Question

Possibly a dumb question, but I don’t want to screw this up. Let’s say I have two Java classes, Class1 and Class2, where Class2 extends Class1. I want to override Object.hashcode() using Guava for both classes. For the superclass, I’ve got

@Override
public int hashCode() {
    return Objects.hashcode(mField1, mField2);
}

For Class2, what’s the right way to implement hashcode() that takes the members of Class1 into consideration? Is it like this?

@Override
public int hashcode() {
    return Objects.hashcode(super.hashcode(), mField3, mField4);
}  

That SEEMS right to me, but I’m looking for some validation. Joshua Bloch doesn’t address this situation in Effective Java, and the Guava docs don’t either.

Answer 1

Yes, that looks correct. It would be the same if you had Objects.hashCode(f1, f2, f3, f4). If you look at the implementation, it’s something like result += 31 * result + hashcodeOfCurrentObject. Which means that your result will be 31 + the super hashcode, which is not exactly the same, but would not be a problem.