Question 15 of 27: Structural vs. Behavioural Patterns
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
98-Comp-B11 Advanced Software Design — National Exams, May 2015. 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 four (4) questions in Part I, any three (3) in Part II, any three (3) in Part III, any two (2) in Part IV, and any four (4) 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 16 questions actually marked (4+3+3+2+4 of 27) each count for 100/16 ≈ 6.25% of the paper. All 27 questions are answered below for completeness.
Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, agile methods, design principles, 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 — structural/behavioural pattern catalogue (Adapter, Bridge, Strategy, Observer, Template Method, Composite, etc.); Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, inheritance and language-level object semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/invariants, the open–closed principle; Barbara Liskov's 1987 substitutability paper for Question 11; Rogers, Sharp & Preece, Interaction Design, and Nielsen, Usability Engineering, for Question 21's HMI-specific non-functional requirements.
PART I — General Principles (answer any 4 of 7)
Question 15: Structural vs. Behavioural Patterns (Part III)
Structural patterns (a GoF category) are concerned with how classes and objects are COMPOSED into larger structures — simple, static ways of assembling objects while keeping the resulting structure flexible and efficient (Adapter, Bridge, Composite, Decorator, Façade, Proxy). Their defining "shape" is a static COMPOSITION relationship between the participant classes.
Behavioural patterns are concerned with algorithms and the assignment of RESPONSIBILITY / communication between objects — how objects interact and distribute work at runtime (Strategy, Observer, Command, Template Method, State, Iterator). Their defining "shape" is a runtime PROTOCOL of collaboration.
Bridge is structural because its core contribution is a composition structure: it decouples an abstraction from its implementation by having the abstraction HOLD a reference to a separately-varying implementation interface, so the two class hierarchies (abstraction and implementation) can evolve independently. The pattern is about the static WIRING between two hierarchies via object composition, not about any runtime algorithm choice.
Strategy is behavioural because its core contribution is encapsulating an interchangeable ALGORITHM (or family of algorithms) behind a common interface, and letting a context object delegate to whichever concrete strategy it is currently configured with. The pattern is about WHICH behaviour executes and how that choice is made or changed at runtime, not about a static object structure.
Structurally, Bridge and Strategy look almost identical (in both, a context/abstraction object holds a reference to an interface and delegates to a concrete implementation) — the classification difference is one of INTENT: Bridge intends to let two class hierarchies evolve independently (a structural, architectural concern), while Strategy intends to make an algorithm swappable at runtime (a behavioural, algorithmic concern).
