Java

Since you aren't a complete novice, I think you're ready for C++, for the following reasons:
It's dangerous. Some may think this is a reason to avoid it, but these are things you are going to have to learn.
It's close to the hardware. This makes it fast and optimization and also suitable for tiny little boxes.
It has a good preprocessor. It's as dangerous as juggling chainsaws, but when you start developing a code base that has to compile on multiple systems, you'll really appreciate it.
It has objects without pointers. Traditional objects always have pointers. C++ lets you have objects that are not allocated and not accessed by pointers. Instead, they look like any other data type. This allows some choices that you might not ordinarily make when thinking in terms of objects, which, because of 2, you can optimize to zero cost for production. My favorite example is units. We lost a $328 million Mars orbiter because of different units of measurement (see MARS CLIMATE ORBITER TEAM FINDS LIKELY CAUSE OF LOSS). It's easy to make a units system in C++ and tweak it for hacking, debugging, and running. Also think currencies.
It has a template system. I think this is the most amazing thing about C++ per se. It has been reported that the system is so powerful because of an early implementation made it more powerful than specified. Good. When you can use templates effectively in C++, you're an expert in C++.

Best books for JAVA -
Head First Java, 2nd Edition - OReilly Media
Thinking in Java (4th Edition)
Java: A Beginners Guide (Sixth Edition) by Herbert Schildt
Introduction to Programming in Java by Robert Sedgewick
Introduction to Object-Oriented Programming with Java by C.Thomas wu
Java in a Nutshell
Beginning Programming with Java For Dummies, 4th Edition by Barry Burd
"The Java Programming Language" by Ken Arnold,James, Gosling,David Holmes.
Core Java Volume I Fundamentals (10th Edition) by Cay S. Horstmann
Java How To Program (late objects) by Paul Deitel, Harvey Deitel
Effective Java (2nd Edition): Joshua Bloch
Java Performance: Charlie Hunt, Binu John
Head First Servlets and JSP
SCJP by Kathy and Sierra
Java - The Complete Reference by Herbert Schildt.
Java Concurrency in Practice
Java Performance
Java Puzzlers : Traps, Pitfalls and Corner Cases.

For a Java developer, having a strong understanding of Object-Oriented Programming is a must.
Master the core APIs- It doesn't matter how strong you are in terms of theoretical knowledge if you don't know the language constructs and core APIs.
Keep coding- Things look simpler when talking about them theoretically. We can give a solution to a problem very easy in theory. But we can realize the depth of the problem when we start implementing our approach. You will come to know the language limitations, or design best practices while coding. So keep coding.
Subscribe to forums -When starting to work on a new technology the best and first thing to do is subscribe to the relevant technology forums. Whatever the issue you are facing, someone else in the world might have already faced it earlier and might have found the solution. And it would be really really great if you can answer the questions asked by other forum users.
Read open source frameworks source code- A good developer will learn how to use a framework. But if you want to be an outstanding developer you should study the source code of various successful, popular frameworks where you can see the internal working mechanism of the framework and lot of best practices. It will help a lot in using the frameworks in a very effective way.
Know the technology trends- In the open source software development technology trends keep on changing.
Know different development methodologies- Nowadays choosing the development methodology depends on the client. Some clients prefer Agile and some clients are happy with waterfall model. So having an idea of various methodologies would be great.
J2EE would be a better choice as its an extended version. So, definitely opt for that.

Some questions for JAVA developers -
Which is the superclass of all classes?
Differentiate between J2SDK 1.5 and J2SDK 5.0?
What are wrapper classes?
What is the ThreadLocal class? How and why would you use it?
What is the volatile keyword? How and why would you use it?
What is runtime polymorphism or dynamic method dispatch?
What is the number of bits used to represent ASCII, Unicode, the UTF-16, and the UTF-8 characters?
Is it possible to import same package or class twice? Will the JVM load the package twice at runtime?
What is the difference between System.out, System.err, and System.in?
What is the Java Classloader? List and explain the purpose of the three types of class loaders?
What are the differences between Heap and Stack Memory?
What are the differences between Get and Post methods?
How do cookies work in Servlets?

Python is the older of the two languages. In programming language evolution, it is common to maintain backward compatibility indefinitely. This is not the case with Python. Python versions are largely compatible but have enough functionality- and syntax-breaking differences that they can be treated as different languages. Python is adapted because of Simplicity, Open source, object-oriented and General purpose language. It is an interpreted language available on many platforms, making it a portable option for general development.
Java on the other hand
Its attraction and value is its portability and relative efficiency. Java is a compiled language, unlike C/C+. Its execution model is more machine-centered than purely interpreted languages. Java is more than just a language and libraries. It is also a virtual machine and, therefore, an ecosystem. The Java Virtual Machine (JVM) is an idealized and portable platform for running Java code. Rather than worrying about hardware specifics and having to port code to new platforms, the promise of Java has been written once, run anywhere. That is so that as long as a JVM is present, anything compiled into its bytecode can run and interact easily with anything else written for the JVM.