25-Comp-B11 Advanced Software Design · Undated paper
Question 15 of 28: Factory Method vs. Template Method
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 15: Factory Method vs. Template Method (Part III)
Both patterns let a subclass customize behaviour defined in a superclass, and both are class-inheritance-based (Question 14) — the difference is WHAT the subclass is customizing.
Factory Method is a CREATIONAL pattern: the subclass decides WHICH CONCRETE CLASS TO INSTANTIATE. Example: an abstract Exporter declares createWriter() as the factory method; CsvExporter overrides it to return a CsvWriter, while XmlExporter overrides it to return an XmlWriter. The rest of Exporter's logic calls only createWriter() and never names a concrete writer class directly.
Template Method is a BEHAVIOURAL pattern: the subclass decides HOW ONE OR MORE STEPS of an already-fixed algorithm are carried out, with no object creation involved. Example: an abstract DataExporter.export() defines the fixed step sequence open() → writeHeader() → writeRows() → close(); CsvExporter overrides writeHeader()/writeRows() to produce comma-separated output, while XmlExporter overrides the same two steps to produce tagged output — the overall algorithm's STEP ORDER never changes, only the content of individual steps.
In short: Factory Method varies WHICH OBJECT gets created; Template Method varies HOW STEPS of a fixed procedure are performed. The two are often used together — a Template Method's step can itself call a Factory Method to obtain the object that step needs.
