Memento Design Pattern in Java – Example Tutorial

Memento pattern is one of the behavioral design pattern. Memento design pattern is used when we want to save the state of an object so that we can restore later on. Memento pattern is used to implement this in such a way that the saved state data of the object is not accessible outside of the object, this protects the integrity of saved state data.

Memento pattern is implemented with two objects – Originator and Caretaker. Originator is the object whose state needs to be saved and restored and it uses an inner class to save the state of Object. The inner class is called Memento and its private, so that it can’t be accessed from other objects.

Caretaker is the helper class that is responsible for storing and restoring the Originator’s state through Memento object. Since Memento is private to Originator, Caretaker can’t access it and it’s stored as a Object within the caretaker.

One of the best real life example is the text editors where we can save it’s data anytime and use undo to restore it to previous saved state. We will implement the same feature and provide a utility where we can write and save contents to a File anytime and we can restore it to last saved state. For simplicity, I will not use any IO operations to write data into file.

Originator Class


public class FileWriterUtil {

	private String fileName;
	private StringBuilder content;
	public FileWriterUtil(String file){
		this.content=new StringBuilder();
	public String toString(){
		return this.content.toString();
	public void write(String str){
	public Memento save(){
		return new Memento(this.fileName,this.content);
	public void undoToLastSave(Object obj){
		Memento memento = (Memento) obj;
		this.fileName= memento.fileName;
	private class Memento{
		private String fileName;
		private StringBuilder content;
		public Memento(String file, StringBuilder content){
			//notice the deep copy so that Memento and FileWriterUtil content variables don't refer to same object
			this.content=new StringBuilder(content);

Notice the Memento inner class and implementation of save and undo methods. Now we can continue to implement Caretaker class.

Caretaker Class


public class FileWriterCaretaker {

	private Object obj;
	public void save(FileWriterUtil fileWriter){;
	public void undo(FileWriterUtil fileWriter){

Notice that caretaker object contains the saved state in the form of Object, so it can’t alter its data and also it has no knowledge of it’s structure.

Memento Test Class

Lets write a simple test program that will use our memento implementation.


public class FileWriterClient {

	public static void main(String[] args) {
		FileWriterCaretaker caretaker = new FileWriterCaretaker();
		FileWriterUtil fileWriter = new FileWriterUtil("data.txt");
		fileWriter.write("First Set of Data\n");
		// lets save the file;
		//now write something else
		fileWriter.write("Second Set of Data\n");
		//checking file contents

		//lets undo to last save
		//checking file content again


Output of above program is:

First Set of Data

First Set of Data
Second Set of Data

First Set of Data

The pattern is simple and easy to implement, one of the thing needs to take care is that Memento class should be accessible only to the Originator object. Also in client application, we should use caretaker object for saving and restoring the originator state.

Also if Originator object has properties that are not immutable, we should use deep copy or cloning to avoid data integrity issue like I have used in above example. We can use Serialization to achieve memento pattern implementation that is more generic rather than Memento pattern where every object needs to have it’s own Memento class implementation.

One of the drawback is that if Originator object is very huge then Memento object size will also be huge and use a lot of memory.

About Pankaj

If you have come this far, it means that you liked what you are reading. Why not reach little more and connect with me directly on Google Plus, Facebook or Twitter. I would love to hear your thoughts and opinions on my articles directly.

Recently I started creating video tutorials too, so do check out my videos on Youtube.

I read a lot of books, below is the one I am currently reading.

Below are some of the recommended books in different technologies.

Some of the video tutorials on Udemy I would recommend are;

Join our email newsletter to get promo codes for almost 90% discount on these courses, and yes you will get a lot of free eBooks too, absolutely Free.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

Subscribe to JournalDev Newsletter
Get the FREE access to JournalDev Newsletter and Free PDF eBooks
*No Spam Guaranteed. By entering your email address, you agree also subscribing to our newsletter.
Oops! - Something went wrong.