Design Pattern: Memento Pattern in Java

Memento pattern is used to store an object’s history of state changes so that that history can be used to track back the change or be used for some other purpose.

Check Details of Memento Pattern

This article demonstrates Memento pattern implementations in Java. Check the following examples.

Examples

Here are few examples of Memento pattern implementation in Java-

Example #1: General Memento

Let’s consider a general example of a simple Memento pattern demonstration.

Class Diagram

Take a look at the class diagram.

Memento

// Memento.java

package com.bigboxcode.designpattern.memento.memento;

public class Memento {

    private String state;

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

    public String getState() {
        return state;
    }

}

Originator

// Originator.java

package com.bigboxcode.designpattern.memento.memento;

public class Originator {

    private String state;

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

    public String getState() {
        return state;
    }

    public Memento setMemento() {
        System.out.println("Memento Saved with timestamp => " + state);

        return new Memento(state);
    }

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

Caretaker

// Caretaker.java

package com.bigboxcode.designpattern.memento.memento;

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

public class Caretaker {

    private List<Memento> mementoList = new ArrayList<>();

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

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

    public Memento getCurrent() {
        return mementoList.get(mementoList.size() - 1);
    }

    public void undo() {
        mementoList.remove(mementoList.size() - 1);
    }
}

Demo

// Demo.java

package com.bigboxcode.designpattern.memento.memento;

public class Demo {

    public static void main(String[] args) throws InterruptedException {
        Caretaker caretaker = new Caretaker();
        Originator originator = new Originator();

        originator.setState("Time - 1 : " + System.currentTimeMillis());
        caretaker.add(originator.setMemento());

        // Add delay if required for testing
        // Thread.sleep(1000);

        originator.setState("Time - 2 : " + System.currentTimeMillis());
        caretaker.add(originator.setMemento());

        // Add delay if required for testing
        // Thread.sleep(1000);

        originator.setState("Time - 3 : " + System.currentTimeMillis());
        caretaker.add(originator.setMemento());

        System.out.println("---------------------------------------------");
        System.out.println("Check state at index 1 (index starts at 0):");

        Memento stateAtIndex1 = caretaker.getByIndex(1);
        System.out.println(stateAtIndex1.getState());

        System.out.println("---------------------------------------------");
        System.out.println("Check last state:");

        Memento lastState = caretaker.getCurrent();
        System.out.println(lastState.getState());

        System.out.println("---------------------------------------------");
        System.out.println("Undoing last state");

        caretaker.undo();

        System.out.println("---------------------------------------------");
        System.out.println("Check last state after undo:");

        Memento lastStateAfterUndo = caretaker.getCurrent();
        System.out.println(lastStateAfterUndo.getState());
    }

}

Output

Memento Saved with timestamp => Time - 1 : 1677468449701
Memento Saved with timestamp => Time - 2 : 1677468450713
Memento Saved with timestamp => Time - 3 : 1677468451726

---------------------------------------------

Check state at index 1 (index starts at 0):
Time - 2 : 1677468450713

---------------------------------------------

Check last state:
Time - 3 : 1677468451726

---------------------------------------------

Undoing last state

---------------------------------------------

Check last state after undo:
Time - 2 : 1677468450713