If it matters, I've been programming more in Kotlin than in Java nowadays, and I have no trouble at all going back to Java... it's a nice language for its age and it gets the job done robustly. I even prefer Java for small applications that I want to distribute to users as small jars, for example. Kotlin has some nice benefits, but it's not a panacea that makes Java irrelevant - it actually strengthens the Java ecosystem in my opinion - it's another tool for the Java developer to use when appropriate.