It requires firing up Android Studio or IntelliJ IDEA, copy the two sample classes and run the converter to turn them from. The lab begins where the automatic conversion of two simple Java classes, User and Repository, to their Kotlin counterparts ends, cherry picking the converted code to not just improve it but also teach Kotlin in the process. The Refactoring to Kotlin Codelab aims to change that. Moreover, given you are naturally predisposed to continue writing Kotlin as if it was Java, following Java's practices and idioms, this won't make the most of Kotlin's fresh approach.

While the latest IDE's do a good job in automatically converting Java code into Kotlin, this doesn't necessarily help you understand the mapping from the one code base to the other. Refactoring to Kotlin, which is available in English, Brazilian and Chinese, provides a guided, hands-on coding experience in a running time of just less than an hour.

Learn how to convert Java to Kotlin using Kotlin's idioms with Google Developers Codelabs.

Java to kotlin conversion online