Cast error on a Kotlin class that extends a Java class


This Content is from Stack Overflow. Question asked by Antamack

I have a Java project that uses an internal company library made in Kotlin. In this library, I have a Kotlin class that extends the Feign.Builder. This is how the decompiled code looks like

public final class CustomizedFeign public constructor() {
    public final class Builder public constructor() : feign.Feign.Builder {

And this is how the original Kotlin code looks like:

class CustomizedFeign {

    fun builder(): Builder = Builder()

    class Builder : Feign.Builder() {

When I try to cast the CustomizedFeign class to Feign.Builder in my Java project

final Feign.Builder builder = (Feign.Builder) new CustomizedFeign.Builder();

IntelliJ shows an error:
Inconvertible types; cannot cast 'CustomizedFeign.Builder' to 'feign.Feign.Builder'

I don’t get why I’m having this error. In java, you can cast from a child class to a parent class without problems. In this case, CustomizedFeign is a subtype of Feign.Builder, so I should be allowed to cast them… I guess the “Kotlin factor” has something to do with this error.


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