Java developers

I have to an an assignment for my Operating Systems class. part of the assignment i need to display the user name of the person. for windows, it would be the account they are logged in as, and linux, it would be the username.
<# puser> ---> where puser is the username that i have to display.

what class in java will allow me to display the user name?

also, i am working with file navigation. people have told me that i should use java.io.file for file navigation, can i set it for default path for it in a computer hiearchy that i havn't seen before? (ie: my teachers computer) is there a class better then io.file or is there a good tutorial that will explain what i need to do/keep in mind when i program this.

Thanks in advance.

This bug report was entered earlier this week, it landed on my desk:

"Login Failed" message is not meaningful, please change it to "Login not Active"

So of course, I couldn't pass this one up. I made the change, and then waited for people to start asking the help desk how to "Activate their Logins". At least when it said login failed, they knew that they screwed up their password and they tried again.