Momento Pattern

Momento Pattern

The memento pattern is a software design pattern that provides the ability to restore an object to itsprevious state (undo via rollback).

The memento pattern is implemented with two objects: the originator and a caretaker. The originator is some object that has an internal state. The caretaker is going to do something to the originator, but wants to be able to undo the change. The caretaker first asks the originator for a memento object. Then it does whatever operation (or sequence of operations) it was going to do. To roll back to the state before the operations, it returns the memento object to the originator. The memento object itself is an opaque object (one which the caretaker cannot, or should not, change). When using this pattern, care should be taken if the originator may change other objects or resources – the memento
pattern operates on a single object.

Classic examples of the memento pattern include the seed of a pseudorandom number generator (it will always produce the same sequence thereafter when initialized with the seed state) and the state in a finite state machine.

Example
The following Java program illustrates the “undo” usage of the Memento Pattern.

Class Originator

public class Originator {
	private String state;

	// The class could also contain additional data that is not part of the
	// state saved in the memento.
	public void set(String state) {
		System.out.println("Originator: Setting state to " + state);
		this.state = state;

	}

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

	public void restoreFromMemento(Memento memento) {
		state = memento.getSavedState();
		System.out.println("Originator: State after restoring from 	Memento: " + state);
	}

	public static class Memento {
		private final String state;

		public Memento(String stateToSave) {
			state = stateToSave;
		}

		public String getSavedState() {
			return state;
		}
	}
}

Class Caretaker

import java.util.ArrayList;
import java.util.List;

public class Caretaker {
	public static void main(String[] args) {
		List<Originator.Memento> savedStates = new ArrayList<Originator.Memento>();
		Originator originator = new Originator();
		originator.set("State1");
		originator.set("State2");
		savedStates.add(originator.saveToMemento());
		originator.set("State3");
		// We can request multiple mementos, and choose which one to roll back
		// to.
		savedStates.add(originator.saveToMemento());
		originator.set("State4");
		originator.restoreFromMemento(savedStates.get(1));
	}
}

Below example uses a String as the state, which by default is an immutable type in java. In real life scenarios the state will almost always be an object. In which case the state has to be cloned before putting in the memento.

private Memento(State state)
{
//state has to be cloned before returning the
//memento, or successive calls to get Memento
//return a reference to the same object
this.mementoState = state.clone();
}

It must be said that this latter implementation has a drawback: it declares an internal class. Better would be that the memento strategy could apply on more that one object.
There are mainly 3 other ways to achieve Memento: 1- Serialization. 2- A class declared in the same package. 3- The
object can also be accessed via a proxy, which can achieve any save/restore operation on the object.