An interface is a contract that says any implementing class must implement the methods defined in the interface. It can't include any code to actually send and receive messages.
That said, creating an interface to define the notification methods is a good idea. An interface is a nice abstraction. The JDK includes an Observer interface along with an Observable class that might do the job for you. Look into the JavaDoc and see if they'll fit.
Before I learned about Observable/Observer I wrote my own little pub-sub routine - a one-abstract-class-framework. If Observable doesn't work out for you I'd be happy to discuss my code. We usually don't hand out complete solutions in this forum - it's much more fun and educational if you work out the tricky bits yourself. So see what you can get going, post some code, we'll talk some more.

[ April 04, 2005: Message edited by: Stan James ]