Hi! I am trying to call a `suspend fun` from Java,...

# coroutines

[jossiwolf]

Hi! I am trying to call a suspend fun from Java, but I can't access any properties of Result - when implementing Continuation, I only get an Object in resumeWith. Can anybody help with that? 🙂

[jossiwolf]

service.myApiCall(10, new Continuation<List<String>>() {
            @NotNull
            @Override
            public CoroutineContext getContext() {
                 ...
            }

            @Override
            public void resumeWith(@NotNull Object result) {
                   // result should acutally be of type Result<List<String>>
            }
        });

[Marc Knaup]

Maybe it's because it is an inline class, so you just have to cast it to the actual List.
But I have no idea how to access the exception from Java 🤔

[jossiwolf]

Yes, that's what I thought (although that's super weird). The thing is while casting works, I still can't access any properties then 😞

[Marc Knaup]

What did you cast it to?

[Dico]

You should be able to perform operations on the object through Result.getValue(instance) for example

[Dico]

I don't think that particular function exists

[Dico]

But the object is what the Result wraps, it's just inlined

[Dico]

If it's an exception then the exception is wrapped by another wrapper class in order to distinguish it

[Dico]

Look at static functions available on Result type

[jossiwolf]

I'm not quite sure what you mean @Dico. I really can't call anything on result except for equals, hashCode and toString, even if I cast it 😕

[Dico]

I said static functions.

[Dico]

On the Jvm, that's a part of how inline classes are implemented. Static functions taking the wrapped value or reference as the first parameter.

[Dico]

If you cant find anything, it happens to be that, if there wasn't an error, the object will be null or a List, which is your continuation's T. However, if you rely on that, you're relying on implementation details of Result.

[jossiwolf]

Right. Still, I don't understand why resumeWith is being called with an Object instead of my T. There seems to be a static method called constructor-impl on the class Result, but I can't call it from Java because of the dash, so this sadly doesn't help me.

[Dico]

If nothing else, this seems like a failure to consider java compatibility.

[Dico]

I tried it myself and, unfortunately, I can't figure it out either.

[Marc Knaup]

The wrapped value of Result is Any?
So Result<Whatever> is Object in Java because it doesn't use the inline class's wrapper class.

[Dico]

@SuppressWarnings("KotlinInternalInJava")
@Override
public void resumeWith(@NotNull Object o) {
    if (o instanceof Result.Failure) {
        Throwable exception = ((Result.Failure) o).exception;
    } else {
        List<String> result = (List<String>) o;
    }
}

[Dico]

sadly, you have to do it like this.

[Marc Knaup]

The @NotNull seems to be wrong though

[Dico]

that's a good point. IntelliJ generated that.

[Dico]

Seems like a bug in IntelliJ.

[Dico]

Maybe not, according to IntelliJ the super method's parameter is marked with @NotNull

[Dico]

which is true if you consider the Result type, but not the erased type of Any?

[Marc Knaup]

So it's wrong in the Continuation interface in Java?
Makes sense

[Dico]

yeah, that's what I think

[jossiwolf]

Thanks for trying! Isn't resumeWith called with a Result in any case? If I got it correctly, casting Object o to the T wouldn't work because it's still a Result.

[Dico]

It's a Result in Kotlin code, but because Result is an inline class, the erased type or the type that's visible from Java is the type of the only field that Result has.

[Dico]

that type is Any? or @Nullable Object, which is used as an intersection of T and Result.Failure.

[jossiwolf]

Neat, thanks for the explanation!