Up-casting template argument types

Question

Why does Java not support automatic up-casting for template argument types?

For example, the following class will not compile unless the newly created Derived instance will be manually casted to a Base instance:

public class Example implements Iterable<Base> {

    @Override
    public Iterator<Base> iterator() {
        return Arrays.asList(new Derived()).iterator();
    }

    private class Base {
    }

    private class Derived extends Base {

    }
}

Return `Iterator extends Base>` – Aleksandr Dubinsky Aug 29 '15 at 11:06 — Aleksandr Dubinsky, Aug 29 '15 at 11:06

score 3 · Accepted Answer · edited May 23 '17 at 12:16

No need to cast. The problem here is that Arrays.asList(new Derived()) naturally tries to create a List<Derived>, and then calling .iterator() on a List<Derived> naturally gives an Iterator<Derived>, which is not a sub-type of Iterator<Base>, so you get a compilation error.

You can specify that you want a List<Derived>, using Arrays.<Base>asList. This works, because you can certainly put a Derived instance into a List<Base>, and then calling .iterator() on a List<Base> naturally gives an Iterator<Base>.

    class Example implements Iterable<Base> {
        @Override
        public Iterator<Base> iterator() {
            return Arrays.<Base>asList(new Derived()).iterator();
        }
    }

Up-casting template argument types

1 Answers1