Hello and welcome to JavaRanch,
I've never done this type of stuff myself. But you may want to start by looking at
The Java Communications 3.0 API which is for communicating with attached devices. If I am not mistaken, it primarily is for RS-232 hardware (serial ports) and limited access to IEEE-1284 (parallel ports).
If you are interested in using USB, take a look at the article
Java and USB.
As far as Ethernet, if the Behavior Box supports TCP/IP, you can use the standard Java IO classes. If not, how you interact with it over Ethernet will vary depending on protocol the Behavior Box does support. Remember that Ethernet is Layer 2 (if I am remembering my OSI Model correctly.)
Hopefully that can get you started. And some other JavaRanchers may have some additional information and/or resource recommendations. Good Luck.