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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2015/imperative-programming/5/b/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2015 / Imperative programming / 5 / b / Solution by

Codex  0 2026-10-31

Write each Scala trait linearization from most specific to least specific. Merge the parents' Scala trait linearizations right-to-left, removing a duplicate from the earlier portion when it also appears in the later portion. This retains every base once while respecting its required position after derived traits.^[ref]

CityMan, City, Earth, Man, Mammal, Animal, Describe

BirdMan, City, Earth, Man, Mammal, Bird, Animal, Describe

BatMan, Man, Gotham, City, US, Earth, Bat, Mammal, Bird, Animal, Describe

For example, combining the final City with Gotham does not put City before its derived trait Gotham; duplicate removal places it after Gotham. Similarly Mammal and Bird remain after Bat, despite their occurrence through other parents.

Each describe calls super.describe before appending its own label. Thus execution enters implementations along the displayed Scala trait linearization, and appends labels in reverse order when calls return. The three printed lines are

Describe, Animal, Mammal, Man, Earth, City, 
Describe, Animal, Bird, Mammal, Man, Earth, City, 
Describe, Animal, Bird, Mammal, Bat, Earth, US, City, Gotham, Man,

Each line has a final comma and space, because every printed string fragment includes them. The essential result is

Output labels follow the reverse linearization, excluding the concrete class.

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