ANONYMOUS wrote:

If Student.getMark(x) returns null, does this mean they haven't taken unit x?

E.g, for task 9 we should filter over Students that return null?

 * @return An iterator over the students who have taken `unit`, from newest to oldest.
 */

I will draw your attention to the documentation of the Student.getMark() method

    /**
     * Returns the mark this student got in a given unit.
     *
     * <p>Returns `null` if the student has not completed the unit.
     *
     * @param unit The unit code to get the mark for.
     * @return The mark this student got in the given unit, or `null` if the student has not
     *     completed the unit.
     */
    Integer getMark(String unit);

which explicitly states that it returns null if the student has not completed the unit. If a student has taken a unit, then they will have completed it and received a mark. Otherwise they will not have.