Why doesn't SingletonSet implement SortedSet

Question

In order so reduce memory consumption, I'm rewriting a class that has a SortedSet<Integer>. In 80% of cases, this collection contains only a single element . So I thought I could use a SingeltonSet in these cases and a normal TreeSet in other cases. Now I noticed that SingletonSet, as returned by Collections.singleton(), does not implement SortedSet. Is there any reason for this shortcoming? A single element can always be considered sorted I'd say. Do I have to write my own SingletonSet implementation?

Answer 1

The SortedSet interface defines methods that require a Comparator for the set elements. Thus elements maintained by a SortedSet have to be comparable. If the singleton returned by Collections.singleton() would implement SortedSet, then Collections.singleton() could accept Comparables only (which is not what we want).