What do you do with a .jar file?

[ke6isf <ke6isf@chez-vrolet.net>]

Running Sarge with the Sun Java package installed, per the instructions 
provided on the IRC channel.

OK, so I have this .jar file.  If I open it using 'java 
hidejournal-0.3.1.jar', it gives me:

Exception in thread "main" java.lang.NoClassDefFoundError: 
hidejournal-0/3/1/jar

If I open it from Nautilus (I run Gnome), it pops it open in File Roller.

Just what should I be using to run this?

-Dennis Carr