Pretty much everything - Java is reasonably fast, reasonably safe, and reasonably expressive. I wouldn't call it the best at any of those things. The real advantage to me is that the virtual machine is ubiquitous and many people can understand it. Since I have the most experience in this language, it's my tool of choice for most projects.
I've also been learning JavaFx so that I can build user interfaces without the web. I've started several single-page-application projects that worked, but felt like workarounds or hacks and would be better-served as self-contained applications.
To complement Java. The REPL lets me interactively exercise Java code. I can write performant and safe libraries in Java, and then use them in Clojure. I also find the data-centric aspect of Clojure (excellent build-in structures, literal syntax for easily creating those structures, functions that act well on abstractions of those structures) good for data processing.
This fits a sweet spot between Ruby and Java.