I wrote a blog post about how nice Kotlin works with RxJava (compared to Java). I hope you enjoy it: https://pspdfkit.com/blog/2017/writing-concise-code-with-kotlin-and-rxjava/

so the issue is only in java7..rt? Java 8 can convert SAM to lambda.

Yes, but more advanced cleanup (e.g. with extension) is only doable in Kotlin