2925. Design Pattern - Memento
Memento Pattern

Behavioral Pattern: Memento Pattern.

1. Memento Pattern

The Memento pattern is used to restore state of an object to a previous state. Normally, this pattern defines three distinct roles:

  • Originator - the object that knows how to save itself.
  • Caretaker - the object that knows why and when the Originator needs to save and restore itself.
  • Memento - the lock box that is written and read by the Originator, and shepherded by the Caretaker.

2. Example

public class Memento {
    private String state;

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

    public String getState() {
        return state;
    }
}

public class Caretaker {
    private ArrayList<Memento> mementos = new ArrayList<>();

    public void addMemento(Memento m) {
        mementos.add(m);
    }

    public Memento getMemento() {
        return mementos.get(1);
    }
}

public class Originator {
    private String state;

    public void setState(String state) {
        System.out.println("Originator: Setting state to " + state);
        this.state = state;
    }

    public Memento save() {
        System.out.println("Originator: Saving to Memento.");
        return new Memento(state);
    }

    public void restore(Memento m) {
        state = m.getState();
        System.out.println("Originator: State after restoring from Memento: " + state);
    }
}

2.2 Client

public class Client {
    public void run() {
        Caretaker caretaker = new Caretaker();
        Originator originator = new Originator();
        originator.setState("State1");
        originator.setState("State2");
        caretaker.addMemento( originator.save() );
        originator.setState("State3");
        caretaker.addMemento( originator.save() );
        originator.setState("State4");
        originator.restore( caretaker.getMemento() );
    }
}

Output.

Originator: Setting state to State1
Originator: Setting state to State2
Originator: Saving to Memento.
Originator: Setting state to State3
Originator: Saving to Memento.
Originator: Setting state to State4
Originator: State after restoring from Memento: State3

3. Source Files

4. References