public static FixedWindows of(Duration size) {
    return new FixedWindows(size, Duration.ZERO);
  }
@Test
  public void testDefaultWindowMappingFnGlobalWindow() {
    PartitioningWindowFn<?, ?> windowFn = FixedWindows.of(Duration.standardMinutes(20L));
    WindowMappingFn<?> mapping = windowFn.getDefaultWindowMappingFn();

    thrown.expect(IllegalArgumentException.class);
    mapping.getSideInputWindow(GlobalWindow.INSTANCE);
  }