int my_random_arr[] = new int[4];
        int cur = 0;
        Random r = new Random();
        while(cur < 4){
            // get random number
            int new_number = r.nextInt(9);
            // check the new number is in my_random_arr
            int idx = 0;
            while(idx < cur){
                // try to find same number in here
                if (new_number == my_random_arr[idx]) {
                    break;
                }
                idx++;
            }
            if ( idx == cur ) {
                my_random_arr[idx] = new_number;
                cur++;
            }
        }
        for (int num : my_random_arr) {
            System.out.print(num);
        }