Ask HN: How do you keep up with learning new tech conceptually as a developer?

Graduated with a major in CS roughly 2 years ago, and my stack had been largely Java related as a student for everything. Right now I am working on Apache Spark with Java 8, and it involves fairly basic concepts of functional programming; however, I find it difficult to easily get resources that would help me learn things from the very fundamentals. As a student, the textbooks helped a lot in understanding not just the language, but the overall view, background and the concepts implemented in it. However, as a professional, and maybe for a lack of better books, I mostly visit sites like Stack Overflow, find a related issue and solve it for "getting it to work". Very few do I get an in depth view of the fundamentals of the tech I am working on. How do you keep up learning in such scenarios?