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. Kotlin can be used on the back-end (server-side), but it's getting a lot of attention right now as a language for Android app development. Kotlin is now supported by Google as a first-class language for Android development, so the popularity of Kotlin is set to explode!
In this first tutorial in the Kotlin From Scratch series, you'll learn about the language fundamentals: everything from variables to lambda functions and object-orientation.