https://android.jlelse.eu/effective-migration-to-kotlin-on-android-cfb92bfaa49b

There are countless resources available to help you jumpstart on Kotlin: official documentation from Google and JetBrains, Google-IO talks, codelabs, and an endless supply of blogpost and tutorial provided by the community. All these resources can introduce you to Kotlin’s type system, null-safety, extension functions, data classes, high order functions and all the other cool features that Kotlin provides.

However, when you start applying all this to a real Android codebase, not everything turns out to be as simple as it appears. Although Kotlin’s syntax is easy to pick up for a Java developer, it takes some time and practice in order to be able to write idiomatic code and effectively translate some of the widely-used patterns on Android development to Kotlin. Furthermore, many libraries are made redundant or at least require some adaptations to be made on how they are used, due to the features that Kotlin offers out of the box.