I invested several years at coming up to speed with Java 8+ (using templates, rx, streams, the new time APIs). It was not hard, but difficult to unlearn some C++ idioms around templates, consts, reference model and object lifecycle management.
Clearly this driven by Oracle's continuous poisoning of Java ecosystem (eg https://headcrashing.wordpress.com/2019/05/03/negotiations-f... )
And I think Kotlin is a solid step forward from the Java without the baggage of pre JDK 1.7 idioms.
But for a small (often single dev) shops trying to compete in Android marketspace, it is quite hard to maintain this rate of change (plus incorporating all the new (and better ways) of doing android with androidx )