java.util
Class IdentityHashMap

java.lang.Object
  java.util.AbstractMap
      java.util.IdentityHashMap

All Implemented Interfaces:

Cloneable, Map, Serializable

public class IdentityHashMap

extends AbstractMap

implements Map, Serializable, Cloneable

IdentityHashMap This is a variant on HashMap which tests equality by reference instead of by value. Basically, keys and values are compared for equality by checking if their references are equal rather than by calling the "equals" function. IdentityHashMap uses open addressing (linear probing in particular) for collision resolution. This is different from HashMap which uses Chaining. Like HashMap, IdentityHashMap is not thread safe, so access by multiple threads must be synchronized by an external mechanism such as Collections.synchronizedMap. It accepts null keys and null values, including null key-value pairs.

See Also:

Serialized Form

Nested Class Summary

Nested classes inherited from class java.util.Map

Map.Entry

Constructor Summary

IdentityHashMap()
Create an IdentityHashMap with default maximum size

IdentityHashMap(int maxSize)
Create an IdentityHashMap with the given maximum size parameter

IdentityHashMap(Map map)
Create an IdentityHashMap using the given Map as initial values.

Method Summary

void	clear() Removes all elements from this Map, leaving it empty.
Object	clone() Answers a new IdentityHashMap with the same mappings and size as this one.
boolean	containsKey(Object key) Searches this Map for the specified key.
boolean	containsValue(Object value) Searches this Map for the specified value.
Set	entrySet() Answers a Set of the mappings contained in this IdentityHashMap.
boolean	equals(Object object) Compares this map with other objects.
Object	get(Object key) Answers the value of the mapping with the specified key.
boolean	isEmpty() Answers if this IdentityHashMap has no elements, a size of zero.
Set	keySet() Answers a Set of the keys contained in this IdentityHashMap.
Object	put(Object key, Object value) Maps the specified key to the specified value.
Object	remove(Object key) Removes a mapping with the specified key from this IdentityHashMap.
int	size() Answers the number of mappings in this IdentityHashMap.
Collection	values() Answers a Collection of the values contained in this IdentityHashMap.

Methods inherited from class java.util.AbstractMap

hashCode, putAll, toString

Methods inherited from class java.lang.Object

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

Methods inherited from interface java.util.Map

hashCode, putAll

Constructor Detail

IdentityHashMap

public IdentityHashMap()

Create an IdentityHashMap with default maximum size

IdentityHashMap

public IdentityHashMap(int maxSize)

Create an IdentityHashMap with the given maximum size parameter

Parameters:

maxSize - The estimated maximum number of entries that will be put in this map.

IdentityHashMap

public IdentityHashMap(Map map)

Create an IdentityHashMap using the given Map as initial values.

Parameters:

map - A map of (key,value) pairs to copy into the IdentityHashMap

Method Detail

clear

public void clear()

Removes all elements from this Map, leaving it empty.

Specified by:

clear in interface Map

Overrides:

clear in class AbstractMap

Throws:

UnsupportedOperationException - when removing from this Map is not supported

See Also:

isEmpty(), size()

containsKey

public boolean containsKey(Object key)

Searches this Map for the specified key.

Specified by:

containsKey in interface Map

Overrides:

containsKey in class AbstractMap

Parameters:

key - the object to search for

Returns:

true if key is a key of this Map, false otherwise

containsValue

public boolean containsValue(Object value)

Searches this Map for the specified value.

Specified by:

containsValue in interface Map

Overrides:

containsValue in class AbstractMap

Parameters:

value - the object to search for

Returns:

true if value is a value of this Map, false otherwise

get

public Object get(Object key)

Answers the value of the mapping with the specified key.

Specified by:

get in interface Map

Overrides:

get in class AbstractMap

Parameters:

key - the key

Returns:

the value of the mapping with the specified key

put

public Object put(Object key,
                  Object value)

Maps the specified key to the specified value.

Specified by:

put in interface Map

Overrides:

put in class AbstractMap

Parameters:

key - the key

value - the value

Returns:

the value of any previous mapping with the specified key or null if there was no mapping

remove

public Object remove(Object key)

Removes a mapping with the specified key from this IdentityHashMap.

Specified by:

remove in interface Map

Overrides:

remove in class AbstractMap

Parameters:

key - the key of the mapping to remove

Returns:

the value of the removed mapping, or null if key is not a key in this Map

entrySet

public Set entrySet()

Answers a Set of the mappings contained in this IdentityHashMap. Each element in the set is a Map.Entry. The set is backed by this Map so changes to one are relected by the other. The set does not support adding.

Specified by:

entrySet in interface Map

Specified by:

entrySet in class AbstractMap

Returns:

a Set of the mappings

keySet

public Set keySet()

Answers a Set of the keys contained in this IdentityHashMap. The set is backed by this IdentityHashMap so changes to one are relected by the other. The set does not support adding.

Specified by:

keySet in interface Map

Overrides:

keySet in class AbstractMap

Returns:

a Set of the keys

values

public Collection values()

Answers a Collection of the values contained in this IdentityHashMap. The collection is backed by this IdentityHashMap so changes to one are relected by the other. The collection does not support adding.

Specified by:

values in interface Map

Overrides:

values in class AbstractMap

Returns:

a Collection of the values

equals

public boolean equals(Object object)

Compares this map with other objects. This map is equal to another map is it represents the same set of mappings. With this map, two mappings are the same if both the key and the value are equal by reference. When compared with a map that is not an IdentityHashMap, the equals method is not necessarily symmetric (a.equals(b) implies b.equals(a)) nor transitive (a.equals(b) and b.equals(c) implies a.equals(c)).

Specified by:

equals in interface Map

Overrides:

equals in class AbstractMap

Parameters:

object - the object to compare with this object

Returns:

whether the argument object is equal to this object

See Also:

AbstractMap.hashCode()

clone

public Object clone()

Answers a new IdentityHashMap with the same mappings and size as this one.

Overrides:

clone in class AbstractMap

Returns:

a shallow copy of this IdentityHashMap

See Also:

Cloneable

isEmpty

public boolean isEmpty()

Answers if this IdentityHashMap has no elements, a size of zero.

Specified by:

isEmpty in interface Map

Overrides:

isEmpty in class AbstractMap

Returns:

true if this IdentityHashMap has no elements, false otherwise

See Also:

size()

size

public int size()

Answers the number of mappings in this IdentityHashMap.

Specified by:

size in interface Map

Overrides:

size in class AbstractMap

Returns:

the number of mappings in this IdentityHashMap