org.apache.commons.collections
Class ReferenceMap

java.lang.Object
  |
  +--java.util.AbstractMap
        |
        +--org.apache.commons.collections.ReferenceMap

public class ReferenceMap

extends AbstractMap

Hashtable-based Map implementation that allows mappings to be removed by the garbage collector.

When you construct a ReferenceMap, you can specify what kind of references are used to store the map's keys and values. If non-hard references are used, then the garbage collector can remove mappings if a key or value becomes unreachable, or if the JVM's memory is running low. For information on how the different reference types behave, see Reference.

Different types of references can be specified for keys and values. The keys can be configured to be weak but the values hard, in which case this class will behave like a WeakHashMap. However, you can also specify hard keys and weak values, or any other combination. The default constructor uses hard keys and soft values, providing a memory-sensitive cache.

The algorithms used are basically the same as those in HashMap. In particular, you can specify a load factor and capacity to suit your needs. All optional Map operations are supported.

However, this Map implementation does not allow null elements. Attempting to add a null key or or a null value to the map will raise a NullPointerException.

As usual, this implementation is not synchronized. You can use Collections.synchronizedMap(java.util.Map) to provide synchronized access to a ReferenceMap.

Since:

2.1

Version:

$Id: ReferenceMap.java,v 1.7 2002/10/12 22:15:18 scolebourne Exp $

Author:

Paul Jack

See Also:

Reference

Field Summary

static int	HARD Constant indicating that hard references should be used.
static int	SOFT Constant indiciating that soft references should be used.
static int	WEAK Constant indicating that weak references should be used.

Constructor Summary

ReferenceMap()
Constructs a new ReferenceMap that will use hard references to keys and soft references to values.

ReferenceMap(int keyType, int valueType)
Constructs a new ReferenceMap that will use the specified types of references.

ReferenceMap(int keyType, int valueType, int capacity, float loadFactor)
Constructs a new ReferenceMap with the specified reference types, load factor and initial capacity.

Method Summary

void	clear() Clears this map.
boolean	containsKey(Object key) Returns true if this map contains the given key.
Set	entrySet() Returns a set view of this map's entries.
Object	get(Object key) Returns the value associated with the given key, if any.
boolean	isEmpty() Returns true if this map is empty.
Set	keySet() Returns a set view of this map's keys.
Object	put(Object key, Object value) Associates the given key with the given value.
Object	remove(Object key) Removes the key and its associated value from this map.
int	size() Returns the size of this map.
Collection	values() Returns a collection view of this map's values.

Methods inherited from class java.util.AbstractMap

containsValue, equals, hashCode, putAll, toString

Methods inherited from class java.lang.Object

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

Field Detail

HARD

public static final int HARD

Constant indicating that hard references should be used.

SOFT

public static final int SOFT

Constant indiciating that soft references should be used.

WEAK

public static final int WEAK

Constant indicating that weak references should be used.

Constructor Detail

ReferenceMap

public ReferenceMap()

Constructs a new ReferenceMap that will use hard references to keys and soft references to values.

ReferenceMap

public ReferenceMap(int keyType,
                    int valueType)

Constructs a new ReferenceMap that will use the specified types of references.

Parameters:

keyType - the type of reference to use for keys; must be HARD, SOFT, WEAK

valueType - the type of reference to use for values; must be HARD, SOFT, WEAK

ReferenceMap

public ReferenceMap(int keyType,
                    int valueType,
                    int capacity,
                    float loadFactor)

Constructs a new ReferenceMap with the specified reference types, load factor and initial capacity.

Parameters:

keyType - the type of reference to use for keys; must be HARD, SOFT, WEAK

valueType - the type of reference to use for values; must be HARD, SOFT, WEAK

capacity - the initial capacity for the map

loadFactor - the load factor for the map

Method Detail

size

public int size()

Returns the size of this map.

Overrides:

size in class AbstractMap

Returns:

the size of this map

isEmpty

public boolean isEmpty()

Returns true if this map is empty.

Overrides:

isEmpty in class AbstractMap

Returns:

true if this map is empty

containsKey

public boolean containsKey(Object key)

Returns true if this map contains the given key.

Overrides:

containsKey in class AbstractMap

Returns:

true if the given key is in this map

get

public Object get(Object key)

Returns the value associated with the given key, if any.

Overrides:

get in class AbstractMap

Returns:

the value associated with the given key, or null if the key maps to no value

put

public Object put(Object key,
                  Object value)

Associates the given key with the given value.

Neither the key nor the value may be null.

Overrides:

put in class AbstractMap

Parameters:

key - the key of the mapping

value - the value of the mapping

Returns:

the last value associated with that key, or null if no value was associated with the key

Throws:

NullPointerException - if either the key or value is null

remove

public Object remove(Object key)

Removes the key and its associated value from this map.

Overrides:

remove in class AbstractMap

Parameters:

key - the key to remove

Returns:

the value associated with that key, or null if the key was not in the map

clear

public void clear()

Clears this map.

Overrides:

clear in class AbstractMap

entrySet

public Set entrySet()

Returns a set view of this map's entries.

Overrides:

entrySet in class AbstractMap

Returns:

a set view of this map's entries

keySet

public Set keySet()

Returns a set view of this map's keys.

Overrides:

keySet in class AbstractMap

Returns:

a set view of this map's keys

values

public Collection values()

Returns a collection view of this map's values.

Overrides:

values in class AbstractMap

Returns:

a collection view of this map's values.