java对文件名的几个操作，获取文件扩展名，去掉扩展名

**     * Return the extension portion of the file's name .     *     * @see #getExtension     */    public static String getExtension(File f) {        return (f != null) ? getExtension(f.getName()) : "";    }

    public static String getExtension(String filename) {        return getExtension(filename, "");    }

        public static String getExtension(String filename) {        return getExtension(filename, "");    }

    public static String getExtension(String filename, String defExt) {        if ((filename != null) && (filename.length() > 0)) {            int i = filename.lastIndexOf('.');

            if ((i >-1) && (i < (filename.length() - 1))) {                return filename.substring(i + 1);            }        }        return defExt;    }

    public static String trimExtension(String filename) {        if ((filename != null) && (filename.length() > 0)) {            int i = filename.lastIndexOf('.');            if ((i >-1) && (i < (filename.length()))) {                return filename.substring(0, i);            }        }        return filename;    }