interface I {
     void f();
     void g();
 }
 
 class A implements I {
     public void f() { System.out.println("A: doing f()"); }
     public void g() { System.out.println("A: doing g()"); }
 }
 
 class B implements I {
     public void f() { System.out.println("B: doing f()"); }
     public void g() { System.out.println("B: doing g()"); }
 }
 
 class C implements I {
     // delegation
     I i = new A();
 
     public void f() { i.f(); }
     public void g() { i.g(); }
 
     // normal attributes
     void toA() { i = new A(); }
     void toB() { i = new B(); }
 }