Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Imperative programming / 6 / d

Count each invocation before delegating, including duplicate insertions, and reset after clearing:

class IntegerSetWithCount extends IntegerSet() {
  private var attempts = BigInt(0)

  override def insert(x: Int): Unit = {
    attempts += 1
    super.insert(x)
  }
  override def clear: Unit = {
    super.clear
    attempts = BigInt(0)
  }
  def getCount: BigInt = attempts
}

The inherited implementation already maintains size; the new counter measures insertion attempts, not successful increases in cardinality. Deletion and popping do not subtract from it. Incrementing before delegation also counts an attempted insertion that subsequently fails a capacity check. Arbitrary-precision arithmetic avoids silently wrapping the historical count, though its arithmetic is not constant-cost for arbitrarily large counts.

As the source supplies no object constructor signature for the concrete set, this code uses a fresh default-constructible IntegerSet, with matching default capacity. Its correct behavior also assumes the base implementation does not internally invoke these overridable operations in a way that interferes with the counter. Part e exhibits that dependency.

Source-signature qualification for the following set implementations: the intended membership query must be declared def contains(x: Int): Boolean. The printed abstract declaration omits its result type; interpreted literally as Scala 2 procedure syntax, it returns Unit. The solutions use the explicit Boolean query contract, and Unit for the two mutators. Parameterless clear is overridden without adding parentheses.^[ref]