Hello. I have a question regarding the compatibili...

# getting-started

[Seokjae Lee]

Hello. I have a question regarding the compatibility between Kotlin and Java.

I was wondering if you could shed some light on why the code below is working...

// Java
public class TestRepository {
    
    public <T> List<T> getCollectionWhichHasNull() {
        return Collections.singletonList(null);
    }
}

// Kotlin
fun main() {
    // type is List<String>, which means that this collection has non-null value (I think...)
    // There's any compilation error is occurred, why?
    val result: List<String> = TestRepository().getCollectionWhichHasNull()

    // Although result variable's type is List<String>, it contains null item...
    for (s in result) {
        println(s != null) // false
    }
}

If you have any references or blog posts, I'd appreciate it 🙂

[ephemient]

https://kotlinlang.org/docs/java-to-kotlin-nullability-guide.html#platform-types
Kotlin treats Java declarations (that don't have known Nullable or NotNull annotations) as having "platform types", or unknown nullability. it'll let you treat them as either nullable or non-nullable without warning, and if you assume they're non-nullable you get the same potential for NullPointerException as you would in Java.

[Seokjae Lee]

Now I understand that because compiler can't identify the type contains null item or not because it does not have some identifier things like @NotNull .

Thanks for your explanation with document thank you color