/** Constructor that creates an empty list; O(1) time */
  public NodeList() {
    numElts = 0;
    header = new DNode(null, null, null);	// create header
    trailer = new DNode(header, null, null);	// create trailer
    header.setNext(trailer);	// make header and trailer point to each other
  }
  /** Checks if position is valid for this list and converts it to
    *  DNode if it is valid; O(1) time */
  protected DNode checkPosition(Position p) throws InvalidPositionException {
    if (p == null)
      throw new InvalidPositionException
	("Null position passed to NodeList");
    if (p == header)
	throw new InvalidPositionException
	  ("The header node is not a valid position");
    if (p == trailer)
	throw new InvalidPositionException
	  ("The trailer node is not a valid position");
    try {
      DNode temp = (DNode)p;
      if ((temp.getPrev() == null) || (temp.getNext() == null))
	throw new InvalidPositionException
	  ("Position does not belong to a valid NodeList");
      return temp;
    } catch (ClassCastException e) {
      throw new InvalidPositionException
	("Position is of wrong type for this list");
    }
  }