The Façade Pattern for a Personal Address Book Application: 25-Comp-B11 Advanced Software Design, Undated paper, Q17

Question 17 of 28: The Façade Pattern for a Personal Address Book Application

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 17: The Façade Pattern for a Personal Address Book 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.

Reusing Question 4's decomposition, the application already has three separate subsystems that must be coordinated: an AddressBookManager (add/delete/edit), a ContactValidator (checks phone/email format before a contact is accepted), and a FileStorage (save/load). Without a Façade, the GUI client would need to know about, and correctly sequence calls across, all three.

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

Where and how Façade is implemented. AddressBookFacade exposes exactly the operations the use cases require (addContact(), deleteContact(), editContact(), save(), load() — one per use case) and internally coordinates the subsystem calls each requires — e.g. addContact(c) internally calls ContactValidator.validate(c) and then, only if valid, AddressBookManager.add(c), without the GUI ever needing to know ContactValidator 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.