public static <T> List<T> setOrPadding(List<T> list, int index, T element) {
		return setOrPadding(list, index, element, null);
	}
@Test
	public void setOrPaddingNullTest() {
		final List<String> list = new ArrayList<>();
		list.add("1");

		// 替换原值
		ListUtil.setOrPadding(list, 0, "a");
		assertEquals("[a]", list.toString());

		//append值
		ListUtil.setOrPadding(list, 1, "a");
		assertEquals("[a, a]", list.toString());

		// padding null 后加入值
		ListUtil.setOrPadding(list, 3, "a");
		assertEquals(4, list.size());
	}