String result = StringUtil.join(delimiter, "one", "two");
        assertEquals("one | two", result);
    }

    @Test
    @DisplayName("Should handle single null element")
    void testJoinSingleNullElement() {
        String result = StringUtil.join(",", (CharSequence) null);
        assertEquals("null", result);
    }

    @Test
    @DisplayName("Should handle whitespace delimiter")