To me Guava is still the most essential, general purpose Java library. Not just for its collection package but because of the caching stuff, graph API, and well

To me Guava is still the most essential, general purpose Java library. Not just for its collection package but because of the caching stuff, graph API, and well written general purpose utilities. But I would also mention Spring 5 which I assume will be released soon, even though many people probably consider it too heavy or opaque at this point. But their major adoption and push towards reactive programming is interesting. And it will be interesting to see if java programmers embrace it. I'm a little skeptical that it provides enough tangible value to justify a more complex mental model, but maybe I'm wrong.