Kotlin is completely interoperable with Java. It hasn't changed any of the existing Java functionality. This means that if we are in the middle of an application or microservice written in Java, we can switch to Kotlin and use its new features.

If we have an existing Java project and want to leverage the advantage that Kotlin offers, we can add new code in Kotlin to an existing Java project. In an application, part of the code can be written in Java, part in Kotlin, and they will work seamlessly together.