Set in hibernate sort all elements by cast the field set to org.hibernate.type.SortedSetType,which wrap the  java.util.SortedSet when persite In the SortedSetType.wrap(SessionImplementor session, Object collection).
it  return new PersistentSortedSet( session, (java.util.SortedSet) collection ); so if you wanted the set sortable,
must use the class that implements the java.util.SortedSet ,and in java.util package,TreeSet  is  validated.
if use HashSet ,you will get error report always in insert time.