Should I have Java installed on my iMac ?

If you need it then yes you should install it. If you don't need it then don't install it. If you don't know if you need it then I would suspect you don't.

Sure, you may have installed java on your mac, and you also can uninstall it, if you no longer use it or need it, just don't worry about the java tools that are part of the macOS, see the following:

https://www.java.com/en/uninstall/uninstall_instructions.html#macOS

A resounding NO! Don’t install unless some app calls for it which could happen but is not likely to.