Java Community | Help. Code. Learn.•11mo ago

Week 58 — What is the "identity hash code" and how does it differ from other `hashCode()`s?

Question of the Week #58

What is the "identity hash code" and how does it differ from other hashCode()s?

2 Replies

The class Object provides a method called hashCode() which returns an int which should return an int that: - Is the same for two equal objects, i.e. if a.equals(b), then a.hashCode() should return the same result as b.hashCode() - ideally be different for most different objects. In case a.equals(b) returns false, then a.hashCode() and b.hashCode() should likely be different. This method is used by data structures that require hashing like HashMap/HashSet. If this method is not overridden, its hashCode returns the "identity hash code" of the object. When an object is created and its identity hash code is requested, the JVM will generate a random number and save it into the object. When it is requested again, the same int will be returned. Two different objects with the same data may have different identity hash codes.

Object someObject = new Object();
System.out.println(someObject.hashCode());//generates the identity hash code of that objects and prints it
System.out.println(someObject.hashCode());//prints the same number again

Object otherObject = new Object();
System.out.println(otherObject.hashCode());//generates a new identity hash code for the second object - this is likely different from the identity hash code of someObject

Object someObject = new Object();
System.out.println(someObject.hashCode());//generates the identity hash code of that objects and prints it
System.out.println(someObject.hashCode());//prints the same number again

Object otherObject = new Object();
System.out.println(otherObject.hashCode());//generates a new identity hash code for the second object - this is likely different from the identity hash code of someObject

The method System.identityHashCode(Object) always returns the identity hash code of the object passed as a parameter, even if the hashCode(). method is overridden.

class SomeClass{
  @Override
  public int hashCode(){
    return 0;//technically a valid hashCode() but not a good one
  }
}

class SomeClass{
  @Override
  public int hashCode(){
    return 0;//technically a valid hashCode() but not a good one
  }
}

SomeClass obj = new SomeClass();
System.out.println(obj.hashCode());//0
System.out.println(System.identityHashCode(obj));//likely prints a different value

SomeClass obj = new SomeClass();
System.out.println(obj.hashCode());//0
System.out.println(System.identityHashCode(obj));//likely prints a different value

📖 Sample answer from dan1st

Eric McIntyre•11mo ago

The identity hash code in Java is provided by the System.identityHashCode(Object obj) method. It returns the same hash code for the given object, regardless of its hashCode() implementation. The hashCode() method is typically overridden by classes to generate hash codes based on the object's contents, but the identity hash code is solely based on the object's memory address.

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Week 58 — What is the "identity hash code" and how does it differ from other `hashCode()`s?