Memento pattern

Introduction

The Memento pattern lets an object expose a snapshot of its state without exposing the details of how that state is represented. The object that owns the state creates and restores the snapshot, while another object can store it for undo, redo, or checkpointing.

The Problem It Solves

You are building a document editor with undo. The naive approach: let the undo manager reach into the document's private fields and copy them.

Two problems. First, NaiveUndoManager depends on every private field, so any change to the document's internals breaks the undo system. Second, the snapshot is a raw Map<String, Object> with no type safety, and nothing prevents other code from reading or mutating it.

Here is what changes when you apply the Memento pattern.

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Structure

Three distinct roles. DocumentEditor (originator) knows how to snapshot and restore its own state. DocumentMemento (memento) is an immutable capsule; its contents are meaningful only to the originator. HistoryManager (caretaker) stores mementos in undo/redo stacks without ever inspecting the contents.

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Implementation

The originator owns the save/restore logic. The caretaker only manages the stack. Memento fits when an object has complex internal state and needs checkpointing without exposing those internals.