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25-Comp-B11 Advanced Software Design · Undated paper

Question 22 of 28: Favor Object Composition Over Class Inheritance

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)

PART V — C++/Java and Modular Programming (answer any 5 of 7)

Question 22: Favor Object Composition Over Class Inheritance (Part V)

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.

The principle recommends building a class's behaviour by HOLDING A REFERENCE to another object and delegating to it (composition, a "has-a" relationship), rather than by subclassing to inherit that behaviour (a fixed, compile-time "is-a" relationship), whenever the behaviour might need to vary independently of the class using it.

Inheritance-based (less flexible):

class GasolineCar {
    void run() { System.out.println("burning fuel"); }
}
class Car extends GasolineCar { }

Here Car's propulsion is locked in at compile time by which class it extends. Supporting an electric variant requires either a whole parallel class hierarchy (ElectricCar extends ElectricPropulsion) with no shared Car code, or fragile multiple inheritance.

Composition-based (favoured):

interface Engine { void run(); }
class GasolineEngine implements Engine {
    public void run() { System.out.println("burning fuel"); }
}
class ElectricEngine implements Engine {
    public void run() { System.out.println("drawing from battery"); }
}
class Car {
    private Engine engine;           // has-a Engine
    Car(Engine engine) { this.engine = engine; }
    void run() { engine.run(); }
}

Now Car HAS an Engine rather than IS a particular engine type. A single Car class works with any Engine implementation, the engine can be swapped at construction time (or even at runtime with a setter) without touching Car's own code, and a brand-new engine type never requires modifying or resubclassing Car at all.