How can I properly and completely uninstall Java?

I tried installing the Java Developers' Kit to install veraPDF. It didn't work. I don't know Java, and can't make sense of the articles regarding the error messages, so I can't debug it for this one application. I don't want to keep it and waste disk space for *nothing*.

So how can I properly and completely uninstall Java? I understand that there are official instructions which don't work.