Java is a long way from Kotlin feature-wise and at the current rate of "innovation" we're more likely to have expired before Java catches up with Kotlin. I mean, come on, in the 25 years since 1.0 Java hasn't even fixed the requirement to escape metacharacters in a regex. The SAM lambdas of Java 8 are also a feeble compromise with FP at best. Kotlin has a much cleaner approach to FP, immutability and null safety.