I am learning coroutines and I got confused when I...

# getting-started

[ckchen]

I am learning coroutines and I got confused when I see the code coroutineContext[Job]. Isn’t Job an interface? Why can it be used for indexing? I have checked the source (public operator fun <E : Element> get(key: Key<E>): E? of CoroutineContext) and still don’t have a clue.

[karelpeeters]

What happens when you do Ctrl+B on Job?

[ckchen]

public interface Job : CoroutineContext.Element

[ckchen]

I mean, Job is an interface, not an instance of Key<E>. Why can it be passed to public operator fun <E : Element> get(key: Key<E>): E?

[karelpeeters]

Dammit I remember reading about this, I completely forgot though. You might have more luck asking in #coroutines

[elizarov]

It is called companion object.

[elizarov]

https://kotlinlang.org/docs/reference/object-declarations.html#companion-objects

[karelpeeters]

Huh might be misremembering then, wasn't there something special about [Job]?

[ckchen]

I see this public companion object Key : CoroutineContext.Key<Job> in Job. So, why not coroutineContext[Job.Key]?

[elizarov]

You can use Job.Key. But since Key is a companion object to Job, simply using Job works, too.

[ckchen]

Is this a hidden secret? 😀 I don’t see it addressed in the document of companion objects.

[karelpeeters]

They explain it the other way around, first with a custom name and then "you can omit it".

[ckchen]

I know. But I thought they are talking about calling the method of the companion object. I didn’t expect that we can refer to the companion object by omitting it and just use the class name.

[elizarov]

Indeed, it is not explicitly spelled in the docs. It would be nice to add such an example there

[ckchen]

Yes. So I made a simple pull request yesterday (https://github.com/JetBrains/kotlin-web-site/pull/1192). Hope it would be helpful.