hashcode()

[Kaz Yosh]

Hi all,
can anybody explain this?
Given the following class, which are correct implementations of the hashCode() method?

class ValuePair {
    public int a, b;
    public boolean equals(Object other) {
        try {
            ValuePair o = (ValuePair) other;
            return (a == o.a && b == o.b)
                || (a == o.b && b == o.a);
        } catch (ClassCastException cce) {
            return false;
        }
    }
    public int hashCode() {
        // Provide implementation here.
    }
}

[ May 29, 2003: Message edited by: Thomas Paul ]

[Thomas Paul]

Two ValuePair objects are equal if they contain the same valuer in their ints. (a == a and b == b) or (a == b and b == a)
So the hashCode method must return an int so that equal objects have equal hashcodes. So a possible valid implementation might be a+b. Returning either a or b would not be valid.

[Kaz Yosh]

Can you explain a bit more about hashcode?

[Thomas Paul]

There is an article in the JavaRanch newsletter about the hashCode method.

[Brian Joseph]

I've recently read a chapter on this. My understanding with implementing hashCode() is that you take a look at the equals method and see which member variables are being used to compare the "value" of the instance of the class, and then create a hashCode implementation like: a+b.
Now, I've been out of college for about 4 years, so my CS knowledge is not quite as sharp, but I was surpised to learn that a valid hashcode could also just be a constant, just "return 123;", but this would not be "efficient" because then every object would have the same hashcode, and I guess that this means when hashing your objects you won't get a good distribution in a Hashtable.