### Question:
FileUtil extends cn.hutool.core.io.FileUtil { public static String getExtensionName(String filename) { if ((filename != null) && (filename.length() > 0)) { int dot = filename.lastIndexOf('.'); if ((dot >-1) && (dot < (filename.length() - 1))) { return filename.substring(dot + 1); } } return filename; }  static File toFile(MultipartFile multipartFile); static String getExtensionName(String filename); static String getFileNameNoEx(String filename); static String getSize(long size); static File inputStreamToFile(InputStream ins, String name); static File upload(MultipartFile file, String filePath); static String fileToBase64(File file); static void downloadExcel(List<Map<String, Object>> list, HttpServletResponse response); static String getFileType(String type); static void checkSize(long maxSize, long size); static boolean check(File file1, File file2); static boolean check(String file1Md5, String file2Md5); static void downloadFile(HttpServletRequest request, HttpServletResponse response, File file, boolean deleteOnExit); static String getMd5(File file); static String readJsonFile(String fileName); static BufferedImage inputImage(MultipartFile file); static float getAccuracy(long size); static StringBuffer fileRename(); static String transformStyle(String source); static boolean isChinese(char c);  }### Answer:
@Test public void testGetExtensionName() { Assert.assertEquals("foo", FileUtil.getExtensionName("foo")); Assert.assertEquals("exe", FileUtil.getExtensionName("bar.exe")); }