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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2017/imperative-programming-resit/5/a/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Imperative programming (resit) / 5 / a / Solution by

Codex  0 2026-10-31

Replace the overloaded entry point by one forwarding method and add a virtual callback on the drink. The callbacks choose a reaction overload using the concrete drink class's statically known this type:

class Drink {
  def serveTo(b: Boy): Unit = b.react(this)
}
class Milk extends Drink {
  override def serveTo(b: Boy): Unit = b.react(this)
}
class Boy {
  def concept(d: Drink): Unit = d.serveTo(this)
  def react(d: Drink): Unit = println("Drink")
  def react(m: Milk): Unit = println("Milk")
}
class Baby extends Boy {
  override def react(d: Drink): Unit = println("Food")
  override def react(m: Milk): Unit = println("White drink")
}

The original b.concept(d) call now dynamically selects Milk.serveTo. Inside that method this has static type Milk, selecting react(Milk); virtual dispatch on b then selects Baby.react(Milk). Thus the output is White drink. This is double dispatch: changing only an overload's parameter name or the reference's runtime object would not change static overload selection.

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