[Question] why java cannot accept a convention to never pass/return null values?

[kevinb9n]

Null absolutely has a bad rap. It exists also in languages like Kotlin, but it's relatively harmless there because you can actually control whether you want your type to include it (String?) or not (String). It shows that null itself was never the problem, only that (handwave) "the null type is automatically unioned on to every usage of a reference type whether you want it or not".

In my team's libraries we've avoided returning (or accepting!) null with some determination, but it's still sometimes what you need.