String result = StringUtil.join(",", sb1, sb2);
        assertEquals("first,second", result);
    }

    @Test
    @DisplayName("Should handle StringBuffer as CharSequence")
    void testJoinWithStringBuffer() {
        StringBuffer sb1 = new StringBuffer("alpha");
        StringBuffer sb2 = new StringBuffer("beta");
        String result = StringUtil.join("-", sb1, sb2);
        assertEquals("alpha-beta", result);