Java - The IdentityHashMap Class

This class implements AbstractMap. It is similar to HashMap except that it uses reference equality when comparing the elements.

This class is not a general-purpose Map implementation. While this class implements the Map interface, it intentionally violates Map's general contract, which mandates the use of the equals method when comparing objects.

This class is designed for use only in rare cases wherein reference-equality semantics are required. This class provides constant-time performance for the basic operations (get and put), assuming the system identity hash function (System.identityHashCode(Object)) disperses elements properly among the buckets.

This class has one tuning parameter (which affects performance but not semantics): expected maximum size. This parameter is the maximum number of key-value mappings that the map is expected to hold.

Following is the list of the constructors supported by the IdentityHashMap.

Apart from the methods inherited from its parent classes, IdentityHashMap defines following methods −

Example

The following program illustrates several of the methods supported by this collection −

import java.util.*;
public class IdentityHashMapDemo {

   public static void main(String args[]) {
      // Create a hash map
      IdentityHashMap ihm = new IdentityHashMap();
 
      // Put elements to the map
      ihm.put("Zara", new Double(3434.34));
      ihm.put("Mahnaz", new Double(123.22));
      ihm.put("Ayan", new Double(1378.00));
      ihm.put("Daisy", new Double(99.22));
      ihm.put("Qadir", new Double(-19.08));
      
      // Get a set of the entries
      Set set = ihm.entrySet();
      
      // Get an iterator
      Iterator i = set.iterator();
      
      // Display elements
      while(i.hasNext()) {
         Map.Entry me = (Map.Entry)i.next();
         System.out.print(me.getKey() + ": ");
         System.out.println(me.getValue());
      }
      System.out.println();
      
      // Deposit 1000 into Zara's account
      double balance = ((Double)ihm.get("Zara")).doubleValue();
      ihm.put("Zara", new Double(balance + 1000));
      System.out.println("Zara's new balance: " + ihm.get("Zara"));
   }
}

This will produce the following result −

Output

Ayan: 1378.0
Qadir: -19.08
Mahnaz: 123.22
Daisy: 99.22
Zara: 3434.34

Zara's new balance: 4434.34