Interoperability
Kotlin -> Java
https://kotlinlang.org/docs/reference/java-interop.html
Kotlin can almost use any Java code without issue.
Instanciate a new user and display its email
Java -> Kotlin
https://kotlinlang.org/docs/reference/java-to-kotlin-interop.html
Using Kotlin in Java is almost quite easy.
Check if the password of the user is valid (at least 8 characters)
