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I am learning coroutines and I got confused when I see the code `coroutineContext[Job]`. Isn’t `Job`...
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c
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.
k
What happens when you do 
Ctrl+B
on 
Job
?
c
public interface Job : CoroutineContext.Element
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
?
k
Dammit I remember reading about this, I completely forgot though. You might have more luck asking in #coroutines
e
It is called companion object.
k
Huh might be misremembering then, wasn't there something special about 
[Job]
?
c
I see this 
public companion object Key : CoroutineContext.Key<Job>
in 
Job
. So, why not 
coroutineContext[Job.Key]
?
e
You can use 
Job.Key
. But since 
Key
is a companion object to 
Job
, simply using 
Job
works, too.
c
Is this a hidden secret? 😀 I don’t see it addressed in the document of companion objects.
k
They explain it the other way around, first with a custom name and then "you can omit it".
c
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.
e
Indeed, it is not explicitly spelled in the docs. It would be nice to add such an example there
c
Yes. So I made a simple pull request yesterday (https://github.com/JetBrains/kotlin-web-site/pull/1192). Hope it would be helpful.
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