Java's Timer is a good way to schedule periodic operations like checking for new files. As Fred pointed out, deciding what's new may be as simple as timestamp > last run or as complex as your business rules make it.
And "listener" is also a good concept, too. Rather than making the directory checker do any real work with the files it finds, have it broadcast a "New File Found" event to one or more listeners. That helps the checker and the listener both do just one thing well.
If you were going to be doing a lot of this, you could wind up with a package containing a directory checker, an interface for plug-in Strategies for deciding what's new and a listener interface that clients could implement and subscribe to new file events. You could use it for any number of purposes without ever touching the code again. That's just an old geek wandering off the path ... do the simplest thing that works for now.