If you have a Java interface ```public interface Transformer

Question

If you have a Java interface ```public interface Transformer lt OUT IN gt OUT transform IN in ``` And a Java method ```filter Transformer lt String String gt var1 ``` And Kotlin wrongly assumes the second parameter to be non `null` How can you call it with a lambda that returns `null`

Joffrey · Answer

Kotlin should infer platform types for those type parameters Do you have a reproducible example of Kotlin assuming non null here I tested your code and I can correctly call it with a lambda returning null

Vampire · Answer

Gradle build script with a Copy task there the filter method

Vampire · Answer

```plugins `java library` tasks processResources filter it ifBlank null ```

Vampire · Answer

So besides somewhere having a wrong assumption or configuration is there a way to force it to work

Vampire · Answer

In this specific case it comes to the rescue that there is a `Closure` variant so I can do ```tasks processResources filter object Closure lt String gt null null fun doCall line String = line ifBlank null ``` but the question is more general In other situations there might not be an appropriate alternative

ephemient · Answer

<https github com gradle gradle blob master subprojects core api src main java org gradle api file package info java|https github com gradle gradle blob master subprojects core api src main java org gradle api file package info java> package level NonNullApi

ephemient · Answer

I think Kotlin is correct here the Gradle API is declaring that everything is non null

Vampire · Answer

`subprojects core api src main java org gradle api package info java` not `subprojects core api src main java org gradle api file package info java` but yeah That explains where still wrong non nullness comes from The JavaDoc clearly says that returning `null` is a valid option and will delete the transformed line So it is a Gradle bug But the question remains can I somehow trick the Kotlin compiler so that `null` is returned without writing a Java or Groovy source file that returns `null` on behalf

ephemient · Answer

not as far as I know disappointed

ephemient · Answer

this works even if a `Closure lt gt ` API isn t provided ```groovy util Eval xy this line String gt line ifBlank null x filter y invoke it ``` admittedly nauseated face

Vampire · Answer

Ah figured out the nice version of it ```tasks processResources filter it ifBlank Supplier null get ```

Vampire · Answer

Interestingly enough this does not work in the anonymous object version

Jason5lee · Answer

For by passing the null check here is a trick ``` Suppress UNCHECKED CAST NOTHING TO INLINE inline fun lt T gt `null but only when you know what you re doing` T = This won t throw because all casting to generics is unchecked null as T```

Jason5lee · Answer

I think I found it from a JetBrains library so they allowed this usage but again it s only for special case

Vampire · Answer

Ah thanks will remember for non JVM case as pure Kotlin variant

Vampire · Answer

Hm maybe I just didn t try to execute it While the IDE complains about ```tasks processResources filter it ifBlank null ``` it actually works just fine

Vampire · Answer

So I think we are back to it being a Kotlin Kotlin IntelliJ plugin issue slightly smiling face

Vampire · Answer

ephemient · Answer

actually that is interesting I wonder if kotlinc doesn t have the same classpath as intellij

ephemient · Answer

Gradle defines ``` javax annotation Nonnull public interface NonNullApi``` and `javax annotation Nonnull` is one of the annotations that Kotlin is supposed to recognize but if can t see that far