The IdentityHashMap Class

IdentityHashMap extends AbstractMap and implements the Map interface. It uses reference equality (==) instead of object equality (equals()) when comparing keys and values.

IdentityHashMap obtains hash codes via System's static int identityHashCode(Object x) method instead of via each key's hashCode() method. The hash code for the null reference is zero.

IdentityHashMap is a generic class that has this declaration:

class IdentityHashMap<K, V>

K specifies the type of key
V specifies the type of value.

 
import java.util.IdentityHashMap;
import java.util.Map;

public class Main {
  public static void main(String[] argv) throws Exception {
    Map<Object, Object> objMap = new IdentityHashMap<Object, Object>();

    Object o1 = new Integer(123);
    Object o2 = new Integer(123);
    objMap.put(o1, "first");
    objMap.put(o2, "second");

    Object v1 = objMap.get(o1); // first
    Object v2 = objMap.get(o2); // second
  }
}

IdentityHashMap supports mutable keys. The mutable keys are objects used as keys and whose hash codes change when their field values change while in the map. The following code contrasts IdentityHashMap with HashMap in a mutable key context

 
import java.util.HashMap;
import java.util.IdentityHashMap;
import java.util.Map;

public class Main {
  public static void main(String[] args) {
    Map<Employee, String> map1 = new IdentityHashMap<Employee, String>();
    Map<Employee, String> map2 = new HashMap<Employee, String>();
    Employee e1 = new Employee("J", 28);
    map1.put(e1, "SALES");
    System.out.println(map1);
    Employee e2 = new Employee("J", 26);
    map2.put(e2, "MGMT");
    System.out.println(map2);
    System.out.println(map1.containsKey(e1));//true
    System.out.println(map2.containsKey(e2));//true
    e1.setAge(29);
    e2.setAge(27);
    System.out.println(map1);
    System.out.println(map2);
    System.out.println(map1.containsKey(e1));//true
    System.out.println(map2.containsKey(e2));//false
  }
}

class Employee {
  private String name;
  private int age;

  Employee(String name, int age) {
    this.name = name;
    this.age = age;
  }

  @Override
  public boolean equals(Object o) {
    if (!(o instanceof Employee))
      return false;
    Employee e = (Employee) o;
    return e.name.equals(name) && e.age == age;
  }

  @Override
  public int hashCode() {
    int hashCode = 19;
    hashCode = hashCode * 31 + name.hashCode();
    hashCode = hashCode * 31 + age;
    return hashCode;
  }

  void setAge(int age) {
    this.age = age;
  }

  void setName(String name) {
    this.name = name;
  }

  @Override
  public String toString() {
    return name + " " + age;
  }
}

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