I tried this app out recently, and would like to commend you on a job well done as far as the interface and game elements are concerned. This app is more than just lists of questions and ads to generate revenue, which I really liked.
However, there are times when the question format fouls up and you get many questions on one screen and cannot see the question being asked. Also, there are a couple of essential Java questions that need correcting (they are in the minority).
Therefore, I would like to know if you are still working on this, and if you would benefit from some input. As I said, I really like the app, and I think with some minor fixes could be the go-to app for drilling Java concepts.