My guess is that they didn't really know what they were talking about, but that they probably meant the method Collections.sort(List).
I would say that a List is always ordered, period. A list always has an order, which behaves in a consistent, predictable manner. Whether it happens to be the same order as what you
want it to be in, that's another question.
Perhaps they want a way to make sure it is
sorted, as opposed to merely ordered? In that case, use Collections.sort(). Easy.
Note that Collections.sort() doesn't really
maintain sort order. It just sorts the list, once. If you need something that maintains sort order continually, consider using a SortedSet, like TreeSet, rather than a List. Note that this is not possible if you need to be able to allow duplicates in the List. If that's a problem, consider Google Guava's
TreeMultiset instead.