### Question:
StringUtils { static String separator(String... sep) { if (sep != null && sep.length > 0 && sep[0] != null) return sep[0]; return "."; }  static String arrayFormat(String format, Object[] items); static Optional<String> cdr(String name, String... sep); static String car(String name, String... sep); static String lastOf(String qualifiedName, String... sep); static String untill(String qualifiedName, String... sep); static String dotToUpperCamelCase(String input); static String dotToDash(String input); static String camelCaseToDash(String input); static boolean isPackageFriendly(String moduleName); static String withoutTypeArgs(String javaTypeString); static String notEmptytrimmed(String param); static Optional<Tuple<String, String>> splitIntoTwo(String mappingString); static int countMatches(String theString, String... sep);  }### Answer:
@Test void separator_null_test() { assertEquals(".",separator()); }