This is a difference in the implementation: the iterator returned by the array list detects concurrent modifications even when it is positioned at the end, because it checks the length; iterators of the HashSet, TreeSet and LinkedList, on the other hand, do not detect this condition, because they check for being positioned at the end before checking for concurrent modification. The documentation allows iterators not to throw on concurrent modifications, so both approaches are valid.
- Demo for the TreeSet.
- Demo for the LinkedList.
