@Override
        public int compare(Book b1, Book b2) {
            int bookIdx1 = b1.getbookIndex();
            int bookIdx2 = b2.getbookIndex();
            
            if (bookIdx1 < bookIdx2)
                return -1;
            if (bookIdx1 > bookIdx2)
                return 1;
            else
                return 0;
        }