Design patterns in examples - Memento

Design patterns in examples - Memento

Memento is a design pattern that represents one of the possibilities to implement save and restore functionality.

The pattern consists of the following elements:

Memento is an interface that represents an object that can be saved and restored but it exposes only choses properties without revealing details.

  // Memento
 public interface MailMemento {
 }

Originator is a class that knows details of Memento because actual implementation of Memento (Draft in this case) is an inner class of it. It exposes methods to create a Memento (createDraft()) and to restore it (load(MailMemento mailMemento)).

 // Originator
 public class MailEditor {
    private String content;
 
    public MailMemento createDraft() {
       return new Draft(content);
    }
 
    public void load(MailMemento mailMemento) {
       content = ((Draft) mailMemento).content;
    }
 
    public void setContent(String content) {
       this.content = content;
    }
 
    private static class Draft implements MailMemento {
       private final String content;
 
       private Draft(String content) {
          this.content = content;
       }
    }
 }

Caretaker is a class that operates on Memento, for example can store and restore it but it has a limited access to Memento's properties as it accesses it via the interface only.

 // Caretaker
 public class MailBox {
    private MailMemento draft;
 
    public void save(MailMemento mailMemento) {
       draft = mailMemento;
    }
 
    public MailMemento loadDraft() {
       return draft;
    }
 }

Memento pattern can be used by other code in the following way.

public class Main {
   public static void main(String[] args) {
      MailBox mailBox = new MailBox();
 
      MailEditor mailEditor = new MailEditor();
      mailEditor.setContent("body");
      MailMemento draft = mailEditor.createDraft();
      mailBox.save(draft);
 
      MailMemento restoredDraft = mailBox.loadDraft();
      mailEditor.load(restoredDraft);
   }
}