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Added in API level 1

IdentityHashMap

public class IdentityHashMap
extends AbstractMap<K, V> implements Map<K, V>, Serializable, Cloneable

java.lang.Object
   ↳ java.util.AbstractMap<K, V>
     ↳ java.util.IdentityHashMap<K, V>


IdentityHashMap is a variant on HashMap which tests equality by reference instead of equality by value. Basically, keys and values are compared for equality by checking if their references are equal rather than by calling the "equals" function.

Note: This class intentionally violates the general contract of Map's on comparing objects by their equals method.

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.

Summary

Public constructors

IdentityHashMap()

Creates an IdentityHashMap with default expected maximum size.

IdentityHashMap(int maxSize)

Creates an IdentityHashMap with the specified maximum size parameter.

IdentityHashMap(Map<? extends K, ? extends V> map)

Creates an IdentityHashMap using the given map as initial values.

Public methods

void clear()

Removes all elements from this map, leaving it empty.

Object clone()

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

boolean containsKey(Object key)

Returns whether this map contains the specified key.

boolean containsValue(Object value)

Returns whether this map contains the specified value.

Set<Entry<K, V>> entrySet()

Returns a set containing all of the mappings in this map.

boolean equals(Object object)

Compares this map with other objects.

V get(Object key)

Returns the value of the mapping with the specified key.

boolean isEmpty()

Returns whether this IdentityHashMap has no elements.

Set<K> keySet()

Returns a set of the keys contained in this map.

V put(K key, V value)

Maps the specified key to the specified value.

void putAll(Map<? extends K, ? extends V> map)

Copies all the mappings in the specified map to this map.

V remove(Object key)

Removes the mapping with the specified key from this map.

int size()

Returns the number of mappings in this IdentityHashMap.

Collection<V> values()

Returns a collection of the values contained in this map.

Inherited methods

From class java.util.AbstractMap
From class java.lang.Object
From interface java.util.Map

Public constructors

IdentityHashMap

Added in API level 1
IdentityHashMap ()

Creates an IdentityHashMap with default expected maximum size.

IdentityHashMap

Added in API level 1
IdentityHashMap (int maxSize)

Creates an IdentityHashMap with the specified maximum size parameter.

Parameters
maxSize int: The estimated maximum number of entries that will be put in this map.

IdentityHashMap

Added in API level 1
IdentityHashMap (Map<? extends K, ? extends V> map)

Creates an IdentityHashMap using the given map as initial values.

Parameters
map Map: A map of (key,value) pairs to copy into the IdentityHashMap.

Public methods

clear

Added in API level 1
void clear ()

Removes all elements from this map, leaving it empty.

See also:

clone

Added in API level 1
Object clone ()

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

Returns
Object a shallow copy of this IdentityHashMap.

See also:

containsKey

Added in API level 1
boolean containsKey (Object key)

Returns whether this map contains the specified key.

Parameters
key Object: the key to search for.
Returns
boolean true if this map contains the specified key, false otherwise.

containsValue

Added in API level 1
boolean containsValue (Object value)

Returns whether this map contains the specified value.

Parameters
value Object: the value to search for.
Returns
boolean true if this map contains the specified value, false otherwise.

entrySet

Added in API level 1
Set<Entry<K, V>> entrySet ()

Returns a set containing all of the mappings in this map. Each mapping is an instance of Map.Entry. As the set is backed by this map, changes in one will be reflected in the other.

Returns
Set<Entry<K, V>> a set of the mappings.

equals

Added in API level 1
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 neither necessarily symmetric (a.equals(b) implies b.equals(a)) nor transitive (a.equals(b) and b.equals(c) implies a.equals(c)).

Parameters
object Object: the object to compare to.
Returns
boolean whether the argument object is equal to this object.

get

Added in API level 1
V get (Object key)

Returns the value of the mapping with the specified key.

Parameters
key Object: the key.
Returns
V the value of the mapping with the specified key.

isEmpty

Added in API level 1
boolean isEmpty ()

Returns whether this IdentityHashMap has no elements.

Returns
boolean true if this IdentityHashMap has no elements, false otherwise.

See also:

keySet

Added in API level 1
Set<K> keySet ()

Returns a set of the keys contained in this map. The set is backed by this map so changes to one are reflected by the other. The set does not support adding.

Returns
Set<K> a set of the keys.

put

Added in API level 1
V put (K key, 
                V value)

Maps the specified key to the specified value.

Parameters
key K: the key.
value V: the value.
Returns
V the value of any previous mapping with the specified key or null if there was no such mapping.

putAll

Added in API level 1
void putAll (Map<? extends K, ? extends V> map)

Copies all the mappings in the specified map to this map. These mappings will replace all mappings that this map had for any of the keys currently in the given map.

Parameters
map Map: the map to copy mappings from.
Throws
NullPointerException if map is null.

remove

Added in API level 1
V remove (Object key)

Removes the mapping with the specified key from this map.

Parameters
key Object: the key of the mapping to remove.
Returns
V the value of the removed mapping, or null if no mapping for the specified key was found.

size

Added in API level 1
int size ()

Returns the number of mappings in this IdentityHashMap.

Returns
int the number of mappings in this IdentityHashMap.

values

Added in API level 1
Collection<V> values ()

Returns a collection of the values contained in this map. The collection is backed by this map so changes to one are reflected by the other. The collection supports remove, removeAll, retainAll and clear operations, and it does not support add or addAll operations.

This method returns a collection which is the subclass of AbstractCollection. The iterator method of this subclass returns a "wrapper object" over the iterator of map's entrySet(). The size method wraps the map's size method and the contains method wraps the map's containsValue method.

The collection is created when this method is called for the first time and returned in response to all subsequent calls. This method may return different collections when multiple concurrent calls occur, since no synchronization is performed.

Returns
Collection<V> a collection of the values contained in this map.
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