java.util
public class WeakHashMap<K,V> extends AbstractMap<K,V>
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.
HashMap
,
WeakReference
,
LinkedHashMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>
Constructor and Description |
---|
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.
|
Modifier and Type | Method and Description |
---|---|
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.
|
clone, equals, hashCode, toString
public WeakHashMap()
public WeakHashMap(int initialCapacity)
initialCapacity
- the initial capacityIllegalArgumentException
- if initialCapacity is negativepublic WeakHashMap(int initialCapacity, float loadFactor)
initialCapacity
- the initial capacity.loadFactor
- the load factor (see class description of HashMap).IllegalArgumentException
- if initialCapacity is negative, or
loadFactor is non-positivepublic WeakHashMap(Map<? extends K,? extends V> m)
m
- the map to copyNullPointerException
- if m is nullpublic int size()
size
in interface Map<K,V>
size
in class AbstractMap<K,V>
Set.size()
public boolean isEmpty()
isEmpty
in interface Map<K,V>
isEmpty
in class AbstractMap<K,V>
AbstractMap.size()
public boolean containsKey(Object key)
containsKey
in interface Map<K,V>
containsKey
in class AbstractMap<K,V>
key
- the key to look forAbstractMap.containsValue(Object)
public V get(Object key)
get
in interface Map<K,V>
get
in class AbstractMap<K,V>
key
- the key to look upAbstractMap.containsKey(Object)
public V put(K key, V value)
put
in interface Map<K,V>
put
in class AbstractMap<K,V>
key
- the key, may be nullvalue
- the value, may be nullAbstractMap.containsKey(Object)
public V remove(Object key)
remove
in interface Map<K,V>
remove
in class AbstractMap<K,V>
key
- the key. This may be null.Iterator.remove()
public Set<Map.Entry<K,V>> entrySet()
public void clear()
clear
in interface Map<K,V>
clear
in class AbstractMap<K,V>
Set.clear()
public boolean containsValue(Object value)
containsValue
in interface Map<K,V>
containsValue
in class AbstractMap<K,V>
value
- the value to search forAbstractMap.containsKey(Object)
public Set<K> keySet()
keySet
in interface Map<K,V>
keySet
in class AbstractMap<K,V>
Set.iterator()
,
AbstractMap.size()
,
AbstractMap.containsKey(Object)
,
AbstractMap.values()
public void putAll(Map<? extends K,? extends V> m)
putAll
in interface Map<K,V>
putAll
in class AbstractMap<K,V>
m
- the map to copy inAbstractMap.put(Object, Object)
public Collection<V> values()
values
in interface Map<K,V>
values
in class AbstractMap<K,V>
Collection.iterator()
,
AbstractMap.size()
,
AbstractMap.containsValue(Object)
,
AbstractMap.keySet()