Question

I’m trying to find a way in which I can return a generic set, which will accept subtypes. The code below returns a Set<? extends A> to which I’m trying to add a B.

import java.util.Set;
import java.util.HashSet;

public class Test {
    private static class A {}
    private static class B extends A {}

    public Set<? extends A> getSet() {
        return new HashSet<B>();
    }

    public void work() {
        getSet().add(new B()); // compile error
    }
}

Which results in the following error:

Test.java:13: cannot find symbol
symbol  : method add(Test.B)
location: interface java.util.Set<capture#320 of ? extends Test.A>
                getSet().add(new B());

The generic wildcard tutorial states that B cannot be added to a Set<? extends A> because we don’t know if B is a subtype of A, but I have just defined it as such. Can someone clarify this for me?

Answer 1

That’s fundamentally impossible.

What if the method actually returns a HashSet<C>?
You just added a B to a set of Cs.

The method’s contents (return statement) are ignored when checking typesafety.

If you want to do this, you should just return an ordinary HashSet<A>.