Mastering High Performance with Kotlin

First-class functions

In computer science, first-class describes programming language entities that can be declared and used anywhere. For instance, a class with the public modifier is a first-class citizen in Java.

In Kotlin, you don't need to create a class to declare a function; you can just do it in the file. In the preceding sample, the main function is a top-level function that's just declared in the Main.kt file. It can be used as the starting point for a program because first-class functions compile to the same bytecode as public final static functions in Java. To ensure this, go to Tools in IntelliJ IDEA and the following dialog window will open:

Go to the Kotlin submenu and click Show Kotlin Bytecode in the following context menu: ...