java.lang.RuntimeException: Uncompilable source code - what can cause this?

Question

This error is just bizarre, my code compiles fine, I can see there are no problems with it, yet this error has just popped up. I have tried re-starting NetBeans and there is

Answer 1

Implementing my own functional interfaces resolved this for me (so instead of using java.util.function.* just create your own single-method interface with the parameters and return-type you want).