How do I get the username of the person who is currently using the computer?

I'm developing a Java application that is supposed to be able to figure out who's currently using the computer and log him or her out. The app is supposed to run under an administrator account that the person who is using the computer is not logged in as.

I want the app to work on non-Windows PCs. For now, I think I'll stick to supporting Unix-based OSes. Windows users will have to install Cygwin to use my app.