I have read in the API that TreeSet is a Red-black implementation of the NavigableSet interface. And the NavigableSet is a SortedSet extended with navigation methods.
Following with SortedSet the API says that is a Set whose elements are automatically sorted when the SortedSet is created.
Probably the answer to my previous answer could be that : Does not make sense to sort several times a TreeSet! , it is sorted using the sorting we specified in the moment of its creation. And when we add new elements it continues sorted.