Yes, Java is relevant, especially in large companies. A lot of back-end systems are using Java. For learning, you can start with reading Java Language Specific

Yes, Java is relevant, especially in large companies. A lot of back-end systems are using Java. For learning, you can start with reading Java Language Specification. And then you can read Java Virtual Machine Specification. After that, you should have very good understanding of big picture. Then it is just about writing code using APIs. As for back-end, there are a lot of topics (not specific to language). File system, networking, distributed computing and others. In real production systems, it is about how to compute/process within certain time-frame. And how to operate the production systems.