Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
98-Comp-B11 Advanced Software Design — National Exams, May 2014. 3 hours, open book, no calculator permitted. The paper is organized into five parts, and candidates were instructed to answer any three (3) questions in Part I, any four (4) in Part II, any three (3) in Part III, any one (1) in Part IV, and any one (1) 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 12 questions actually marked (3+4+3+1+1 of 25) each count for 100/12 ≈ 8.3% of the paper. All 25 questions are answered below for completeness.
Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, agile methods, design principles; 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 (Proxy, Bridge, Strategy, Observer, Template Method, Composite, etc.); Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, inheritance and language-level object semantics (also underpins the Java/C++ discussion in Part V). Bertrand Meyer's Object-Oriented Software Construction is cited by name where the paper's own vocabulary (design by contract, open–closed principle) originates there; Barbara Liskov's 1987 substitutability paper is likewise cited by name for Question 11.
(Implementation) inheritance creates a static, compile-time is-a relationship in which the subclass automatically receives — and can be affected by — the ENTIRE public and protected interface of its superclass, including operations irrelevant or even harmful to the subclass's own abstraction. This is exactly the "message leakage" problem: a Stack implemented by extending List inherits insertAt(index)/removeAt(index), letting client code bypass the Stack's push/pop discipline entirely and violate its own invariant.
Delegation (composition plus forwarding) creates a runtime has-a relationship: the using class holds a private reference to an instance of the used class and explicitly forwards only the specific operations it chooses to expose through its own interface — the used object's full interface never leaks through.
When delegation is necessary in place of inheritance:
When the is-a relationship doesn't actually hold in the LSP sense (Question 11) — the subclass cannot honestly satisfy the superclass's full contract without weakening it.
When only a PARTIAL subset of the superclass's interface is wanted or appropriate — the Stack-via-List message-leakage case above; delegation lets the class expose only push()/pop()/peek(), hiding insertAt/removeAt entirely.
When the relationship needs to change at RUNTIME — which object is delegated to can be swapped after construction, while inheritance's is-a binding is fixed at compile time; this is precisely the mechanism behind the Strategy pattern (Question 15).
When avoiding tight coupling to a superclass's IMPLEMENTATION details across a fragile-base-class boundary — a change to the superclass's internals can silently break subclasses that rely on its exact behaviour in ways delegation's narrower forwarding interface never exposes.
Left: inheritance leaks List's full interface into Stack. Right: delegation exposes only push/pop/peek, forwarding internally to a private List.
