static void BookExample()
        {
            Book book = null;

            BookShelf bookshelf = new BookShelf();

            book = BookFactory.CreateBook("mystery");
            Console.WriteLine("----");
            bookshelf.OrderBook("mystery");
        }
 public BookShelfIterator(BookShelf bookShelf)
 {
     this.bookShelf = bookShelf;
     index          = 0;
 }