Design Patterns - Memento Pattern

Memento pattern is used to restore state of an object to a previous state. Memento pattern falls under behavioral pattern category.

Implementation

Memento pattern uses three actor classes. Memento contains state of an object to be restored. Originator creates and stores states in Memento objects and Caretaker object is responsible to restore object state from Memento. We have created classes Memento, Originator and CareTaker.

MementoPatternDemo, our demo class, will use CareTaker and Originator objects to show restoration of object states.

Step 1

Create Memento class.

Memento.java

pubpc class Memento {
   private String state;

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

   pubpc String getState(){
      return state;
   }	
}

Step 2

Create Originator class

Originator.java

pubpc class Originator {
   private String state;

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

   pubpc String getState(){
      return state;
   }

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

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

Step 3

Create CareTaker class

CareTaker.java

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

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

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

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

Step 4

Use CareTaker and Originator objects.

MementoPatternDemo.java

pubpc class MementoPatternDemo {
   pubpc 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("First saved State: " + originator.getState());
      originator.getStateFromMemento(careTaker.get(1));
      System.out.println("Second saved State: " + originator.getState());
   }
}

Step 5

Verify the output.

Current State: State #4
First saved State: State #2
Second saved State: State #3