Command and Memento for High-Quality GUI Design: 25-Comp-B11 Advanced Software Design, Undated paper, Q20

Question 20 of 28: Command and Memento for High-Quality GUI Design

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

17-Comp-B11 Advanced Software Design — National Exams, May 2019. 3 hours, closed book exam with two aid sheets allowed (written on both sides), no calculator permitted. The paper is organized into five parts, and candidates were instructed to answer any five (5) questions in Part I, any three (3) in Part II, any four (4) in Part III, any two (2) in Part IV, and any five (5) in Part V — only the first questions answered, in each part, as they appear in the answer book are marked. All questions carry equal weight, so the 19 questions actually marked (5+3+4+2+5 of 28) each count for 100/19 ≈ 5.26% of the paper. All 28 questions are answered below for completeness.

Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, design principles, testing, dependability, reuse; Pressman, Software Engineering: A Practitioner's Approach (9th ed.) — supplementary process/metrics/quality coverage; Gamma, Helm, Johnson & Vlissides (GoF), Design Patterns: Elements of Reusable Object-Oriented Software — pattern-language structure, the GoF pattern catalogue, and the "favor object composition over class inheritance" / "program to an interface, not an implementation" principles; Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, visibility, encapsulation, interfaces; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/class invariants; Barbara Liskov's 1987 substitutability paper for Question 12; Stroustrup, The C++ Programming Language, for friend/access-control semantics (Question 25).

Question 12 prints “Liskpv substitution principle”, a typo in the paper; it is answered as the Liskov substitution principle.

PART I — General Principles (answer any 5 of 7)

Question 20: Command and Memento for High-Quality GUI Design (Part IV)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Command encapsulates a request as an object with an execute() method (and typically an undo() method), decoupling the invoker (a toolbar button or menu item) from the receiver (the object that actually performs the action). Example: a DeleteContactCommand object wraps "delete this contact"; the toolbar's "Delete" button simply calls command.execute() without knowing what specifically happens, and the same command object can be queued, logged, or placed on an undo stack.

Memento captures and externalizes an object's internal state, without violating its encapsulation, so that state can later be restored. Example: before DeleteContactCommand.execute() removes a contact, it asks the contact to produce a ContactMemento snapshot of its full data; undo() restores the contact directly from that memento.

How they combine for GUI quality. Command structures the WHAT — a discrete, nameable, undoable/redoable user action, which is the standard mechanism behind a GUI's Undo/Redo menu and toolbar; Memento supplies the HOW of undo without exposing a class's private fields to the command objects that need to restore them. A command's undo() implementation is very often literally "restore the receiver from the Memento captured just before execute() ran," directly improving usability (Question 21's "help prevent and/or fix user errors" goal) since a mistaken deletion is trivially reversible.

Concept & Theory: Structuring Undo Without Breaking Encapsulation

Topic: Command names the action; Memento supplies its rollback state, without either pattern exposing internal representation to the other.

Key relations: Command.execute() mutates state → Memento (captured just before) holds the pre-mutation snapshot → Command.undo() restores from it.

Why this works: Without Memento, undo logic would need direct access to the receiver's private fields, breaking encapsulation exactly where a GUI most needs many independent, composable undoable actions.

Common pitfall: Implementing undo as "re-run the inverse operation" instead of restoring a captured Memento — the inverse is not always well-defined (e.g. undoing a "delete" needs the deleted data, which only a Memento captured beforehand actually has).

Source: Gamma, Helm, Johnson & Vlissides, Design Patterns, "Command" and "Memento."