Memento Pattern — Short Notes

Problem Overview

The TextEditor class keeps its text in one content field. write() replaces that text.
The class has no way to:

• Save previous states.
• Restore older versions of the content.

So undo/redo is impossible.

Design Issues

1. No State Preservation

   • Each call to write() replaces content, so the old text is lost.
   • There is no way to go back to a previous state.

2. Encapsulation Violation Risk

   • If we add undo/redo inside TextEditor, other code would need to see its internal state.
   • This would break encapsulation (the data should stay private). It would also break the Single Responsibility Principle (SRP), which says a class should do one job. The editor should not manage history.

3. Tight Coupling

   • If TextEditor also managed history, editing and history code would mix in one class. It would be harder to maintain or extend.

Core Problem

How can we add undo/redo, or restore an old state, without exposing the internal state of TextEditor and without violating SRP?

The Memento Pattern solves this by introducing:

• A separate Memento class (EditorMemento in After) to hold snapshots of the editor’s state.
• A Caretaker (e.g., a History class) to keep the mementos.

Each class has one job. The editor's data stays private.