Polymorphic collections

Polymorphic collections. ArrayList<Employee> holds any mix of Employees and Managers; the enhanced for-loop calls the override for whatever each element really is:

int total = 0;
for (Employee e : staff) {
    total += e.pay();   // Manager? +bonus. Employee? base.
}

The tracing discipline for these: one row per element, columns for "reference type / object type / which pay runs / contribution." The hard version adds a conditional override — a Manager only counts the bonus when a flag holds — so the contribution column depends on state you must also trace.

And the cast trap: ((Manager) e).bonus compiles but throws ClassCastException when e holds a plain Employee. On the exam, "runtime error" is a possible answer, not a distraction — check whether every element really is the cast type before trusting any option that casts.