Learning Java

What's the best resource for learning Java? I assume the one asking this question has some experience with the language. I probably asked this question several times since I first wrote my first line of Java a while ago.

Sure, "learning Java" is a bit broad. You can "learn", for example, how to add a tool tip to a JButton. StackOverflow is obviously the top choice for these very specific things. Then the Oracle tutorials are also good at introducing numerous topics and the API. But I am talking more about the language itself than its APIs. 

There has been one resource in particular that greatly improved my understanding and knowledge of the language in a way that nothing else came even close: THE. JAVA. LANGUAGE. SPECIFICATION. (along with the JVM specification)

There are some things about the language that you may not learn at university or from a random tutorial / article online, but all will be covered in the language specification. It's a document I constantly read to refresh my memory and understand things better and I think all Java developers should have a copy handy.