code: public static void shuffle(int[] arr, Random rand) {
          int size = arr.length;
          for (int i = size; i > 1; i--) {
              int tmp = arr[i-1];
              int r = rand.nextInt(i);
              arr[i-1] = arr[r];
              arr[r] = tmp;
          }
      }
docs: Randomly shuffles the contents of the provided array