In a somewhat recent interview, I was asked how I would add Roman numbers together without first converting them into decimal notation. I had to work completely within the string domain. I stumbled through the question and did not present a good answer. I went home and made an attempt to code a working solution. I sent it in the solution. I looked it over months later and discovered it had a bug. I missed a fairly important test case. My bad. I made another attempt with a better set of test cases. This is my solution.
There is a crude executable Main class that you can execute. It takes two Roman numerals from the command line, adds them together, and prints the result to stdout.
If you have the intent to use this body of code as a library, consider using the class RomanAdder as your entry point. It has one public method, Add. This method takes two strings as arguments and returns the sum in string format. It will throw Runtime exceptions if you pass it invalid Roman numerals.
There are unit tests located here.
Enjoy.