Same here. On my my most recent project the client requested we use Java, since after they take over they can use devs with Java knowledge.

It will take a week or two to convert experienced Java devs to Kotlin.

Yea I agree. I like Kotlin a lot and found the transfer from Java to Kotlin fairly straightforward.

Just not all clients are open to one more thing for their devs to do/learn. In the specific case for us, the devs were new to Android so they wanted to use a language they were comfortable in.

If they are new to Android how are they comfortable in Androids' take on Java? Trying to use paradigms from enterprise/server side Java in Android is going to backfire hard on all but the most trivial projects.

you really wont be, it's like a couple of weeks and you'll be up and running in kotlin.

I don't mind Kotlin, but I don't want to get a big middle finger from Google for choosing to remain with Java for this project I already started in Java.

Kotlin & Java interop really seamlessly, that's one of kotlin's biggest features. Not interops like there's an FFI that is obviously an FFI, but interops like there's not much obvious difference between calling either way.

You won't get the kotlin syntax sugar, of course, but you're not going to be missing any features, either.

I’ve used Kotlin enough to be comfortable but Java is still my go to for personal projects.

Java + StreamSupport + RxJava (and formerly + Retrolambda) is a very modern feeling language.

Heavy use of Optional makes the null safety issues a little less worrying and all my apps are built around AutoValue objects which are a bit like Kotlin data classes.

And apparently Java 10 has ,`var` keyword. It won't be long before Java has all the goodness.