I would never want to stifle someone else’s creativity or enthusiasm, but having written several editors I can tell you that it is not a trivial undertaking. Most things are possible if you’re willing to invest enough time and energy. I suggest you get started by learning to compile and run straight java code. You can do this with a couple of text areas, one for the code and one for the error messages, and then using ‘java’ and ‘javac’ from the command line to compile and run. Of course you won’t be using Processing for this, just raw java or javafx. If you want to take advantage of the Processing core library then Intellij might be a good place to get your feet wet. You will have to learn how to import the needed core libraries from the Processing.app, but you will have a nice editor to work in. If you still want to write your own editor after this experience go for it. Personally, I have the utmost respect for Ben Fry and his contributors. I’m relatively new to java programming and I doubt that I would be engaged in it if it were not for Processing and its editor. I rarely use my own java and javafx editors due to all the bugs that I never could resolve.
Example of gui for simple java editor:
