Retro library from my outdated book "Java für Mac OS X".
The Sys class lets you determine on which system your Java application is running.
Furthermore, it offers methods for cross-platform queries for paths such as the preferences folder.
With this information, you can tune your applications for certain operating systems.
Please note: This library is deprecated, just like the Apple API it uses. However, there are several alternatives, e.g. the Apache Commons library.