Run the attached Test against commons-collections-3.2.1. While the JavaDoc for CollatingIterator claims
@param comp the comparator to use to sort, or null to use natural sort order
the test fails, because the comparator is null.
Fixing this requires a change in org.apache.commons.collections.iterators.CollatingIterator.least():
could be replaced with
The workaround is to provide a comparator, of course.