private StringJoinFunction() {
        super("string join");
    }
@Test
    void stringJoinFunction() {
        FunctionTestUtil.assertResult(stringJoinFunction.invoke(Arrays.asList("a", "b", "c"), "_and_"), "a_and_b_and_c");
        FunctionTestUtil.assertResult(stringJoinFunction.invoke(Arrays.asList("a", "b", "c"), ""), "abc");
        FunctionTestUtil.assertResult(stringJoinFunction.invoke(Arrays.asList("a", "b", "c"), null), "abc");
        FunctionTestUtil.assertResult(stringJoinFunction.invoke(Arrays.asList("a"), "X"), "a");
        FunctionTestUtil.assertResult(stringJoinFunction.invoke(Arrays.asList("a", null, "c"), "X"), "aXc");
        FunctionTestUtil.assertResult(stringJoinFunction.invoke(Arrays.asList("a", "b", "c")), "abc");
        FunctionTestUtil.assertResult(stringJoinFunction.invoke(Arrays.asList("a", null, "c")), "ac");
    }