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

# stdlib

[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> getNullId() {
        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().getNullId()

    // 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 🙂

[Seokjae Lee]

If this is not the place to post, please point me to the right channel.

[jw]

I would recommend #getting-started

[Seokjae Lee]

Thanks. I'll post to that channel 🙂

[jw]

👍 it has to do with something called "platform types" which someone will point you to. it's a compromise, basically.

[Seokjae Lee]

I'll check the document. Thanks for your kind guidance kodee greetings

[kevin.cianfarini]

Platform types are denoted in the IDE as SomeType! and nullability enforcement from the kotlin side doesn't apply.

Unless the Java interface is annotated with something like @NonNull or @Nullable I usually try to hedge against these problems by always treating the types as nullable.

[kevin.cianfarini]

So in your example the type you're dealing with is List<String!> but it would be safer to treat it as List<String?>