Dynamically changing the behavior of an object via composition (the 'State' design pattern) : Class Fields « Class Definition « Java Tutorial

1. Java Tutorial
2. Class Definition
3. Class Fields

abstract class Actor {
  public abstract void act();
}

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

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

class Stage {
  private Actor actor = new HappyActor();

  public void change() {
    actor = new SadActor();
  }

  public void performPlay() {
    actor.act();
  }
}

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

HappyActor
SadActor