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 Incoming links: Fragile base class problem

Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Imperative programming / 7 / c / ii / Solution 2026-10-25

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One base-class implementation changes the private fields directly:

def flip(): Unit = {
  val oldH = _h
  _h = _w
  _w = oldH
}

The completed operation leaves the area unchanged. It calls no overriding setters, so the logging subclass's count stays unchanged and does not need an override. The invariant is about completed public updates, not transient internal states between the two assignments.

A second implementation uses public setters:

def flip(): Unit = {
  val oldH = height
  val oldW = width
  height = oldW
  width = oldH
}

These setter calls use dynamic dispatch on a logging receiver. Starting at height two and width three, the first call increases the temporary area from six to nine and wrongly increments the count, while the final area returns to six. This version needs a subclass override. With the counter name from part (i), the following override discards the internal setter contributions:

override def flip(): Unit = {
  val before = increases
  super.flip()
  increases = before
}

For this specified base implementation the two assignments complete normally and the final area equals the initial area, so restoring the saved count is correct. Another design could suppress logging during the compound operation. The difference between the two base implementations illustrates the fragile base class problem: an internal refactoring can change subclass behavior through newly introduced virtual calls.

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Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Imperative programming / 6 / e / Solution 2026-10-25

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A base-class change can preserve its own set abstraction while breaking a subclass that intercepts internal calls. For example, suppose IntegerSet.delete(x) is reimplemented by collecting the elements to retain, calling clear, then calling insert for each retained element. Conceptually its body is

val retained = snapshotOfElements.filter(_ != x)
clear
retained.foreach(insert)

Here snapshotOfElements denotes the base implementer's own internal snapshot operation in this hypothetical revision, not an API supplied by the examination. The algorithm still removes exactly the requested value and preserves all other set members.

On an IntegerSetWithCount, however, these unqualified calls use dynamic dispatch. The internal clear resets the subclass counter and the internal insertions increase it. After client calls insert(10), insert(20), insert(10), the correct count is three. Calling delete(10) with this revised base implementation leaves the right set, containing only the other value, but resets the counter and reinserts that value, leaving count one instead of three. The client made no further insertion attempt and requested no clear.

This is the fragile base class problem: subclass correctness depends on the base implementation's internal call structure, beyond its public set postconditions. A change need not alter the base API to expose that dependence.

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