Why Separate Entity, Control, and Boundary Classes: 25-Comp-B11 Advanced Software Design, December 2018, Q19

Question 19 of 28: Why Separate Entity, Control, and Boundary Classes

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

17-Comp-B11 Advanced Software Design — National Exams, December 2018. 3 hours, closed book exam with up to 2 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, 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, inheritance and language-level object semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/invariants; Barbara Liskov's 1987 substitutability paper for Question 11; Karl Wiegers, Software Requirements (3rd ed.) — the functional/quality/process/implementation/business requirements taxonomy of Question 2; Kruchten, The Rational Unified Process: An Introduction, for Question 1; Myers, The Art of Software Testing, for Questions 6 and 28.

PART I — General Principles (answer any 5 of 7)

Question 19: Why Separate Entity, Control, and Boundary Classes (Part IV)

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.

Boundary classes model the interface between the system and its actors and are, by their nature, VOLATILE — they change whenever the presentation technology changes even when the underlying business logic and data are completely unchanged. Keeping Boundary separate from Entity (persistent domain data) and Control (use-case orchestration logic) confines presentation-layer churn to the Boundary classes alone.

Implementation. The same business logic can be reused behind a different interface (e.g., exposing the same rules through a REST API alongside a desktop GUI) since the logic is never welded to one specific UI technology's event model.

Testing. Business logic kept out of UI event handlers can be exercised by fast, headless unit tests against a pure Entity/Control class with no UI dependency at all, instead of slow, brittle tests that must drive the actual UI — this is the single largest practical quality/efficiency gain from separating.

Maintenance. A UI framework upgrade or platform port (e.g., desktop to web) does not force re-verifying or re-writing business logic that merely happened to live inside now-obsolete UI classes, multiplying the cost and risk of what should have been a purely cosmetic change.

This is the same requirements-analysis-level rationale underlying Question 4's use-case-to-class derivation and Question 18's MVC View/Model split — ECB is the general responsibility-assignment principle both are specific instances of.