API design question! I'm writing a API/library in Kotlin. For a method if it returns a sealed class ...

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[shaktiman_droid]

API design question!
I'm writing a API/library in Kotlin. For a method if it returns a sealed class object then is there anything I can do to make it nicer when the API is used from Java side ?

For example

sealed class Response {
    object Success : Response()
    data class Failure(val error: String) : Response()
}

object Test {
    @JvmStatic
    fun returnSealed(): Response {
        return Response.Success
    }
}

On Java side, caller would have to use instanceOf if-else logic. Is there a way to make it better for them ?

Response response = Test.returnSealed();
if (response instanceof Response.Failure) {
    ((Response.Failure) response).getError();
} else {
    // handle success
}

[randomcat]

You could add an onSuccessOrElse(() -> blah, err -> blah) method to Response

[randomcat]

Probably accompanied by onSuccess and onError

[shaktiman_droid]

is this a known pattern on Java side ? Can you share any example ?
I want to make the API feel more natural for Java users.

[randomcat]

It would be similar to what Optional does

[shaktiman_droid]

Thanks. I'll give it a try.

[randomcat]

np

[shaktiman_droid]

aah so that would be restrictive in a sense that I would be defining a strict method for API user especially that lambda signature. If they want to return their own object then they can't use it.

[randomcat]

Add a method that takes a Supplier<T> and a Function<String, T> then

[shaktiman_droid]

Yeah so something like this based on what you suggested ?

sealed class Response {
    abstract fun <T> onSuccessOrElse(onSuccess: () -> T, onError: (String) -> T)

    object Success : Response() {
        override fun <T> onSuccessOrElse(onSuccess: () -> T, onError: (String) -> T) {
            onSuccess()
        }
    }

    data class Failure(val error: String) : Response() {
        override fun <T> onSuccessOrElse(onSuccess: () -> T, onError: (String) -> T) {
            onError(error)
        }
    }
}

[randomcat]

Yeah that would work

[randomcat]

Though it won't accept void-producing lambdas

[shaktiman_droid]

yeah I realized that. Is there a way to solve it ? I think that's usually the problem I have seen where I have to explicitly return Unit.INSTANCE from Java

[randomcat]

I don't know if overloading works properly (test it), but if it doesn't you could just write another function taking a nullary void functional interface and a one-ary void functional interface

[shaktiman_droid]

With functional interface, final version would look like this

fun interface SuccessHandler {
    fun invoke()
}

fun interface FailureHandler {
    fun invoke(error: String)
}

sealed class Response {
    abstract fun <T> onSuccessOrElse(onSuccess: SuccessHandler, onError: FailureHandler)

    object Success : Response() {
        override fun <T> onSuccessOrElse(onSuccess: SuccessHandler, onError: FailureHandler) {
            onSuccess.invoke()
        }
    }

    data class Failure(val error: String) : Response() {
        override fun <T> onSuccessOrElse(onSuccess: SuccessHandler, onError: FailureHandler) {
            onError.invoke(error)
        }
    }
}

[shaktiman_droid]

calling code from Java would like this

response.onSuccessOrElse(() -> {
            // Handle success
        }, error -> {
            // Handle failure with error
        });

[shaktiman_droid]

I think this is a good solution to my problem. Thanks @randomcat

[randomcat]

np

[shaktiman_droid]

bway @randomcat
Thanks for the help.
I wrote a blog post about it and thanked you there as well. I hope I found your correct github profile
https://jabbar.hashnode.dev/kotlin-sealed-class-api-for-java-consumption

[randomcat]

cool, and you did

[randomcat]

although, technically SuccessHandler and FailureHandler's invoke methods do return Unit in Kotlin, they just get generated to return void in Java because the return type is not generic

[shaktiman_droid]

Yeah looks like I didn't do good job at clarifying that somewhere?

[randomcat]

Since our invoke method in the interface doesn't return Unit like our Lambda, our Java code doesn't need to return anything anymore.

this could be more explicit, but I'm just a pedant

[randomcat]

good post overall

[shaktiman_droid]

aah makes sense. I'll 100% update it tonight. It's better to use correct words and add more details so someone somewhere doesn't take the incorrect thing literally and get incorrect knowledge.

Thank you for catching that.

[randomcat]

np