Its easy to get the modules work individually, but when we plug all the network components together, things go haywire. This problem was encountered because of
a. the real time nature of the project
b. Due to repeated testing, connections became loose on the robot car.

The solution:
We took apart all the accessories and reconfigured the robot car from the basic operations.

First we tried to make only two Xbee boards to talk to each other, with server code and client code.  When those parts worked,  we put the receiving Xbee on the robotcar.

We finally figured out making java client code sending signal to server c code.
One Xbee board connecting via USB to server, and sending signal to another Xbee on robotCar.