Java IdentityHashMap Class

---

---

Introduction

The Java 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

Sr.No.	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

Sr.No.	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

Adding a Key-Value Mapping in an IdentityHashMap Example

The following example shows the usage of Java IdentityHashMap put() method to put few values in a Map. We've created a Map object of Integer,Integer. Then few entries are added using put() method and then map is printed.

package com.tutorialspoint;

import java.util.IdentityHashMap;

public class IdentityHashMapDemo {
   public static void main(String args[]) {
      
      // create identity map
      IdentityHashMap<Integer,Integer> newmap = new IdentityHashMap<>();

      // populate identity map
      newmap.put(1, 1);
      newmap.put(2, 2);
      newmap.put(3, 3); 

      System.out.println("Map elements: " + newmap);
   }    
}

Let us compile and run the above program, this will produce the following result.

Map elements: {2=2, 3=3, 1=1}