Porting From C To Java App

how to structure a very simple (but computationally intensive) app which has the following general characteristics (no details needed).The app has some data (say 2MB) statically stored.When the app starts this data is preprocessed and a large structure of about 40Mb is created.

After the initialization, the app has one text field for input, one for output and a button (for start/stop).The user enters a query then press the start button.The app computes a result using the large array.If the calculation on the input takes too long, the user can stop the processing with the stop button and start again with a new input.

When I was doing this under windows/linux with C/C++, I had no problems: I used a large global array and two threads, one to draw interface and respond to user (mouse, clicks, etc.) and one to perform the computation.Which is the standard (best) way to structure this kind of app in Java/ Android ?

Should I use an Activity for the graphic interface and a Service for the computation ? Can I build the large array in the Activity when the app starts and pass a reference to it to the Service every time Start button is pushed ?

I just need to be pointed in the right direction, to avoid some blind alleys. In a sense using c/c++ would be better, but I read that developing in Java (for Android) is much easier so I will try first with Java and revert to C only if the app will turn out to be too slow.