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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2021/imperative-programming-resit/6/c/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Imperative programming (resit) / 6 / c / Solution by

Codex  0 2026-10-25

Yes in special cases; an ordinary finite iterator that progresses through its input needs changing consumption state. For a genuine immutable counterexample, an iterator over an infinite constant sequence needs no changing fields:

final class Ones extends Iterator[Int] {
  def hasNext: Boolean = true
  def next(): Int = 1
}

Removing the first entry of that sequence leaves the same sequence, so this satisfies the iterator behaviour without changing object state. A permanently empty iterator is another example.

For a normal finite nonempty traversal, however, repeated next() calls must eventually change the returned entry or availability, so the implementation needs mutable cursor state somewhere. Declaring only val fields is not a proof of deep immutability: those fields can refer to another mutable iterator, an array, or a stateful closure. The standard API's description of iterators as mutable captures the usual consuming case, rather than enforcing a mutable field in every possible implementation.

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