IdentityHashMap in Java

The IdentityHashMap implements the Map interface. HashMap compares the equality of keys by comparing its content but IdentityHashMap compares the references address of the keys.

Key Properties of IdentityHashMap

It uses reference equality (==) instead of the equals() method. It performs relatively faster than HashMap.

It also supports null keys and values.

Map<String, String> map1 = new IdentityHashMap<>();  
String key1 = "key";  
String key2 = "key";  
map1.put(key1, "value1");  
map1.put(key2, "value2");  

for (String key: map1.keySet()) {  
    System.out.println(key + " -> " + map1.get(key));  
}  

/*
Outputs:
key -> value2
*/

Map<String, String> map2 = new IdentityHashMap<>();  
String key3 = new String("key");  
String key4 = new String("key");  
map2.put(key3, "value1");  
map2.put(key4, "value2");  

for (String key : map2.keySet()) {  
    System.out.println(key + " -> " + map2.get(key));  
}

/*
Outputs:
key -> value2
key -> value1
*/
 

Notice that in the second map, the key3 and key4 have the same content but are different objects. Hence, IdentityHashMap treats them as two different keys, as a result, the map contains two entries.