I prefer functional programming, and have a background in functional programming, but I still primarily work with Java, and it is currently my go-to for any project. In my opinion, Java as a language is gross and unlovable, but the runtime, the development tooling, and selection of libraries and other integrations is about as good as it gets.

I feel like I am continuously looking for a better alternative, but it's hard to overcome the practicality of using something with such immense buy-in. For example, nearly all major SaaS offer a Java SDK, or have Java-specific docs. They want the big companies to be their customers, and they know they use Java. I am all too happy to benefit from that.

It should also be noted that Java the language is rapidly being enhanced with features inspired by functional programming. For example, sum types (in the form of sealed classes) have been added to the language, and ML-style pattern matching with compile-time exhaustiveness checking is in the pipeline. More on the platform side, Java now has a REPL (JShell).

Of course, Java codebases you encounter in the wild are very likely to make you want to cry. The world of Java development has a lot of bad taste and questionable practices enshrined as best practices. That said, the Java community is slowly shifting toward a better style.