abstract class State {
  abstract public void handle(Context context);
}

public class ConcreteStateA extends State{
  public void handle(Context context) {
    context.setState(new ConcreteStateB());
  }
}

public class ConcreteStateB extends State {
  public void handle(Context context) {
    context.setState(new ConcreteStateA());
  }
}

public class Context {
  private State state;

  public Context(State state) {
    this.state = state;
  }
  public State getState() {
    return state;
  }
  public void setState(State state) {
    this.state = state;
  }

  public void request() {
    state.handle(this);
  }
  public void displayState() {
    System.out.println("状態は" + state.getClass().getName() + "です");
  }
}

public class Client {
  public static void main(String[] args) {
    Context c = new Context(new ConcreteStateA());
    c.displayState();

    c.request();
    c.displayState();

    c.request();
    c.displayState();
  }
}

C:\JavaHello\State>java Client
状態はConcreteStateAです
状態はConcreteStateBです
状態はConcreteStateAです

C:\JavaHello\State>