public static FixedWindows of(Duration size) {
    return new FixedWindows(size, Duration.ZERO);
  }
@Test
  public void testSimpleFixedWindow() throws Exception {
    Map<IntervalWindow, Set<String>> expected = new HashMap<>();
    expected.put(new IntervalWindow(new Instant(0), new Instant(10)), set(1, 2, 5, 9));
    expected.put(new IntervalWindow(new Instant(10), new Instant(20)), set(10, 11));
    expected.put(new IntervalWindow(new Instant(100), new Instant(110)), set(100));
    assertEquals(
        expected,
        runWindowFn(
            FixedWindows.of(Duration.millis(10)), Arrays.asList(1L, 2L, 5L, 9L, 10L, 11L, 100L)));
  }