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Is this course really 100% online? Do I need to attend any classes in person?
How much math do I need to know to take this Specialization?
Can I take the course for free?
I would take a project you've implemented in Python and try converting it to Java. Since you already know basic programming fundamentals, it'll probably be easier if you take things you know how to do and figure out how you'd do the same sort of operations in Java (or whatever new language you want to learn).
In the end, the only way to learn to write code, is to write more code.
Java and C# will be less of a step away from Python than would C or C++ because Java, C#, and Python all have automatic memory management. A good Java book is Thinking in Java by Bruce Eckel. It starts at an introductory level, but also has a lot of depth.
The big difference with the language coming from Python is the fact that all variables are typed. The other hard thing with Java has to do with the bewildering array of Java APIs out there. The fact that you are interested in Android is an advantage here. After becoming comfortable with the core language, I suggest you start learning the Android API and focus on becoming an Android expert. I think Android will be a growing market for a while.
Good luck!