It doesn't require any Java but the docs do at times sort of assume you understand the JVM to some extent - which was a bit frustrating when first learning the language. It'll use terms like "classpath" without explaining what that is. However nowadays with LLMs these are insignificant speedbumps.

If you want to use Java you also don't really need to know Java beyond "you create instances of classes and call methods on them". I really don't want to learn a dinosaur like Java, but having access to the universe of Java libs has saved me many times. It's super fun and nice to use and poke around mature Java libs interactively with a REPL :)

All that said I'd have no idea how to write even a helloworld in Java

PS: Agreed on Emacs. I love Emacs.. but it's for turbo nerds. Having to learn Emacs and Clojure in parallel was a crazy barrier. (and no, Emacs is not as easy people make it out to be)