public class poly {
    public static void main(String[] args) {
	
	A a1 = new A();
	A a2 = new A();
	B b1 = new B();
	B b2 = new B();

	a1 = b1;
	// b2 = (B) a2; 	// compiles, but won't run
    }

}

class A {}
class B extends A {}