You can use this template to create Processing applications in Kotlin. It also allows to call Java classes from Kotlin, and vice-versa.
To run the sketches
gradle run
To clean the build
gradle clean
Enjoy.
You need to have Gradle (5.4) installed or to use the provided wrapper. You also need to have the Kotlin compiler installed (e.g., 1.3.30). Refer to the official Kotlin documentation for details.
Alternatively, you can open the project using the IntelliJ IDE.