I've been helping a friend of mine with his postgraduate project the last 3 months.
It was a Java based program made in Processing. Though I am not a Java developer and I never used processing before, it wasn't that hard to write the logic of the program.

I noticed that sometimes Java made me use loops for almost everything.
Also I had to communicate between server and client via JSON but I had to write it manually as string due to the lack of keys in Java.

The main trial though was with the logic of the project. It was supposed to be made as a framework to be extended from custom user classes. I had to change the core classes I made many times because the user class had methods that should run while the parent class didn't have them declared. That could be my fault for not knowing how to write desktop application framework but you can't expect a framework to be extended in a compiled state, or so I think. Processing on the other hand doesn't seem to like the idea of an external java library. At least it didn't workout for me, it should be able to work normally.

In the end the project was never as completed as we wanted. It could rum a basic sim but we hadn't the time to test other possibilities.