Java interoperability

You will primarily use Kotlin when developing your application. Since both Kotlin and Java runs on the JVM, it is possible to mix the languages. This allows you to benefit from the rich Java ecosystem.

While it is theoretically possible to use Java when declaring rules and interacting with Klerk, this is not recommended. However, it may make sense to mix Kotlin and Java in your own code if you have a legacy application in Java which doesn't have to interact directly with Klerk, at least not much.