Not sure exactly how cheap you're looking, but....
Raspberry-PI based robot, programmable in Java:
https://www.dexterindustries.com/gopigo/
https://www.dexterindustries.com/gopigo-and-java/
Arduino-based (not java, but usually cheaper):
Sparkfun sells a decent kit based around arduino:
https://www.sparkfun.com/products/13166
There's also some stuff on Banggood - looks like you'd have to get the major components separately, but you might be able to pull something together:
http://www.banggood.com/4WD-Smart-Ro...l?rmmds=search Plus
http://www.banggood.com/2-Channel-Mo...l?rmmds=search Plus your own Arduino plus a few batteries?
Full disclosure, I have not used any of these personally, they are just things I have found or knew about.