org.apache.commons.collections.map

Class ReferenceIdentityMap

public class ReferenceIdentityMap extends AbstractReferenceMap implements Serializable

A Map implementation that allows mappings to be removed by the garbage collector and matches keys and values based on == not equals().

When you construct a ReferenceIdentityMap, 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 default constructor uses hard keys and soft values, providing a memory-sensitive cache.

This map is similar to ReferenceMap. It differs in that keys and values in this class are compared using ==.

This map will violate the detail of various Map and map view contracts. As a general rule, don't compare this map to other maps.

This java.util.Map Map implementation does not allow null elements. Attempting to add a null key or value to the map will raise a NullPointerException.

This implementation is not synchronized. You can use java.util.Collections#synchronizedMap to provide synchronized access to a ReferenceIdentityMap. Remember that synchronization will not stop the garbage collecter removing entries.

All the available iterators can be reset back to the start by casting to ResettableIterator and calling reset().

Note that ReferenceIdentityMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. The simplest approach is to wrap this map using java.util.Collections#synchronizedMap. This class may throw exceptions when accessed by concurrent threads without synchronization.

Since: Commons Collections 3.0 (previously in main package v2.1)

Version: $Revision: 348007 $ $Date: 2005-11-21 22:52:57 +0000 (Mon, 21 Nov 2005) $

Author: Stephen Colebourne

See Also: java.lang.ref.Reference

Constructor Summary
ReferenceIdentityMap()
Constructs a new ReferenceIdentityMap that will use hard references to keys and soft references to values.
ReferenceIdentityMap(int keyType, int valueType)
Constructs a new ReferenceIdentityMap that will use the specified types of references.
ReferenceIdentityMap(int keyType, int valueType, boolean purgeValues)
Constructs a new ReferenceIdentityMap that will use the specified types of references.
ReferenceIdentityMap(int keyType, int valueType, int capacity, float loadFactor)
Constructs a new ReferenceIdentityMap with the specified reference types, load factor and initial capacity.
ReferenceIdentityMap(int keyType, int valueType, int capacity, float loadFactor, boolean purgeValues)
Constructs a new ReferenceIdentityMap with the specified reference types, load factor and initial capacity.
Method Summary
protected inthash(Object key)
Gets the hash code for the key specified.
protected inthashEntry(Object key, Object value)
Gets the hash code for a MapEntry.
protected booleanisEqualKey(Object key1, Object key2)
Compares two keys for equals.
protected booleanisEqualValue(Object value1, Object value2)
Compares two values for equals.

Constructor Detail

ReferenceIdentityMap

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

ReferenceIdentityMap

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

Parameters: keyType the type of reference to use for keys; must be ReferenceIdentityMap, ReferenceIdentityMap, ReferenceIdentityMap valueType the type of reference to use for values; must be ReferenceIdentityMap, ReferenceIdentityMap, ReferenceIdentityMap

ReferenceIdentityMap

public ReferenceIdentityMap(int keyType, int valueType, boolean purgeValues)
Constructs a new ReferenceIdentityMap that will use the specified types of references.

Parameters: keyType the type of reference to use for keys; must be ReferenceIdentityMap, ReferenceIdentityMap, ReferenceIdentityMap valueType the type of reference to use for values; must be ReferenceIdentityMap, ReferenceIdentityMap, ReferenceIdentityMap purgeValues should the value be automatically purged when the key is garbage collected

ReferenceIdentityMap

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

Parameters: keyType the type of reference to use for keys; must be ReferenceIdentityMap, ReferenceIdentityMap, ReferenceIdentityMap valueType the type of reference to use for values; must be ReferenceIdentityMap, ReferenceIdentityMap, ReferenceIdentityMap capacity the initial capacity for the map loadFactor the load factor for the map

ReferenceIdentityMap

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

Parameters: keyType the type of reference to use for keys; must be ReferenceIdentityMap, ReferenceIdentityMap, ReferenceIdentityMap valueType the type of reference to use for values; must be ReferenceIdentityMap, ReferenceIdentityMap, ReferenceIdentityMap capacity the initial capacity for the map loadFactor the load factor for the map purgeValues should the value be automatically purged when the key is garbage collected

Method Detail

hash

protected int hash(Object key)
Gets the hash code for the key specified.

This implementation uses the identity hash code.

Parameters: key the key to get a hash code for

Returns: the hash code

hashEntry

protected int hashEntry(Object key, Object value)
Gets the hash code for a MapEntry.

This implementation uses the identity hash code.

Parameters: key the key to get a hash code for, may be null value the value to get a hash code for, may be null

Returns: the hash code, as per the MapEntry specification

isEqualKey

protected boolean isEqualKey(Object key1, Object key2)
Compares two keys for equals.

This implementation converts the key from the entry to a real reference before comparison and uses ==.

Parameters: key1 the first key to compare passed in from outside key2 the second key extracted from the entry via entry.key

Returns: true if equal by identity

isEqualValue

protected boolean isEqualValue(Object value1, Object value2)
Compares two values for equals.

This implementation uses ==.

Parameters: value1 the first value to compare passed in from outside value2 the second value extracted from the entry via getValue()

Returns: true if equal by identity

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