Problem

The Memento pattern addresses the need to capture and restore an object's internal state without violating its encapsulation. This is useful in scenarios where you want to implement undo/redo functionality, allowing an object to revert to a previous state.

Solution

The Memento pattern involves three main components:

1. Originator: The object whose internal state needs to be saved and restored.
2. Memento: An object that stores the internal state of the originator. The memento is immutable.
3. Caretaker: Responsible for requesting the originator to save or restore its state from a memento.

The originator creates a memento containing a snapshot of its current state. This memento can then be stored by the caretaker and used to restore the originator's state when needed.

Pros and Cons

Pros

• Preserves Encapsulation: Allows an object's internal state to be saved and restored without exposing its implementation details.
• Simple Undo/Redo: Facilitates the implementation of undo/redo functionality, making the system more robust and user-friendly.
• State History: Allows maintaining a history of previous states of the object, enabling navigation between different states.

Cons

• Memory Consumption: Storing multiple mementos can consume significant memory, especially if the object's state is large.
• Additional Complexity: Introduces additional complexity to the code, with the need to manage the creation and restoration of mementos.
• Caretaker Responsibility: The caretaker needs to manage mementos efficiently, which can add responsibility and complexity to the system.

Example of Real-World Application

A practical example of the Memento pattern is in text editors that offer undo/redo functionality. Each change to the document can be saved as a memento, allowing the user to revert to previous states of the document as needed.

Example Code in Java

Memento pattern in code:

// Originator
public class Editor {
    private String content;

    public void setContent(String content) {
        this.content = content;
    }

    public String getContent() {
        return content;
    }

    public Memento save() {
        return new Memento(content);
    }

    public void restore(Memento memento) {
        content = memento.getContent();
    }

    // Memento
    public static class Memento {
        private final String content;

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

        private String getContent() {
            return content;
        }
    }
}

// Caretaker
public class History {
    private final Stack<Editor.Memento> history = new Stack<>();

    public void save(Editor editor) {
        history.push(editor.save());
    }

    public void undo(Editor editor) {
        if (!history.isEmpty()) {
            editor.restore(history.pop());
        }
    }
}

// Client code
public class Client {
    public static void main(String[] args) {
        Editor editor = new Editor();
        History history = new History();

        editor.setContent("Version 1");
        history.save(editor);
        System.out.println(editor.getContent());

        editor.setContent("Version 2");
        history.save(editor);
        System.out.println(editor.getContent());

        editor.setContent("Version 3");
        System.out.println(editor.getContent());

        history.undo(editor);
        System.out.println(editor.getContent());

        history.undo(editor);
        System.out.println(editor.getContent());
    }
}