Have you tried Groovy? Sounds kind of ideal if your main work is with Java. I have a BeakerX (JupyterLab+Groovy) notebook open pretty much continuously next to my IDE while I code so that I can validate all my assumptions as I code. A nice workflow is that you start with a snippet of experimental code, which you tidy up in the REPL, then it splits into the actual function and test code, one part going to the unit test and the other to the real code.