I'd like to add:
1. By getting a Java certification (especially, the developer certification).
2. By converting a non-Java app to Java app.
3. Write a Java app and keep re-writing it (or refactoring it) and improving it and using new APIs; over years you will find out the road map.
4. And, of course make it a must to write at least 5 lines of tested and working Java code, every day.
5. Write a blog about your coding experience.
6. Write a book (well, at least attempt at one; no need to publish)
7. And, learn a new programming language, once in a while