在子类中调用super.equals和super.hashCode？

Question

If I implement equals() and hashCode() in both the parent and child classes, is it necessary to call super.equals() in equals() in the child class, eg

public boolean equals(Object obj) {

  if (obj.getClass() != ChildClass.class) {
    return false;
  }

  return super.equals() && this.var == ((ChildClass) obj).var;

}

I am assuming that the parent class is not Object and is giving the correct definition of equals and hashCode.

Answer 1

No, that's not necessary, and would probably be wrong. Indeed, part of the reason why you're overriding equal is because super.equals doesn't give the correct behaviour (right?).

Or put another way, if super.equals gives the correct behaviour, you probably don't need to go to the trouble of overriding it.

But if you are overriding equals , then yes, you need to override hashCode , too.

Answer 2

If your super class doesn't implement equals, then calling super.equals will get you the Object implementation which only compares references, so in any normal equals implementation where you want to compare a business key it would actually cause a lot of false negatives.

That said, your implementation above really isn't any different semantically than the default equals implementation in Object since you are just using == for comparison.

As for hashCode, if you override equals you really should override hashCode in order to keep the contract of the two. For instance, if you are using a business key for equals you should probably use the same business key for the hashcode so that two objects that are equal generate the same hashcode.