The 3.0 version of Android Studio was released by Google, and it promoted Kotlin as a first class language for Android development. Kotlin is developed by JetBrains in the same way as the Intellij IDEA platform, which is the basis of Android Studio. This language was released in February 2016, it was in development for five years before it was released. It's easy to gradually convert the code base of a project from Java to Kotlin, and a developer that is familiar with Java can learn Kotlin in a few weeks. Kotlin became popular before its release, because this language is full of features and is designed to interoperate with Java. The following diagram shows how Kotlin and Java code are compiled to the same bytecode:

As you can see, part of our application can be written in Java and another part in Kotlin. The kotlinc compiler compiles Kotlin source code to the same bytecode as the javac compiler.