[dec0dedab0de]

Well I have not done any android development, and I know Java has some specific quirks around classes, so maybe this isn't very helpful.

In my general development experience the amount of files/classes/functions/modules/whatever you use is really just about organizing your code in a way that makes it easier for you to work on one part of the code at a time. The idea being that once you know you have part of your code working the way you want it to, then you can forget about that part and focus on the next bit.

As far as when and where to do those splits, it's not always obvious, but it comes with experience in the language, and a greater understanding of what you want your application to be. When in doubt, just get it to work as best you can. Then when you find yourself digging through a giant method to debug 4 lines that could stand on their own, or copy/pasting a blob of code that could be a function, or just wishing to yourself that there was a library that did exactly what you needed, then take a step back and think about how to make it better.

Sometimes the solution won't present itself until you have to go back and make changes years later. In that case, fight the urge to throw it all away and start over, but remember the pain for your next project.