public class IdentityHashMap<K, V>

  1. Object
  2. AbstractMap<K, V>
  3. IdentityHashMap

ImplementsMap<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.

Constructors

public IdentityHashMap()Creates an IdentityHashMap with default expected maximum size.
public IdentityHashMap(int maxSize)Creates an IdentityHashMap with the specified maximum size parameter.
public IdentityHashMap(Map<? extends K, ? extends V> map)Creates an IdentityHashMap using the given map as initial values.

Methods

public void clear()Removes all elements from this map, leaving it empty.
public boolean containsKey(Object key)Returns whether this map contains the specified key.
public boolean containsValue(Object value)Returns whether this map contains the specified value.
public V get(Object key)Returns the value of the mapping with the specified key.
public V put(K key, V value)Maps the specified key to the specified value.
public void putAll(Map<? extends K, ? extends V> map)Copies all the mappings in the specified map to this map.
public V remove(Object key)Removes the mapping with the specified key from this map.
public Set<Map.Entry<K, V>> entrySet()Returns a set containing all of the mappings in this map.
public Set<K> keySet()Returns a set of the keys contained in this map.
public Collection<V> values()Returns a collection of the values contained in this map.
public boolean equals(Object object)Compares this map with other objects.
public boolean isEmpty()Returns whether this IdentityHashMap has no elements.
public int size()Returns the number of mappings in this IdentityHashMap.

Inherited nested types

From AbstractMap

AbstractMap.SimpleImmutableEntry, AbstractMap.SimpleEntry

From Map

Map.Entry

Inherited methods

From AbstractMap

hashCode, toString

From Object

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

From Map

getOrDefault, putIfAbsent, remove, replace, replace, forEach, replaceAll, computeIfAbsent, computeIfPresent, compute, merge

Constructor details

IdentityHashMap

public IdentityHashMap()
Creates an IdentityHashMap with default expected maximum size.

IdentityHashMap

public 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

public IdentityHashMap(Map<? extends K, ? extends V> map)
Creates an IdentityHashMap using the given map as initial values.

Parameters

map Map<? extends K, ? extends V>
A map of (key,value) pairs to copy into the IdentityHashMap.

Method details

clear

public void clear()
Removes all elements from this map, leaving it empty.

Throws

UnsupportedOperationException
if removing from this map is not supported.

See also

containsKey

public boolean containsKey(Object key)
Returns whether this map contains the specified key.

Parameters

key Object
the key to search for.

Returns

true if this map contains the specified key, false otherwise.

containsValue

public boolean containsValue(Object value)
Returns whether this map contains the specified value.

Parameters

value Object
the value to search for.

Returns

true if this map contains the specified value, false otherwise.

get

public V get(Object key)
Returns the value of the mapping with the specified key.

Parameters

key Object
the key.

Returns

the value of the mapping with the specified key.

put

public V put(K key, V value)
Maps the specified key to the specified value.

Parameters

key K
the key.
value V
the value.

Returns

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

putAll

public 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<? extends K, ? extends V>
the map to copy mappings from.

Throws

NullPointerException
if map is null.
UnsupportedOperationException
if adding to this map is not supported.
ClassCastException
if the class of a key or value is inappropriate for this map.
IllegalArgumentException
if a key or value cannot be added to this map.

remove

public 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

the value of the removed mapping, or null if no mapping for the specified key was found.

entrySet

public Set<Map.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

a set of the mappings.

keySet

public 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

a set of the keys.

values

public 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

a collection of the values contained in this map.

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 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

whether the argument object is equal to this object.

isEmpty

public boolean isEmpty()
Returns whether this IdentityHashMap has no elements.

Returns

true if this IdentityHashMap has no elements, false otherwise.

See also

size

public int size()
Returns the number of mappings in this IdentityHashMap.

Returns

the number of mappings in this IdentityHashMap.