How do I use Processing to read and write data to an Arduino

Interfascia looks like oit will do what I want, as soon as I learn more about Java. (Old C/C++ programmer)

Looking at the radio button example, I see how radio buttons are displayed, but how do I determine which has been selected? As in in finding which button was last pushed?
I tried to set a text label with the selected button id/reference/index and could not figure out the syntax for converting an index into a parameter the label would take.
If strings are static, how to I change the text in a string?
This is likely simple JAVA, but new to me.

Documentation is not what I am use to.