# Purpose

Captures and stores the current state of an object to be able to restore it later, without violating encapsulation.

# Problem

• State Restoration: Need to go back without exposing internal implementation.

• Encapsulation Violation: Prevent external objects from manipulating the private state.

# Solution

Delegate the creation of a state copy (Memento) to the object itself (Originator). A Caretaker manages the history without seeing the details.

# Structure

# Participants

• Memento: The state snapshot.
• Originator: Creates the memento and restores its state from it.
• Caretaker: Maintains the memento stack (history).

# When to Use It

To implement "undo/redo" functionality or naturally keep state histories without exposing the origin class.

# Example: Text Editor (Undo)

Store editor content before each change in a stack to allow rollback.

# Java Code

class Memento {
    private final String state;
    public Memento(String s) { state = s; }
    public String getState() { return state; }
}

class Editor {
    private String text;
    public Memento save() { return new Memento(text); }
    public void undo(Memento m) { text = m.getState(); }
}

# Mapping Participants

• TextMemento: Memento.
• TextEditor: Originator.
• Caretaker: Change stack manager.

# Conclusions

Robust solution to implement state rollback in a clean and maintainable way.

# Related Patterns