Java Cookbook

Overriding the Hashcode Method

Problem

You want to use your objects in a hash, and you need to write a hashCode( ).

Discussion

The hashCode() method is supposed to return an int that should uniquely identify different objects.

A properly written hashCode( ) method will follow these rules:

1. It is repeatable: hashCode(x) must return the same int when called again unless set methods have been called.

2. It is symmetric: if x.equals(y), then x.hashCode( ) must == y.hashCode( ), i.e., either both return true, or both return false.

3. If !x.equals(y), it is not required that x.hashCode( ) != y.hashCode( ), but doing so may improve performance of hash tables, i.e., hashes may call hashCode( ) before equals( ).

The default hashCode( ) on Sun’s JDK returns a machine address, which conforms to Rule 1. Conformance to Rules 2 and 3 depends, in part, upon your equals( ) method. Here is a program that prints the hashcodes of a small handful of objects:

/** Display hashCodes from some objects */
public class PrintHashCodes {

    /** Some objects to hashCode(  ) on */
    protected static Object[] data = {
        new PrintHashCodes(  ),
        new java.awt.Color(0x44, 0x88, 0xcc),
        new SomeClass(  )
    };

    public static void main(String[] args) {
        System.out.println("About to hashCode " + data.length + " objects.");
        for (int i=0; i<data.length; i++) {
            System.out.println(data[i].toString(  ) + " --> " + 
                data[i].hashCode(  ));
        }
        System.out.println("All done.");
    }
}

What does it print?

> jikes +E -d . PrintHashCodes.java > java PrintHashCodes About ...