Can you use jdk 16 for a minecraft client 1.8.9 / 1.8.8 or do you need jdk 8?
As far as I know, 1.8.x was written in Java 8 (or 6, I'm not sure at the moment). Just to be on the safe side, I would stick to Java 8 if I were you. At that time I had no problems with client programming. This already contains enough features that could be very helpful (among other things the Diamond operators as well as streams and much more).
OK thank you!