public class E {

interface X { void f(); }


public static void main(String[] Args) {

class B {
public void g() { System.out.print("B.g() "); }
}


class C extends B implements X {
	public void f() { System.out.print("C.f() "); }
  	public void f(Object ref) { System.out.print("C.f(Object) "); }

}

abstract class A extends B implements X {
	public void f() { System.out.print("A.f() "); }
	public void g() { System.out.print("A.g() "); }
	public abstract B f(B ref);
}

class D extends A {
	public static B st = new B();
	public void f() { System.out.print("D.f() "); }
	public B f(B ref) { if (ref instanceof A) return (D)ref; return st; }
}



}

  A a = new D();
B b = a;
b.g();
a.g();
a.f(b);

}