被记录的类：

public class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

备忘录类：

public class Memento {
    private String state;

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

    public String getState() {
        return state;
    }
}

纪录类：

public class CareTaker {
    private List<Memento> mementoList = new ArrayList<>();

    public void add(Memento state) {
        mementoList.add(state);
    }

    public Memento get(int index) {
        return mementoList.get(index);
    }
}

演示类：

public class MementoPaneDemos {
    public static void main(String[] args) {
        Originator originator = new Originator();
        CareTaker careTaker = new CareTaker();
        originator.setState("State #1");
        originator.setState("State #2");
        careTaker.add(originator.saveStateToMemento());
        originator.setState("State #3");
        careTaker.add(originator.saveStateToMemento());
        originator.setState("State #4");

        System.out.println("Current State: " + originator.getState());
        originator.getStateFromMemento(careTaker.get(0));
        System.out.println("Current State: " + originator.getState());
        originator.getStateFromMemento(careTaker.get(1));
        System.out.println("Current State: " + originator.getState());
    }
}

演示结果：

设计模式-行为型-备忘录模式-Memento_设计模式

编辑