Java.util.IdentityHashMap Class


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Introduction

The java.util.IdentityHashMap class implements the Map interface with a hash table, using reference-equality in place of object-equality when comparing keys (and values).Following are the important points about IdentityHashMap:

  • This class provides all of the optional map operations, and permits null values and the null key.

  • This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

  • In an IdentityHashMap, two keys k1 and k2 are considered equal if and only if (k1==k2), while in Map implementations (like HashMap) two keys k1 and k2 are considered equal if and only if (k1==null ? k2==null : k1.equals(k2)).

Class declaration

Following is the declaration for java.util.IdentityHashMap class:

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

Class constructors

S.N. Constructor & Description
1

IdentityHashMap()

This constructs a new, empty identity hash map with a default expected maximum size (21).

2

IdentityHashMap(int expectedMaxSize)

This constructs a new, empty map with the specified expected maximum size.

3

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

This constructs a new identity hash map containing the keys-value mappings in the specified map.

Class methods

S.N. Method & Description
1

void clear()

This method removes all of the mappings from this map.

2

Object clone()

This method returns a shallow copy of this identity hash map: the keys and values themselves are not cloned.

3

boolean containsKey(Object key)

This method tests whether the specified object reference is a key in this identity hash map.

4

boolean containsValue(Object value)

This method tests whether the specified object reference is a value in this identity hash map.

5

Set<Map.Entry<K,V>> entrySet()

This method returns a Set view of the mappings contained in this map.

6

boolean equals(Object o)

This method compares the specified object with this map for equality.

7

V get(Object key)

This method returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

8

int hashCode()

This method returns returns the hash code value for this map.

9

boolean isEmpty()

This method returns true if this identity hash map contains no key-value mappings.

10

Set<K>keySet()

This method returns an identity-based set view of the keys contained in this map.

11

V put(K key, V value)

This method associates the specified value with the specified key in this identity hash map.

12

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

This method copies all of the mappings from the specified map to this map.

13

V remove(Object key)

This method removes the mapping for this key from this map if present.

14

int size()

This method returns the number of key-value mappings in this identity hash map.

15

Collection<V> values()

This method returns returns a Collection view of the values contained in this map.

Methods inherited

This class inherits methods from the following classes:

  • java.util.AbstractMap
  • java.util.Object


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