is Java safe?

Java in your web browser is NOT safe. Mostly because this mode allows someone to send you a program to run, and unless the web browser based Java is perfectly sandboxed, it can allow the Java program delivered via a webpage to probe and maybe penetrate your system. And when I say someone sends the Java program your Web browser runs, that can be any website, and banner Ad hosted on any website, etc... Lots and lots of opportunity to mess with your Mac.

Java in a macOS application is as safe as any other programming language. That is to say, it is a function of the programmers that wrote the Java application. Plus you download it once from hopefully a trusted source, and you get updates from the same trusted source.

Do you actually need Java for anything?

If you don't need it, just remove it completely. It serves very little purpose these days and is not what I would consider 'secure'.