Kotlin: inherit from a nested Java class

Question

I have these Java classes in a .java file:

public class BaseOuter
{
    public class BaseInner
    {
    }
}

And I have these Kotlin classes in a .kt file:

class DerivedOuter : BaseOuter()
{
    class DerivedInner : BaseOuter.BaseInner()
    {
    }
}

This Kotlin code in Android Studio gives me an "Unresolved reference: BaseInner" error. So I can inherit from the BaseOuter class, but deriving from Java's BaseOuter.BaseInner is not syntactically correct (but if I try the same with inheritance from Kotlin classes I get no errors).

Somehow I can't google a same example or the question.

For one thing, this doesn't work, because you have an inner class in Java but a static member class in Kotlin. For a class to be inner in Kotlin, you need to mark it with the inner keyword: inner class DerivedInner ... — Erwin Bolwidt
– Erwin Bolwidt, Commented Nov 14, 2018 at 23:34
You're welcome. I didn't think it would solve it (because the error says it can't resolve the reference which doesn't sound like the problem I mentioned) and I didn't have time to try it. Hence only a comment — Erwin Bolwidt
– Erwin Bolwidt, Commented Nov 15, 2018 at 0:15

syntagma · Accepted Answer · 2018-11-14 23:41:52Z

2

The equivalent of Java's non-static nested class in Kotlin is inner class.

To make your code work, you need to use inner keyword:

class DerivedOuter() : BaseOuter() {
    inner class DerivedInner : BaseOuter.BaseInner() {
    }
}

answered Nov 14, 2018 at 23:41

syntagma

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