this hash map.
See Also:
Set.size()

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.

Specified by:
isEmpty in interface Map<K,V>
Overrides:
isEmpty in class AbstractMap<K,V>
Returns:
true, iff the map is empty.
See Also:
AbstractMap.size()

containsKey

public boolean containsKey(Object key)
Tells if the map contains the given key. Note that the result may change spontanously, if the key was only weakly reachable.

Specified by:
containsKey in interface Map<K,V>
Overrides:
containsKey in class AbstractMap<K,V>
Parameters:
key - the key to look for
Returns:
true, iff the map contains an entry for the given key.
See Also:
AbstractMap.containsValue(Object)

get

public V get(Object key)
Gets the value the key is mapped to.

Specified by:
get in interface Map<K,V>
Overrides:
get in class AbstractMap<K,V>
Parameters:
key - the key to look up
Returns:
the value the key was mapped to. It returns null if the key wasn't in this map, or if the mapped value was explicitly set to null.
See Also:
AbstractMap.containsKey(Object)

put

public V put(K key,
             V value)
Adds a new key/value mapping to this map.

Specified by:
put in interface Map<K,V>
Overrides:
put in class AbstractMap<K,V>
Parameters:
key - the key, may be null
value - the value, may be null
Returns:
the value the key was mapped to previously. It returns null if the key wasn't in this map, or if the mapped value was explicitly set to null.
See Also:
AbstractMap.containsKey(Object)

remove

public V remove(Object key)
Removes the key and the corresponding value from this map.

Specified by:
remove in interface Map<K,V>
Overrides:
remove in class AbstractMap<K,V>
Parameters:
key - the key. This may be null.
Returns:
the value the key was mapped to previously. It returns null if the key wasn't in this map, or if the mapped value was explicitly set to null.
See Also:
Iterator.remove()

entrySet

public Set<Map.Entry<K,V>> 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.

Specified by:
entrySet in interface Map<K,V>
Specified by:
entrySet in class AbstractMap<K,V>
Returns:
a set representation of the entries.
See Also:
Map.Entry

clear

public void clear()
Clears all entries from this map.

Specified by:
clear in interface Map<K,V>
Overrides:
clear in class AbstractMap<K,V>
See Also:
Set.clear()

containsValue

public boolean containsValue(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.

Specified by:
containsValue in interface Map<K,V>
Overrides:
containsValue in class AbstractMap<K,V>
Parameters:
value - the value to search for
Returns:
true if it is found in the set.
See Also:
AbstractMap.containsKey(Object)

keySet

public Set<K> 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.

Specified by:
keySet in interface Map<K,V>
Overrides:
keySet in class AbstractMap<K,V>
Returns:
a set representation of the keys.
See Also:
Set.iterator(), AbstractMap.size(), AbstractMap.containsKey(Object), AbstractMap.values()

putAll

public void putAll(Map<? extends K,? extends V> 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.

Specified by:
putAll in interface Map<K,V>
Overrides:
putAll in class AbstractMap<K,V>
Parameters:
m - the map to copy in
See Also:
AbstractMap.put(Object, Object)

values

public Collection<V> 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.

Specified by:
values in interface Map<K,V>
Overrides:
values in class AbstractMap<K,V>
Returns:
a collection representation of the values.
See Also:
Collection.iterator(), AbstractMap.size(), AbstractMap.containsValue(Object), AbstractMap.keySet()