public class Originator {
  private String state = "OFF";
  public String getState() {
    return state;
  }
  public void setState(String state) {
    this.state = state;
  }

  public Memento createMemento() {
    return new Memento(state);
  }
  public void setMemento(Memento memento) {
    state = memento.getState();
    System.out.println("状態を" + state + "に戻しました");
  }
}

public class Memento {
  private String state;

  public Memento(String state) {
    this.state = state;
  }
  public String getState() {
    return state;
  }
}

public class Caretaker {
  Memento memento;
  public Memento getMemento() {
    return memento;
  }
  public void setMemento(Memento memento) {
    this.memento = memento;
  }
}

public class Client {
  public static void main(String[] args) {
    Originator o = new Originator();
    o.setState("ON");

    Caretaker ct = new Caretaker();
    ct.setMemento(o.createMemento());

    o.setState("OFF");

    o.setMemento(ct.getMemento());
  }
}

C:\JavaHello\Memento>java Client
状態をONに戻しました

C:\JavaHello\Memento>