public boolean checkIsbn(int uid) {

        if(isbn%56000<999) // Checking if the ISBN is valid i.e, within range 56000-56999
        {
        for(int i=1;i<=3;i++)
            if( this.isbn== uid)
            {
                return true;

            }
        }
        else
        System.out.println("Invalid ISBN!!!!");

        return false;
    }