public class Subtypes {
	public static void main(String... args){
		A<X> a = new A<X>();
		B<X> bx = new B<X>();
		B<Y> by = new B<Y>();

		bx = (B<X>)a;
		//by = (B<Y>)a;
		a = bx;
		//a = by;
	}
}

class A<T>{
}

class B<T> extends A<T> {
}

class X{};
class Y extends X{};