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

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

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

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 to hashCode 3 objects.
PrintHashCodes@982741a0 --> -1742257760
java.awt.Color[r=68,g=136,b=204] --> -12285748
SomeClass@860b41ad --> -2046082643
All done.
>

The hashcode value for the Color object is interesting. It is actually computed as something like:

(r<<24 + g<<16 + b<<8 + alpha)

The “high bit” in this word having been set by shifting causes the value to appear negative when printed as a signed integer. Hashcode values are allowed to be negative.