25-Comp-B11 Advanced Software Design · December 2019
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, December 2019. 3 hours, closed book exam with one aid sheet 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; Pressman, Software Engineering: A Practitioner's Approach (9th ed.) — supplementary process and quality coverage; Gamma, Helm, Johnson & Vlissides (GoF), Design Patterns: Elements of Reusable Object-Oriented Software — creational/structural/behavioural pattern catalogue and the "program to an interface, not an implementation" / "favor object composition over class inheritance" principles; Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, visibility, and multiple inheritance semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/class invariants; Barbara Liskov's 1987 substitutability paper for Question 12; Karl Wiegers, Software Requirements (3rd ed.); Myers, The Art of Software Testing, for Question 6.
PART I — General Principles (answer any 5 of 7)
Question 20: Command and Memento for High-Quality GUI Design (Part IV)
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: an AddTransactionCommand object wraps "add this transaction"; the toolbar's "Add" 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 EditTransactionCommand.execute() mutates a transaction's amount, it asks the transaction to produce a TransactionMemento snapshot of its OLD values; undo() restores the transaction 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 edit is trivially reversible.
