Retro library from my outdated book "Java für Mac OS X".
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.