You may find team 254s implementation of a constants.txt file useful:
https://github.com/Team254/FRC-2013/...tantsBase.java
https://github.com/Team254/FRC-2013/...Constants.java
It doesn't write out to a file. It looks for a text file on the crio at startup and allows you to override default values of variables in your program.
I'm assuming you want the ability to calibrate your arm angle without having to recompile code. This could be achieved by uploading new versions of a constants.txt file over FTP to your robot using the above implementation.
You could alternatively extend this implementation to allow the constants.txt file to be written to by the robot. Take a look at
java.io.DataOutputStream