public class factory { 
   factory  
  
  public factory() { }
  
  letter letter() { return new letter(); }
  envelope envelope() { return new envelope(); }
  };
  

  public class envelope { 
   envelope  
  letter impl;
  public envelope() {
  	factory f = new factory();
  	impl = f.letter();   // obtained from factory
  	}
  
  public void message() {
  	impl.message();
  	}
  };