Class is not abstract and does not implement abstract base class member

Question

I'm confused by this Kotlin error associated with providing an implementation for an abstract class that has been imported from a maven package.

I have a maven library that is written in Kotlin and exposes an abstract class called APIGatewayRequestHandler. In my app that imports the library, I provide an implementation of the abstract class:

class GetWelcomeMessageHandler : APIGatewayRequestHandler<WelcomeMessage>()
    fun handleAPIGatewayRequest(input: com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent, context: com.amazonaws.services.lambda.runtime.Context?): WelcomeMessage {
        return WelcomeMessage()
    }
}

The decompiled abstract class from the library looks like this:

public abstract class APIGatewayRequestHandler<T> public constructor() : com.amazonaws.services.lambda.runtime.RequestHandler<com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent, T> {
    public abstract fun handleAPIGatewayRequest(input: com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent, context: com.amazonaws.services.lambda.runtime.Context?): T

    public open fun handleRequest(input: com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent?, context: com.amazonaws.services.lambda.runtime.Context?): T {
        /* compiled code */
    }
}

I get the following error:

Class 'GetWelcomeMessageHandler' is not abstract and does not implement abstract base class member
public abstract fun handleAPIGatewayRequest(input: APIGatewayProxyRequestEvent, context: Context?): WelcomeMessage

Answer 1

I think you're just missing some override keywords. Namely, your abstract class should have it on the handleRequest method:

public abstract class APIGatewayRequestHandler<T> public constructor() : com.amazonaws.services.lambda.runtime.RequestHandler<com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent, T> {
    public abstract fun handleAPIGatewayRequest(input: com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent, context: com.amazonaws.services.lambda.runtime.Context?): T

    public override fun handleRequest(input: com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent?, context: com.amazonaws.services.lambda.runtime.Context?): T {
        /* compiled code */
    }
}

And then your GetWelcomeMessageHandler should have it on its handleAPIGatewayRequest method:

class GetWelcomeMessageHandler : APIGatewayRequestHandler<WelcomeMessage>() { // <-- This curly brace was also missing
    override fun handleAPIGatewayRequest(input: com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent, context: com.amazonaws.services.lambda.runtime.Context?): WelcomeMessage {
        return WelcomeMessage()
    }
}