Legal cast rejected

This program doesn't compile

public class X {

    public interface SuperInterface<A> {
    }

    public interface SubInterface extends SuperInterface<String> {
        public String getString();
    }

    private SuperInterface< ? extends SuperInterface> x = null;

    public void f() {
        ((SubInterface) this.x).getString();
    }
}

###@###.### 2005-06-08 16:13:28 GMT