Kotlin for Java developers pathway.
Kotlin is 100% interoperable with Java, so you can include as little or as much Kotlin code in your app as you want. Through the articles and codelabs in the Kotlin to Java learning pathway, you can learn how to write idiomatic Kotlin code, add Kotlin to an existing app, and make it seamlessly callable from your Java code. You’ll also see how Android Studio can help you convert Java code to Kotlin code.
You can find more information here.