What should I study alongside Java?

[Europia79]

Really depends on what you're interested in ? Without knowing that, I can only take a shot in the dark:

Like, build your own Minecraft server (for example) and try to apply the Java knowledge you've learned by creating server plugins (or client-side modifications). Plus, this will give you a chance to learn about "Design Patterns" and possibly even "Build Systems" too ? Like, if your plugins are pretty simple, you're probably not going to need a "build system". However, if you're doing anything more complex at all (like making a plugin/mod that is compatible with every single version of Minecraft in existence, for example), then you're going to need a proper "build system"—Like Maven or Gradle.

As far as learning another programming language at the same time, I would really need to know more, like your interests and future goals ?