A good rule is that constructors MUST not do work. Writing constructors in Kotlin is possible but not that common. Data classes are typically initialized with just value assignments. Having support for default values also means that it largely removes the need for having multiple constructors (which is very common in Java).

If you need to do validation, there are very decent frameworks that you should use for both Java and Kotlin. We are using a thing called Konform currently. Very nice.

For non negative numbers, you can use the new unsigned numeric types they added in Kotlin with the last release: https://kotlinlang.org/docs/basic-types.html#unsigned-intege...