25-Comp-B11 Advanced Software Design · December 2019
Question 14 of 28: Creational, Structural, and Behavioural Pattern Classification
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 14: Creational, Structural, and Behavioural Pattern Classification (Part III)
Creational patterns are concerned with the PROCESS OF OBJECT CREATION itself — abstracting how, when, and which concrete class gets instantiated (Singleton, Factory Method, Abstract Factory, Builder, Prototype). Their defining concern is controlling instantiation.
Structural patterns are concerned with how classes and objects are COMPOSED into larger structures (Adapter, Bridge, Composite, Decorator, Façade, Proxy). Their defining shape is a static composition relationship between participant classes.
Behavioural patterns are concerned with algorithms and the assignment of RESPONSIBILITY and communication between objects (Strategy, Observer, Command, Template Method, State, Iterator). Their defining shape is a runtime protocol of collaboration.
Creational example — Singleton. Its entire contribution is controlling instantiation: it ensures a class has exactly one instance and provides a single global access point. It says nothing about composing objects into a larger structure, nor about a runtime communication protocol — its purpose begins and ends at controlled, single instantiation.
Structural example — Façade (Question 17). Façade provides one simplified interface object that is statically COMPOSED of references to several subsystem objects, hiding their individual interfaces behind one unified entry point — a pure composition-of-objects concern.
Behavioural example — Observer. Its core contribution is a runtime PROTOCOL of collaboration: a subject notifies an open-ended set of registered observers whenever its state changes, and each observer decides for itself how to react. The pattern is entirely about the assignment of responsibility and the sequencing of communication at runtime.
