org.geotoolkit.util.collection
Class WeakValueHashMap<K,V>

Object
  AbstractMap<K,V>
      WeakValueHashMap<K,V>

Type Parameters:

K - The class of key elements.

V - The class of value elements.

All Implemented Interfaces:

Map<K,V>

@ThreadSafe
public class WeakValueHashMap<K,V>
extends AbstractMap<K,V>

A hashtable-based map implementation that uses weak references, leaving memory when an entry is not used anymore. An entry in a WeakValueHashMap will automatically be removed when its value is no longer in ordinary use. This class is similar to the standard WeakHashMap class provided in J2SE, except that weak references are hold on values instead of keys.

This class is convenient for avoiding the creation of duplicated elements, as in the example below:

K key = ...
V value;
synchronized (map) {
    value = map.get(key);
    if (value != null) {
        value = ...; // Create the value here.
        map.put(key, value);
    }
}

The calculation of a new value should be fast, because it is performed inside a synchronized statement blocking all other access to the map. This is okay if that particular map instance is not expected to be used in a highly concurrent environment.

Note that this class is not a cache, because the entries are discarded as soon as the garbage collector determines that they are no longer in use. If caching service are wanted, or if concurrency are wanted, consider using Cache instead.

Since:

2.0

Version:

3.13

Author:

Martin Desruisseaux (IRD, Geomatys)

See Also:

WeakHashMap, WeakHashSet, Cache

Module:

utility/geotk-utility (download)

View source code for this class

Nested Class Summary

Nested classes/interfaces inherited from class AbstractMap

AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>

Constructor Summary

WeakValueHashMap()
Creates a WeakValueHashMap.

WeakValueHashMap(int initialSize)
Creates a WeakValueHashMap of the requested size and default load factor.

WeakValueHashMap(Map<K,V> map)
Creates a new WeakValueHashMap populated with the contents of the provied map.

Method Summary

void	clear() Removes all of the elements 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 this value.
Set<Map.Entry<K,V>>	entrySet() Returns a set view of the mappings contained in this map.
V	get(Object key) Returns the value to which this map maps the specified key.
V	put(K key, V value) Associates the specified value with the specified key in this map.
V	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.

Methods inherited from class AbstractMap

clone, equals, hashCode, isEmpty, keySet, putAll, toString, values

Methods inherited from class Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

WeakValueHashMap

public WeakValueHashMap()

Creates a WeakValueHashMap.

WeakValueHashMap

public WeakValueHashMap(int initialSize)

Creates a WeakValueHashMap of the requested size and default load factor.

Parameters:

initialSize - The initial size.

WeakValueHashMap

public WeakValueHashMap(Map<K,V> map)

Creates a new WeakValueHashMap populated with the contents of the provied map.

Parameters:

map - Initial contents of the WeakValueHashMap.

Method Detail

size

public int size()

Returns the number of key-value mappings in this map.

Specified by:

size in interface Map<K,V>

Overrides:

size in class AbstractMap<K,V>

containsValue

public boolean containsValue(Object value)

Returns true if this map maps one or more keys to this value.

Specified by:

containsValue in interface Map<K,V>

Overrides:

containsValue in class AbstractMap<K,V>

Parameters:

value - value whose presence in this map is to be tested.

Returns:

true if this map maps one or more keys to this value.

containsKey

public boolean containsKey(Object key)

Returns true if this map contains a mapping for the specified key.

Specified by:

containsKey in interface Map<K,V>

Overrides:

containsKey in class AbstractMap<K,V>

Parameters:

key - key whose presence in this map is to be tested.

Returns:

true if this map contains a mapping for the specified key.

Throws:

NullPointerException - If key is null.

get

public V get(Object key)

Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key.

Specified by:

get in interface Map<K,V>

Overrides:

get in class AbstractMap<K,V>

Parameters:

key - Key whose associated value is to be returned.

Returns:

The value to which this map maps the specified key.

Throws:

NullPointerException - if the key is null.

put

public V put(K key,
             V value)

Associates the specified value with the specified key in this map. The value is associated using a WeakReference.

Specified by:

put in interface Map<K,V>

Overrides:

put in class AbstractMap<K,V>

Parameters:

key - key with which the specified value is to be associated.

value - value to be associated with the specified key.

Returns:

previous value associated with specified key, or null if there was no mapping for key.

Throws:

NullArgumentException - if the key or the value is null.

remove

public V remove(Object key)

Removes the mapping for this key from this map if present.

Specified by:

remove in interface Map<K,V>

Overrides:

remove in class AbstractMap<K,V>

Parameters:

key - key whose mapping is to be removed from the map.

Returns:

previous value associated with specified key, or null if there was no entry for key.

clear

public void clear()

Removes all of the elements from this map.

Specified by:

clear in interface Map<K,V>

Overrides:

clear in class AbstractMap<K,V>

entrySet

public Set<Map.Entry<K,V>> entrySet()

Returns a set view of the mappings contained in this map. Each element in this set is a Map.Entry.

Specified by:

entrySet in interface Map<K,V>

Specified by:

entrySet in class AbstractMap<K,V>

Returns:

a set view of the mappings contained in this map.