void linkLast(final E e) {
    final E l = last;
    last = e;

    if (l == null) {
      first = e;
    } else {
      setNext(l, e);
      setPrevious(e, l);
    }
  }
void linkLast ( final E e ) { final E l = last ; last = e ; if ( l == null ) { first = e ; } else { setNext ( l , e ) ; setPrevious ( e , l ) ; } }
Links the element to the back of the deque so that it becomes the last element.

@param e the unlinked element