I would try to program first something in Java like a small project (without gui) that uses some libraries and has an OOP design. Just experiment with classes,

I would try to program first something in Java like a small project (without gui) that uses some libraries and has an OOP design. Just experiment with classes, structures and maybe access a database. Then when you know what interfaces, subclasses etc are , migrate and take a course on android development from a MOOC such as udacity or coursera. Then build an awesome app :)