Question

I have implemented a class to model rational numbers in java, it has two integers to model numerator and denominator. I am required to override the hashcode method of Object, so the same numbers have the same hash code.

I have defined my equals() method like this:

public boolean equals(Object obj) {
    Racional r = null;
    if (obj instanceof Racional) {
        r = (Racional) obj;
    } else {
        return false;
    }
    return r.getDenominador() * this.numerador == r.getNumerador() * this.denominador;
}

Regarding this:

Would returning numerator * denominator be a good approach?

Should equivalent rational numbers (like 1/4 and 2/8) return the same hashcode?

Answer 1

It all depends on how you implemented your equals method. If obj1.equals(obj2) is true, obj1.hashCode() == obj2.hashCode() should also be true. I would probably just use new Double((double) numerator / denominator).hashCode() for my hash, but your requirements might not allow that.

/edit
Using numerator * denominator for your hash would be an invalid approach given your equals method. Using your example of 1/4 and 2/8, 1/4.equals(2/8) would return true, but 1/4.hashCode() == 2/8.hashCode() would evaluate to 4 == 16, and return false.