25-Comp-B11 Advanced Software Design · Undated paper
Question 23 of 28: Private, Protected, and Public — Meaning and Role in Encapsulation and Modularity
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 23: Private, Protected, and Public — Meaning and Role in Encapsulation and Modularity (Part V)
Private members are accessible only from within the SAME class's own code — not even a subclass can reach them directly. Protected members are accessible from the declaring class and any of its subclasses, but not from unrelated classes. Public members are accessible from any code anywhere that holds a reference to the object.
Why this matters for encapsulation. Encapsulation means bundling data with the operations that maintain its consistency and hiding the data's representation from the outside. Declaring the representation (fields) private and exposing only a deliberate set of public methods enforces this at the language level: no external code can bypass the class's own methods to put its data into an invalid state (e.g. Question 11's BankAccount.balance can only ever be changed through methods that preserve balance ≥ 0).
Why this matters for modularity. A class's public members are its interface contract with the rest of the system; its private members are an implementation detail the class is free to change without affecting any other code, AS LONG AS the public interface's behaviour is preserved. This is what lets one module's internals change independently of every other module — the essential property of modularity. protected occupies a middle ground, deliberately widening access to a class's own subclass family (needed for genuine inheritance-based extension) while still hiding those members from unrelated code.
