Keep improving Kotlin-Java Interop null safety, and constant parameters

[c19x]

[Who] As a Developer

[What] I would like to use Kotlin and take advantage of null safety, and use Java to enforce constant parameters

[Value] In order to achieve cleaner code, and reduce unintentional coding errors

Describe the potential solution you'd like

• Build on @Nullable and @NonNull coverage of Improve Kotlin-Java Interop null safety #191
• Set variables and method parameters to final where applicable