To begin with, java is simply a tool, like all other languages, for completing a specific purpose. Sure, java can do some things better than other languages, which is why it was created. But you must keep in mind that, at the end of the day, it is only a tool. Rather than learning the language, you enhance your skills while learning about the concepts. If you’re interested in networking, for example, look at the basic TCP/IP stack and other routing protocols, then look into what java has to offer utilizing the Oracle docs. Most people believe that learning a language entails remembering methods, APIs, and other information, which is a waste of time and energy when the information can be found with a simple Google search.
Research the architecture of JVM if you’re interested in the inner workings. Inside JVM by Bill Venners is one of the books I recommend. Most crucial, understand why it is that the language was established; no one will commit effort in creating something that does not solve a problem. Java was designed to do things that C++ couldn’t, therefore go to forums and learn about them. This will help you decide when to choose java instead of other languages.
So, first determine whatever topic you want to grasp, then go for Oracle documentation.