The Façade Pattern for a Personal Financial Management Application: 25-Comp-B11 Advanced Software Design, December 2019, Q17

Question 17 of 28: The Façade Pattern for a Personal Financial Management Application

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 17: The Façade Pattern for a Personal Financial Management Application (Part III)

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.

Extending Question 4's decomposition with a separate report-formatting class, the application has three separate subsystems that must be coordinated: a TransactionManager (add/edit/calculate), a ReportGenerator (summaries), and a FileStorage (save/load). Without a Façade, the GUI client would need to know about, and correctly sequence calls across, all three.

FinanceFacade sits between the GUI client and the three subsystems, exposing one simplified method per user-facing operation.

Where and how Façade is implemented. FinanceFacade exposes exactly the five operations the use cases require (addTransaction(), editTransaction(), viewSummary(), save(), load()) and internally coordinates the subsystem calls each requires — e.g. viewSummary() internally calls TransactionManager.calculateBalance() and then ReportGenerator.format(), without the GUI ever needing to know ReportGenerator exists. The GUI client depends on ONLY the Façade's five methods; the three subsystem classes remain independently usable by any advanced caller that bypasses the Façade, since it does not hide them, only simplifies the common path.