public class envelope { 
   envelope  

  letter impl;
  public envelope() {
  	impl = new letter();
  	}
  
  public void message() {
  	impl.message();
  	}
  };
  


  public class letter { 
   letter  

  
  public letter() { }
  
  public void message() {
  	System.out.println("Message in a letter");
  	}
  };
slide: Envelope/Letter