Describe the Memento design pattern and provide an example of its usage.

The Memento design pattern is a behavioral design pattern that allows an object to capture and store its internal state so that it can be restored later without violating encapsulation. It is used to provide the ability to undo or rollback changes made to an object's state.

The Memento pattern consists of three main components: the Originator, the Memento, and the Caretaker. The Originator is the object whose state needs to be saved and restored. The Memento is an object that stores the state of the Originator. The Caretaker is responsible for storing and managing the Mementos.

Here is an example to illustrate the usage of the Memento design pattern:

Let's consider a text editor application that allows users to write and edit documents. The application needs to provide an undo/redo functionality to revert changes made to the document.

1. Originator: The Document class represents the Originator in this example. It has a state that includes the content of the document.

2. Memento: The DocumentMemento class represents the Memento. It stores the state of the Document object at a specific point in time.

3. Caretaker: The DocumentHistory class represents the Caretaker. It is responsible for storing and managing the DocumentMementos.

When a user makes changes to the document, the Document object's state is updated. To enable undo/redo functionality, the Document object creates a DocumentMemento object and passes its current state to it. The DocumentMemento object is then stored in the DocumentHistory object.

When the user wants to undo a change, the DocumentHistory object retrieves the last DocumentMemento object and restores the Document object's state to that of the DocumentMemento. Similarly, when the user wants to redo a change, the DocumentHistory object retrieves the next DocumentMemento object and restores the Document object's state.

By using the Memento design pattern, the text editor application can easily implement the undo/redo functionality without exposing the internal state of the Document object. The Memento pattern helps in maintaining encapsulation and separation of concerns by keeping the responsibility of state management separate from the Originator object.