Why hashcode

Question

What is the use of hash code () method?

Answer 1

hashCode is used as a fast way to compare two objects.
If the hashCode are equal, there is a chance that the
two objects are equal. This is then verified by
calling the equals method.
It is used a lot with the Collection classes, for
example with the HashMap. The HashMap will not allow
duplicate keys, so when you add something, it checks
if the key already exist, and if it does it overrides
the value.This is true, but only upto a point.
It depends on the way the hash code is generated.
The hashcode for java.lang.Object is based on the actual reference (read: pointer value) for that object and you can test for identity when using the standard hash code values but not for probable equality.
This kind of test for identity or probable equality is not reliable however. It is often recommended that you override the equals and hashcode methods for classes so that things like the Collections classes handle instances of your classes in a predictable manner. This can mean that using hash code comparison as a clue to possible equality can often cause you to take an extra step in checking for equality when this would not be the case (reason, you normally check for class equality with instanceof and then equality. The hashcode step can work, see below).
Using hashcode for probable equality checks will only work properly, after instance class comparison, if the hashcode reflects the uniqueness of those member variables for the purposes of comparison.
The problem with this usage of hashcodes is that it can cause clumping of keys inside datastructures like the Hashtable collection class where hashcode is used for working out which 'bucket' the key should land in. This leads to poor performance in your collection as the less uniform the key distribution is the more time it takes to actually find the key you want in the bucket.
In short:-
hashcode should normally be overriden to optimise the distribution of keys in Collections,
equals should be overriden to ensure proper equality and also allow you to optimise the method, instance class equality, primitives then object member equality.
Example
class Simple {
    int a = 1;
    String b = "I am a String";
    public boolean equals(Object ob) {
        return (ob instanceof Simple.class &&
                   this.a == ((Simple)ob).a &&
                   this.b.equals(((Simple)ob).b)); // Casts will be optimised out for the optipurists
    public int hashCode() {
        return this.a ^ this.b.hashCode();