25-Comp-B11 Advanced Software Design · December 2019
Question 15 of 28: Factory Method vs. Abstract Factory
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 15: Factory Method vs. Abstract Factory (Part III)
Factory Method defines an interface for creating a SINGLE object, but lets a subclass decide which CONCRETE class to instantiate — one creation method, one product hierarchy, varied by subclassing the creator. Example: a DocumentApplication declares an abstract createDocument(); PDFApplication overrides it to return a PDFDocument, WordApplication to return a WordDocument.
Abstract Factory provides an interface for creating an entire FAMILY of related objects without naming their concrete classes, and specifically guarantees the created objects are MUTUALLY COMPATIBLE. Example: a UIFactory interface declares both createButton() and createCheckbox(); a concrete WinUIFactory produces a matched Windows-look Button and Checkbox pair, while MacUIFactory produces a matched Mac-look pair — a client using either factory can never accidentally mix a Windows button with a Mac checkbox.
The difference. Factory Method varies ONE product via subclassing; Abstract Factory varies a whole FAMILY of related products via composition/delegation, and is very often implemented internally USING several Factory Methods (one per product in the family) bundled behind a single factory interface. Factory Method's axis of variation is "which concrete class," Abstract Factory's axis is "which whole consistent family."
