Implementing MongoDB CodecProvider with Kotlin giving Type mismatch Compilation Error

Question

I am trying to implement MongoDB CodecProvider interface using Kotlin.

The interface only has the following method (in Java)

    <T> Codec<T> get(Class<T> clazz, CodecRegistry registry);

when I try to implement this with Kotlin.

For example using IntegerCodec (which implements Codec<Integer> in the mongoDB Java driver):

    override fun <T : Any?> get(clazz: Class<T>?, registry: CodecRegistry?): Codec<T> {
        return IntegerCodec()
    }

The Kotlin compiler gives me an error saying:

Type mismatch: inferred type is IntegerCodec but Codec<T> was expected

I do not understand why Kotlin is complaining about this. Is there any special handling we have to do to implement generic Java interface with Kotlin?

Answer 1

As the overridden method signature is still generic, you need to cast the return value

return IntegerCodec() as Codec<T>