Kotlin is an open-source, statically typed programming language backed by JetBrains. Kotlin combines OO and functional features and is focused on interoperability, safety, clarity, and tooling support. It currently targets the JVM and JavaScript, and it's an officially supported language on Android.

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Kotlin is 100% interoperable with Java. In other words, it can be used together with Java in the same project. So you can refactor parts of your Java code to Kotlin and it won't break. In addition to that, it is concise, expressive, and has great tooling.