25-Comp-B11 Advanced Software Design · December 2019
Question 12 of 28: Liskov Substitution Principle — A Design That Follows It and One That Violates It
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
17-Comp-B11 Advanced Software Design — National Exams, December 2019. 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, design principles, testing, 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, visibility, and multiple inheritance semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/class invariants; Barbara Liskov's 1987 substitutability paper for Question 12; Karl Wiegers, Software Requirements (3rd ed.); Myers, The Art of Software Testing, for Question 6.
PART I — General Principles (answer any 5 of 7)
Question 12: Liskov Substitution Principle — A Design That Follows It and One That Violates It (Part II)
The Liskov Substitution Principle (LSP) states that a subtype must be substitutable for its base type: any code written against a reference to the base type must behave correctly no matter which subtype instance it actually receives at runtime. Formally, a subtype's methods may not STRENGTHEN preconditions or WEAKEN postconditions relative to the base type's contract (Questions 8–10) — it may only ask for as little, and promise as much, as the base type did.
Follows LSP. A base class Bird declares makeSound(). Subclasses Sparrow and Duck each override it to produce their own sound, but neither strengthens the (empty) precondition nor weakens the postcondition ("produces some audible sound") — any client code that calls makeSound() on a Bird reference behaves correctly regardless of which subclass it actually holds.
Violates LSP.Rectangle with setWidth(w)/setHeight(h) and area(); Square extends Rectangle overrides both setters so that setting either dimension also changes the other, keeping width = height. Client code that does r.setWidth(5); r.setHeight(4); assert r.area() == 20; is correct for any genuine Rectangle but fails for a Square instance (area becomes 16), because Square WEAKENS the postcondition of setWidth()/setHeight(): Rectangle guarantees that setting one dimension leaves the other unchanged, and Square no longer delivers that guarantee — it is not substitutable for Rectangle in this behaviour, even though it is a mathematically valid "is-a" relationship.
