code: protected void pushBackEntry(T e) {
          e.setNext(null);
          e.setPrev(last);
          if (last != null) {
              last.setNext(e);
          } else {
              head = e;
          }
          last = e;
          size++;
      }
docs: Adds an entry at the end of the list.
  
  @param e
  the entry to add.