code: public static Permutation createForNElements(final int numElements, final Random rng) {
      final int[] permutation = IntUtils.arange(numElements);
      IntUtils.shuffle(permutation, checkNotNull(rng));
      return new Permutation(permutation);
    }
docs: Creates a random permutation of n elements using the supplied random number generator.  Note
  that for all but small numbers of elements most possible permutations will not be sampled by
  this because the random generator's space is much smaller than the number of possible
  permutations.