What’s the Target Platform of Kotlin? How is Kotlin-Java interoperability possible?

Answer 1

There are rules you must follow to go from one to the other. The JVM code supports a lot of things neither language use so to call Kotlin code from Java it must be coded like Java.

Answer 2

Kotlin used to be mostly for Android, but now you can develop for many more platforms.
Kotlin and Java both run on JVM and compile to bytecode, so it doesn't make that big of a difference whether it's Java class or Kotlin class.