Memento pattern

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

The memento pattern is used by 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 rollback 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 can not, 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 and the state in a finite state machine.

Example

The following Java program illustrates the "undo" usage of the Memento Pattern.

import java.util.*;  
 
class Originator {
    private String state;
    /* lots of memory consumptive private data that is not necessary to define the
     * state and should thus not be saved. Hence the small memento object. */
 
    public void set(String state) { 
        System.out.println("Originator: Setting state to "+state);
        this.state = state; 
    }
 
    public Object saveToMemento() { 
        System.out.println("Originator: Saving to Memento.");
        return new Memento(state); 
    }
    public void restoreFromMemento(Object m) {
        if (m instanceof Memento) {
            Memento memento = (Memento)m; 
            state = memento.getSavedState(); 
            System.out.println("Originator: State after restoring from Memento: "+state);
        }
    }
 
    private static class Memento {
        private String state;
 
        public Memento(String stateToSave) { state = stateToSave; }
        public String getSavedState() { return state; }
    }
}
 
class Caretaker {
    private ArrayList savedStates = new ArrayList();
 
    public void addMemento(Object m) { savedStates.add(m); }
    public Object getMemento(int index) { return savedStates.get(index); }
}   
 
class MementoExample {
    public static void main(String[] args) {
        Caretaker caretaker = new Caretaker();
 
        Originator originator = new Originator();
        originator.set("State1");
        originator.set("State2");
        caretaker.addMemento( originator.saveToMemento() );
        originator.set("State3");
        caretaker.addMemento( originator.saveToMemento() );
        originator.set("State4");
 
        originator.restoreFromMemento( caretaker.getMemento(1) );
    }
}

The output is:

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

See also

• Facade pattern
• Facade memento strategy

External links

• Description by Matthew Heaney
• Memento pattern discussion with 1-page examples (C++ and Java) by Vince Huston