interface Addable<E> {
    public E add(E x);
    public E sub(E y);
    public E zero();
}

class SumSet<E extends Addable> implements Set<E> {

    private E element;

    public SumSet(E element) {
        this.element = element;
    }

    public E getSum() {
        return element.add(element.zero());
    }
}

It seems that element.add() doesn't return an E extends Addable but rather an Object. Why is that? Has it anything to do with Java not knowing at run-time what the object types really are, so it just assumes them to be Objects(thus requiring a cast)?

Thanks