public static PTransform<PCollection<?>, PCollection<String>> elements() {
    return new Elements();
  }
@Test
  @Category(NeedsRunner.class)
  public void testToStringOf() {
    Integer[] ints = {1, 2, 3, 4, 5};
    String[] strings = {"1", "2", "3", "4", "5"};
    PCollection<Integer> input = p.apply(Create.of(Arrays.asList(ints)));
    PCollection<String> output = input.apply(ToString.elements());
    PAssert.that(output).containsInAnyOrder(strings);
    p.run();
  }