Question

So, let’s say I have the following classes:

Foo

abstract class Foo{
    .
    .
    .
    public void setKey(FooKey key){
        this.key = key;
    }
}

Bar

final class Bar extends Foo{
    .
    .
    .
    @Override
    public void setKey(BarKey key){ // Can't do this?
        super.setKey(key);
    }
}

In the above example, BarKey is a subclass of FooKey. Here I want to ensure that the key that is set for Bar is a specific subclass of FooKey (i.e. BarKey). How do I go about this? Am I making this overly complicated? Is there a better design approach?

Answer 1

This isn’t correct, because you’re making the subclass more restrictive than the superclass: the superclass makes it possible to set any kind of FooKey, but the subclass only accepts BarKey.

In order to do this, you need to make the superclass generic:

abstract class Foo<K extends FooKey> {
    private K key;

    public void setKey(K key){
        this.key = key;
    }
}

final class Bar extends Foo<BarKey> {
}