Memento Pattern | Design Pattern

Introduction

Memento Design Pattern is a Behavioral design pattern. It offers a solution to implement undoable actions. Memento pattern is used to save and restore the previous state of an object without revealing the details of its implementation.

Intent

• Capture the internal state of an object without violating encapsulation.
• Providing a mean for restoring the object into initial state when needed.
• Save checkpoints in application and restore back to those checkpoints later.

Implementation

Following are the participants in Memento Design Pattern

• Originator : Object for which the state is to be saved. It knows how to create and save it’s state.
• Caretaker : It keeps track of multiple mementos.
• Memento : Maintain the state of Originator. It is written and read by the Originator. A memento must be in immutable object so that no one can change it’s state once created.

Originator can produce and consume a Memento, while Caretaker only keeps the state before restoring it.

Example

In this example, memento is created and restored for Article object.

public class Article 
{
  private String title;
  private String content;
     
  public Article(String title) {
    this.title = title;
  }
     
  //Create Memento     
  public ArticleMemento createMemento() 
  {
    ArticleMemento m = new ArticleMemento(title, content);
    return m;
  }
  
  // Restore Memento
  public void restore(ArticleMemento m) {
    this.title = m.getTitle();
    this.content = m.getContent();
  }

  @Override
  public String toString() {
    return "Article : Title=" + title + ", Content=" + content;
  }

  void setContent(String s){
    content = s;
  }
}

// Memento Class
public final class ArticleMemento
{
  private final String title;
  private final String content;
     
  public ArticleMemento(String title, String content) {
    this.title = title;
    this.content = content;
  }
 
  public String getTitle() {
    return title;
  }
 
  public String getContent() {
    return content;
  }
}

// The Main class is acting as Caretaker which creates and restores the memento objects.
public class MementoTest 
{
  public static void main(String[] args) 
  {
    Article article = new Article("Article1");

    // Add Content
    article.setContent("Content");      
    System.out.println(article);
      
    // Create memento
    ArticleMemento memento = article.createMemento();
      
    // Update Content
    article.setContent("ContentChanged");
    System.out.println(article);
      
    // Restore
    article.restore(memento);
  }
}