The J2K converter assumes some ambiguous fields/params (inherited, generics etc) to be nullable and ...

# intellij

[Chetan Sachdeva]

The J2K converter assumes some ambiguous fields/params (inherited, generics etc) to be nullable and after conversion adds ? to them. Is there a way we can avoid this? Does @ParametersAreNonnullByDefault work?

[Adam S]

my general experience is that it will respect any recognised nullability annotations https://kotlinlang.org/docs/java-interop.html#nullability-annotations

@ParametersAreNonnullByDefault isn’t listed as a supported one though

https://github.com/JetBrains/kotlin/blob/v1.8.22/core/compiler.common.jvm/src/org/jetbrains/kotlin/load/java/JvmAnnotationNames.kt

[Chetan Sachdeva]

Unfortunately it doesn’t. For a java class like:

class TestChild extends TestParent<Object> {
    public TestChild(String a) {
        super(a);
    }
}

J2K converter outputs:

internal class TestChild(a: String?) : TestParent<Any?>(a)

which is incorrect.

[Adam S]

help me out, what’s incorrect?

[Chetan Sachdeva]

Ah! Sorry for the confusion. We use ErrorProne NullAway so we assume all unannotated types are non-null by default. But J2K converter assumes it as nullable.

[Chetan Sachdeva]

So the output in this case should be:

internal class TestChild(a: String) : TestParent<Any>(a)

[Adam S]

ahh okay!

[Adam S]

how does NullAway work? Does it add nullability annotations during compilation?

[Adam S]

I would guess that j2k takes the source code and then converts it to Kotlin, so the `@NonNull`/`@Nullable` annotations would have to be explicitly added to the source code

[Chetan Sachdeva]

Given these annotations, NullAway performs a series of type-based, local checks to ensure that any pointer that gets dereferenced in your code cannot be null. NullAway is similar to the type-based nullability checking in the Kotlin and Swift languages, and the Checker Framework and Eradicate null checkers for Java. source

[Chetan Sachdeva]

Yea guess we need to explicitly add @NonNull. Strangely the “old” J2K converter works as per our requirement but the “new” one takes a safer route.

[hfhbd]

There is a settings class: https://github.com/JetBrains/intellij-community/blob/555d12b0d283068cd5f3e9fa06ad5c71cefdbab6/plugins/kotlin/j2k/old/src/org/jetbrains/kotlin/j2k/ConverterSettings.kt#L4 but I have no idea how to set it in Idea.

[Chetan Sachdeva]

settings is not being used in NewJavaToKotlinConverter.kt

[hfhbd]

Oh you are right. Strange, maybe it is a bug.

[Anton Mefodichev]

https://youtrack.jetbrains.com/issue/KTIJ-26107/NewJavaToKotlinConverter-adds-against-all-non-annotated-types-assuming-them-as-nullable