Driver for Android System for credit card processing

Hi, I have a client who wants to make his own credit card reader which would contact the bank and make the transaction(s), similar to "square" and many other, but he wants his own system. One programmer have advised me that in order to make the whole system working you would have to make the "driver" first and then make other staff like app(s). I am not sure where to begin? This would be for android system, all I know it has to be written in java.
Anyone knows or direct me, so that i will not waist time, or go to do it in the wrong direction.

Thanks in advance.

Hmm, first you can ask client which device is he planning to use and get all the documentation you can get. It would be better to search card processing hardware, which has USB or RS232 communication protocol ability and has excellent documentation and API with examples and then begin with android communications library (for device) and some test app, which will use this library!

He want to use android phone and android tablets I would assume. What is RS232 ? You saying to test app which would use this library? Good Idea. If I have further questions i would get back, thanks.

RS232 is a Serial Protocol.

I tried to search for " card processing hardware", I am not sure what exactly I would need? I think I would like a code in java for an android hardware as well as for PC and MAC, I do not know if it is possible??? Which would be able to identify the card reader if I would plug it in. So as soon as I would plug it in, it would recognize it. And I think it I would want to make it for everything :USB,RS232 and also for the card reader by itself. So if it would use USB I would need to have an USB adapter as well as adapter for the serial port. I have attached a picture of that card reader. Driver is the actual program (code) for the hardware. Could you lead me to some code examples, thanks in advance.Attachment 3522