Question 4 of 28: Use-Case Actors vs. Design Classes (Library Domain)
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
98-Comp-B11 Advanced Software Design — National Exams, May 2016. 3 hours, closed book exam with one aid sheet 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, agile methods, 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 (Singleton, Proxy, Template Method, Observer, etc.); 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, the open–closed principle; Barbara Liskov's 1987 substitutability paper for Question 11; Rogers, Sharp & Preece, Interaction Design, and Nielsen, Usability Engineering, for Question 21's HMI-specific non-functional requirements; Myers, The Art of Software Testing, for Question 28's boundary value analysis.
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; "Librarian" exists in the use-case model purely to scope which external role checks books in/out, manages holds, and issues overdue notices. The design model's internal representation of that work need not be a class named "Librarian" at all — per the Entity/Control/Boundary (ECB) responsibility pattern (Question 19), the system might instead need only a CirculationController Control class coordinating checkout/return logic, with the librarian's own account handled by a generic StaffAccount Entity shared with other employee roles. Conversely, if librarians genuinely have data no other role has (a specialization certification, an assigned collection area), a Librarian Entity class recording exactly that data COULD legitimately exist — but that is an independent design decision driven by what data must be stored, not a consequence of the actor's mere existence in the use-case diagram.
The Patron direction is symmetric: a Patron ENTITY class storing name, address, and borrowed-item history does not imply a Patron actor must exist in the use-case diagram — if, for example, the library's system is used only at the front desk (patrons hand items to a librarian, who keys every checkout/return into the system on the patron's behalf, and patrons never touch a terminal themselves), then only Librarian is an actor, and Patron 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.
