code: protected T popFrontEntry() {
          if (head == null) {
              return null;
          }
          T next = head.getNext();
          if (next != null) {
              next.setPrev(null);
          } else {
              last = null;
          }
          T e = head;
          head = next;
          e.setNext(null);
          size--;
          return e;
      }
docs: Removes and returns the first entry in the list. If the list is empty, it remains unmodified and
  <code>null</code> is returned.
  
  @return the previously first entry in the list, or <code>null</code>.