25-Comp-B11 Advanced Software Design · December 2014
Question 9 of 26: Use-Case Actors vs. Design Classes
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
98-Comp-B11 Advanced Software Design — National Exams, December 2014. 3 hours, closed 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 four (4) in Part III, any two (2) in Part IV, and any two (2) 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 15 questions actually marked (3+4+4+2+2 of 26) each count for 100/15 ≈ 6.7% of the paper. All 26 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); Brown, Malveau, McCormick & Mowbray, AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis — anti-pattern catalogue (Question 18). 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.
PART I — General Principles (answer any 3 of 5)
Question 9: Use-Case Actors vs. Design Classes (Part II)
Disagree, in both directions. Actors and classes belong to different models with different purposes, and nothing requires a 1:1 name correspondence between them.
An actor represents a ROLE played by something external to the system (a human, another system) interacting across the system boundary; it exists in the use-case model purely to scope external interactions. A class with the same name in the design model may or may not correspond to that role's system-side representation — per the Entity/Control/Boundary (ECB) responsibility pattern (Question 20), an actor's data footprint inside the system (their stored account, their session) is typically an Entity or Control class, not literally a class named after the actor. Conversely, the system may need an internal Dispatcher CONTROL class purely to coordinate cab-assignment logic (choosing which driver is sent where) — a class that has nothing to do with a human/external "Dispatcher" actor at all, and may not correspond to any actor whatsoever; it is an internal design decision, not a reflection of who interacts with the system.
The Customer direction is symmetric: a Customer ENTITY class storing name/address/billing data does not imply a Customer actor must exist in the use-case diagram — if, for example, dispatch staff enter customer details on the customer's behalf and the customer never directly operates the system, Customer is a purely internal data-storage class with no corresponding actor at all.
Conclusion. Use-case actors and class-diagram classes are independent naming spaces serving independent modelling purposes (external interaction scope vs. internal responsibility allocation); a name shared between them is coincidental, not a modelling rule. Treating them as necessarily linked collapses the boundary between the analysis (requirements/use-case) model and the design (class) model, a common novice UML error.
