Overriding and Implementation of hashcode()

[Pranav Bhatt]

Hi,
Can anyone please explain me the concept of overriding of hashCode()? I'am clear with the concept of overriding of equals() method. But am still not clear with the hashCode(), it will help if you could provide me with some examples. And why is it necessary to override hashCode() along with equal().

thanks

[Jari Timonen]

Think of buckets (thanks K&B). Buckets represent holding place for objects. Every bucket has a unique number.

HashCode is method which tells Hash-algorithms to which bucket the object should go. If every object is in different bucket, performance is better.

If you don't implement hashCode or it changes every time, your Object will be never found from e.x. HashMap. (remember buckets!)


Example by words (two in same bucket):
You have two objects A B, both has same hashcode. When Object is searched with hash-algorithm, it calculates hashcode and retrieves two objects (A and B since they have same hashcode and they're in same bucket). After this equals method is invoked and the correct object is returned.

Example by code:

    
// very bad implementation, but legal
public class A{
    // equals method here
 
    public int hashCode(){
      return 1;
    }
}
 
public class B{
    // equals method here
 
    public int hashCode(){
      return 1;
    }
}
 
 
// better implementation since hashcode is mostlikely different to every object
public class C{
    // equals method here
    private int verySecretNumber; //assume this get's initialized
    private String firstName; //assume this get's initialized
    public int hashCode(){
      int hashcode = verySecretNumber * firstName.hashCode() * 31;
      return hashcode ;
    }
}
 
public class D{
    // equals method here
    private int verySecretNumber; //assume this get's initialized
    private String firstName; //assume this get's initialized
    public int hashCode(){
      int hashcode = verySecretNumber * firstName.hashCode() * 31;
      return hashcode ;
    }
}

Finally contract of hashcode (you can see this also in api description of Object

* Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
* If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
* It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

Hope this helps.
[ December 06, 2007: Message edited by: Jari Timonen ]