Question

I need a simple scenario to produce a hashing collision in a HashMap. Could someone please provide one.

Is it possible to produce hashing collision if my hashmap keys are immutable?

Regards,

Raju komaturi

Answer 1

You could create your own type and create a bad hash function:

public class BadHash {

    private String aString;

    public BadHash(String s) {
        aString = s;
    }

    public int hashCode() { 
        return aString.length();
    }

    public boolean equals(Object other) {
        // boilerplate stuff
        BadHash obj = (BadHash) other;
        return obj.aString.equals(aString);
    }
}

This will make it easy to create a collision.

An example would be:

BadHash a = new BadHash("a", value1);
BadHash b = new BadHash("b", value2);
hashMap.add(a);
hashMap.add(b);

These two entries would collide because a and b hash to the same value even though they are not equal.