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kotlinlang

there is something i don’t understand. is everything non-nullable by default (unless i use “?”) or is there something specific to write to ensure that something can’t be nulled ?

There is no default. Without `?` you explicitly make it non-nullable.

in Kotlin, a concrete type written as `String` is non-nullable, `String?` is nullable. types coming from Java have unknown nullability unless they have a recognized annotation, <https://kotlinlang.org/docs/java-interop.html#null-safety-and-platform-types|https://kotlinlang.org/docs/java-interop.html#null-safety-and-platform-types>

if you have a generic type `&lt;T&gt;` in Kotlin, without further constraints, it does not have a specified nullability. you can use a constraint such as `&lt;T : Any&gt;` to ensure it is always a non-nullable type, or write `T?` or `T &amp; Any` (the latter is <https://kotlinlang.org/docs/whatsnew17.html#stable-definitely-non-nullable-types|new in Kotlin 1.7>) to make specific use-sites nullable or non-nullable

make sure to know about platform types to understand why extra attention is required when interacting with java code (nulls might leak into your non-nullable variables when interacting with Java). <https://kotlinlang.org/docs/java-interop.html#null-safety-and-platform-types>

i’ve read the documentation but somehow, compared to other part of the documentation, this one isn’t very beginner friendly. i found a lot of explanation that use String as an exemple, but i wasn’t sure if String was a specific case or if everything was non-nullable unless specified otherwise.

anyway, you answered my question. thank you