Understanding the Memento Design Pattern

1. Memento Kendra Kachelein Maureen Thomas

2. Definition • The memento captures and externalizes an object’s internal state, so the object can be restored to that state later.

3. Applicability • Use the Memento pattern when: • A snapshot of an object’s state must be saved so that it can be restore to that state later, and • A direct interface to obtaining the state would expose implementation details and break the object’s encapsulation

4. Participants • There are 3 participants in the Memento pattern: • Originator: whose state has to be maintained • Memento: stores the state of the Originator object • Caretaker: has a handle to the Memento

5. Example 34-55-09 Caretaker Memento Originator

6. UML Diagram Caretaker Originator Memento -State -State +GetState() +SetState() +SetMemento(in Memento) +CreateMemento() Class Example: Security Guard Owner Safety Deposit -State -State +GetCombo() +GiveCombo() +DepositCombination(in Memento) +WriteCombination()

7. Source Code Implementation public class Owner { private int combination public safetyDeposit getCombo() { return new saftetyDeposit(this); } public void giveCombo(safetyDeposit sd) { combination = sd.combination; }