Which functional interfaces exactly are you talking about. The way I understand it () -> Unit or any other functional interface is represented by some synthetic class KFunction0<T> which implements Function0<T> which declares invoke.
So I guess the functional interfaces have invoke already.
Maybe you can give an example of what you are trying to do (I’m not sure if I understand your problem yet).
As far as I know there should be no problem combining kotlin lambdas with java and vice versa.