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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2019/imperative-programming/7/b/i/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Imperative programming / 7 / b / i / Solution by

Codex  0 2026-10-25

The constructor stores a reference, not a copy:

val d = new Dimension(3, 4)
val f = new Frame(d)
d.width = 10

Now f.area is still $12$ , but the product of its stored dimension fields is $40$ . Even without retaining d, a client can break the same invariant through f.dimension.height = 9, because the accessor exposes the same mutable object. This is representation exposure through reference aliasing. A val prevents reassigning the reference; it does not freeze the referenced object.

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