Prerequisites

These articles aren't intended to be step-by-step instructions on how to program for a particular platform - there are already numerous resources that provide this sort of instruction. Instead, I'll be focusing on the differences and similarities between the platforms.

If you don't have a basic working knowledge of Android / iOS development, you might be more comfortable with one of the following resources:

Java:
http://docs.oracle.com/javase/tutorial/
http://developer.android.com/training/basics/firstapp/index.html

Objective-C and iOS:
http://www.bignerdranch.com/book/objective-c_programming_the_big_nerd_ra...
http://www.bignerdranch.com/book/ios_programming_the_big_nerd_ranch_guid...