我在mapreduce中创建了一个wordcount示例，并尝试使用以下命令生成jar文件：

hduseros@MT2012018:/usr/local/hadoop$ jar -cvf playground/wordcount.jar -C playground/classes

但它根本没有运行并给我以下结果：

hduseros@MT2012018:/usr/local/hadoop$ jar -cvf playground/wordcount.jar -C playground/classes
Usage: jar {ctxui}[vfm0Me] [jar-file] [manifest-file] [entry-point] [-C dir] files ...
Options:
    -c  create new archive
    -t  list table of contents for archive
    -x  extract named (or all) files from archive
    -u  update existing archive
    -v  generate verbose output on standard output
    -f  specify archive file name
    -m  include manifest information from specified manifest file
    -e  specify application entry point for stand-alone application 
        bundled into an executable jar file
    -0  store only; use no ZIP compression
    -M  do not create a manifest file for the entries
    -i  generate index information for the specified jar files
    -C  change to the specified directory and include the following file
If any file is a directory then it is processed recursively.
The manifest file name, the archive file name and the entry point name are
specified in the same order as the 'm', 'f' and 'e' flags.

Example 1: to archive two class files into an archive called classes.jar: 
       jar cvf classes.jar Foo.class Bar.class 
Example 2: use an existing manifest file 'mymanifest' and archive all the
           files in the foo/ directory into 'classes.jar': 
       jar cvfm classes.jar mymanifest -C foo/ .

请纠正我的错误。我已经设置了类路径。我可以运行已经出现的例子，但不能使jar文件。