用继承表达行为间的差异，并用字段表达状态上的变化

eg:

public class Actor {
public void act(){}

}

public class HappyActor extends Actor{
public void act(){
System.out.println("HappyActor");
}

}

public class SadActor extends Actor{
public void act(){
System.out.println("SadActor");
}

}

public class Stage {
private Actor actor = new HappyActor();
public void change(){ actor= new SadActor();}
public void performPlay(){ actor.act();}

}

public class Transmogrify {
public static void main(String[] args) {
Stage stage = new Stage();
stage.performPlay();
stage.change();
stage.performPlay();
}

}