### Question:
IrisString { public static String joinIfNotEmpty(String separator, String... params) { if (params.length <= 0) return ""; StringBuilder builder = null; for (String s : params) { if (StringUtils.isEmpty(s)) continue; if (builder == null) { builder = new StringBuilder(s); } else { builder.append(separator); builder.append(s); } } if (builder == null) return ""; return builder.toString(); }  static String joinIfNotEmpty(String separator, String... params);  }### Answer:
@Test public void testCombine() { String result = IrisString.joinIfNotEmpty("_", "1", "2"); Assert.assertEquals("1_2", result); result = IrisString.joinIfNotEmpty("_", "1", "2", "3"); Assert.assertEquals("1_2_3", result); result = IrisString.joinIfNotEmpty("_", "", "2"); Assert.assertEquals("2", result); result = IrisString.joinIfNotEmpty("_", "1", null, "3"); Assert.assertEquals("1_3", result); }