public static String join(List<String> src, String delimiter) {
        return src == null ? null : String.join(delimiter, src.toArray(new String[0]));
    }
@Test
    public void testJoin() {
        assertEquals(join(Arrays.asList("a", "b"), "|"), ("a|b"));
        assertNull(join(null, "|"));
        assertEquals(join(Collections.singletonList("a"), "|"), ("a"));
    }