🧠Memento Pattern

Capture and externalize an object's internal state so it can be restored later — without violating encapsulation.

📌 Key IdeaOriginator creates Memento snapshots; Caretaker stores them (doesn't peek inside). Undo stacks in editors, game save slots, transactional checkpoints.

🖼

Visual

🧠 Snapshot without breaking encapsulation

  Editor.originator.save() → Memento
  Caretaker.stack.push(memento)
  ... edit ...
  Caretaker.pop().restore(editor)

Caretaker holds opaque token; only originator interprets memento.

✅

When to Use

Undo/redo beyond simple command inverse — need full state snapshot.
Checkpoints / rollback for long-running edits.
You must not expose originator's private fields to caretaker.

🚫

When NOT to Use

State is huge — memory pressure (consider incremental command undo instead).
Immutable state — often simpler to rebuild from operations.

🚨

Anti-Pattern (What NOT to Do)

Caretaker pokes private editor fieldsBadjava

void undo(Editor ed) {
    ed.text = stack.pop();      // breaks encapsulation
    ed.cursor = stack.pop();
}

Caretaker shouldn't know how to reconstruct internals.

✅

The Right Way

Opaque memento from originatorGoodjava

final class Editor {
    private String text;
    Memento save() { return new Memento(text); }
    void restore(Memento m) { this.text = m.text(); }

    private record Memento(String text) {}  // narrow visibility
}

Caretaker stores Memento references blindly; only Editor reads fields.

⚖️

Pros & Cons

👍 Pros

Preserves encapsulation boundaries.
Clean separation: originator vs caretaker responsibilities.

👎 Cons

Memory cost if snapshots are large or frequent.
Careful with deep vs shallow copy semantics.

🔀

Common Confusions Cleared Up

Memento vs Command for undo

Command undo often stores reverse operation. Memento stores state snapshot when reversal isn't trivial or history branches.

🎤

Interview Q&A

🔮

Likely Follow-up Questions

Copy-on-write for large documents?

🧾

Quick Cheat Sheet

IntentSnapshot state; restore later.

RolesOriginator, Memento, Caretaker.

Pairs withCommand (undo stack).

🔗