code: public void sort (Comparator<? super T> comp)
      {
          if (_size > 1) {
              QuickSort.sort(_elements, 0, _size-1, comp);
          }
      }
docs: Sorts the elements in this list with the supplied element
  comparator using the quick sort algorithm (which does not involve
  any object allocation). The elements must all be mutually
  comparable.