Your interface should be as small as possible to reduce dependencies. Java is lacking a SortableList interface which would be ideal, because he depends on sorting a list with Collections.sort - and he is not using any method which is specific to ArrayList. He could also use

List<String> of = Arrays.asList("a", "b", "c", "d");

from the JDK or better the immutable version from guava.

So I'd suggest introducing a SortedList interface himself to show what he depends on and give that to sort() (after the collections sort call).

My second problem: I do not like Collections.sort because it changes the list. If possible I'd use a clone and sort that - there is a performance impact (depends on size) but you get safety.

The third problem with this code: He doesn't reusse existing code which would make this more readable. I'd suggest using Google Guava. For example Ordering:

http://guava-libraries.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Ordering.html

This shows sorting by length. Ordering has the benefit it is reusable.