更改此：

fw = new FileWriter("output.txt");

到

fw = new FileWriter("output.txt", true);

参见 javadoc 了解详细原因 - 实际上append默认为false。

See the javadoc for details why - effectively the "append" defaults to false.

请注意 FileWriter 通常不是一个很好的类 - 我更喜欢使用 FileOutputStream 包装在 OutputStreamWriter ，因为它允许您指定要使用的字符编码，而不是使用您的操作系统默认值。

Note that FileWriter isn't generally a great class to use - I prefer to use FileOutputStream wrapped in OutputStreamWriter, as that lets you specify the character encoding to use, rather than using your operating system default.