code: public static void swap(final int[] arr, final int a, final int b, final int n) {
          for (int i = 0; i < n; ++i)
              swap(arr, a + i, b + i);
      }
docs: Swaps arr[a .. (a+n-1)] with arr[b .. (b+n-1)].
  
  @param arr order array
  @param a   first offset
  @param b   second offset
  @param n   number of values