Confused about hashCode() and equals() method contracts

[Jacob Thomas]

Following is a qoute from K & B SCJP 1.4 book:

".... equals() and hashCode() are bound together by a joint contract that specifies if two objects are considered equal using the equals() method, then they must have identical hashcode values..."

Following is a snippet of code from the same book which I have modified to print the hashcode values of the objects and see if the values are equal:

public class EqualsTest {
   public static void main(String[] args) {
       Moof one = new Moof(8);
       Moof two = new Moof(8);
 
       System.out.println("Both are equal: " + one.equals(two));
       System.out.println("Moof one hashcode: " + one.hashCode());
       System.out.println("Moof two hashcode: " + two.hashCode());
       System.out.println("Hashcode equal: " + ((one.hashCode()) == (two.hashCode())));
    }
}
 

class Moof {
    private int moofValue;
 
    Moof(int val) {
       moofValue = val;
    }
 
    public int getMoofValue() {
       return moofValue;
    }
 
    public boolean equals(Object o) {
       if ((o instanceof Moof) && (((Moof)o).getMoofValue() == this.moofValue)) {
          return true;
       } else {
          return false;
       }
    }
}       

If you run this program, you'll find the hashcode values for the objects are different. I know I haven't overridden the hashCode() method and hence the different hashcode values in the output.

Is it only necessary to override the hashCode() method only when it is going to be used by some collection classes? And finally, can this be considered as a "correct or appropriate" implementation as it clearly violating the abovementioned contract? Thanks!

[wise owen]

Java theory and practice: Hashing it out.

[vivekkumar sharma]

Is it only necessary to override the hashCode() method only when it is going to be used by some collection classes? And finally, can this be considered as a "correct or appropriate" implementation as it clearly violating the abovementioned contract? Thanks![/QB]

Yes it is necessary to override hashCode() for proper use of collections which depend on hash keys

I would say bove example is not appropriate ,however it is legal in java