I realize that this example is in LabVIEW but similar APIs should be available in Java. Hopefully this will give you a push in the right direction.
AndyMark released some example code that drives a touch-screen shield on the
RIOduino (the REV Robotics Arduino-compatible MXP board). The screen code won't be of interest, but the last part of their example shows how they communicate between the roboRIO and the RIOduino through the I2C port.
Take a look at the example under the "Files & Documents" tab on the
RIOduino with 2.8" Touchscreen (am-2999) bundle page.
Again, their example is in LabVIEW, but the same APIs should exist in Java.
Hope this helps!
-David