Mastering java takes years. Sure, most of the basic syntax you can learn in a few days. The codecademy course is a good start, but doesn't cover everything.
Java is huge, seriously huge. There is too much to cover in courses on codecademy. Understanding java deeply, getting to know some of the library's, programming logic, that takes a long time. Start with the codecademy course, and take a look here, it contains some good courses a few of which cover java