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 {@link 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
UNKNOWN: updated to 1.4 (partial 1.5)
| 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<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. |
Parameters: initialCapacity the initial capacity
Throws: IllegalArgumentException if initialCapacity is negative
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
Parameters: m the map to copy
Throws: NullPointerException if m is null
Since: 1.3
Parameters: key the key to look for
Returns: true, iff the map contains an entry for the given key.
Parameters: value the value to search for
Returns: true if it is found in the set.
Returns: a set representation of the entries.
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.
Returns: true, iff the map is empty.
Returns: a set representation of the keys.
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.
Parameters: m the map to copy in
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.
Returns: the number of entries in this hash map.
Returns: a collection representation of the values.