Java to Kotlin: A Refactoring Guidebook

It takes a week to travel the 8,000 miles overland from Java to Kotlin. If you're an experienced Java developer who has tried the Kotlin language, you were probably productive in about the same time.

You'll have found that they do things differently in Kotlin, though. Nullability is important, collections are different, and classes are final by default. Kotlin is more functional, but what does that mean, and how should it change the way that you program? And what about all that Java code that you still have to support?

Your tour guides Duncan and Nat first made the trip in 2015, and they've since helped many teams and individuals follow in their footsteps. Travel with them as they break the route down into legs like Optional to Nullable, Beans to Values, and Open to Sealed Classes. Each explains a key concept and then shows how to refactor production Java to idiomatic Kotlin, gradually and safely, while maintaining interoperability.

The resulting code is simpler, more expressive, and easier to change. By the end of the journey, you'll be confident in refactoring Java to Kotlin, writing Kotlin from scratch, and managing a mixed language codebase as it evolves over time.

Nat and Duncan both started programming in Java before its 1.0 release, and have 55 years of combined experience on both the JVM and other platforms. Until they discovered Kotlin in 2015 Java was their language of choice for most applications.

That changed when they fell in love with JetBrains’ new creation and spread the word, first with their own colleagues and clients, then the JVM community in London, and then internationally through conferences. They have both presented at KotlinConf, where they also run a one day workshop “Refactoring to Kotlin,” which forms the basis for the introductory chapters of this book.