97 Things Every Java Programmer Should Know

Chapter 37. Interop with Kotlin

Sebastiano Poggi

In recent years, Kotlin has been a hot-button topic in the JVM community; the usage of the language is constantly increasing, from mobile to backend projects. One of Kotlin’s advantages is its great degree of interoperability with Java right off the bat.

Calling into any Java code from Kotlin just works. Kotlin understands Java perfectly well, but there’s one minor annoyance that may present itself if you’re not following Java best practices to the letter: the lack of non-nullable types in Java. If you don’t apply nullability annotations in Java, Kotlin assumes all those types have unknown nullability—they’re so-called platform types. If you’re certain they will never be null, you can coerce them into a non-null type with the !! operator or by casting them to a non-null type. In either case, you’ll get a crash if the value is null at runtime.  The best way to handle this scenario is to add nullability annotations such as @Nullable and @NotNull to your Java APIs. There are a variety of supported annotations: JetBrains, Android, JSR-305, FindBugs, and more. This way, Kotlin will know the type nullability, and when coding in Java you’ll receive additional IDE insights and warnings about potential nulls. Win-win!

When invoking Kotlin code from Java, you should find that while the majority of the code will work just fine, you may see ...