I am learning coroutines and I got confused when I see the c kotlinlang #getting-started

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Title

ckchen

11/03/2018, 12:44 PM

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?

CoroutineContext

) and still don’t have a clue.

karelpeeters

11/03/2018, 12:49 PM

What happens when you do

Ctrl+B

Job

ckchen

11/03/2018, 12:50 PM

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

karelpeeters

11/03/2018, 12:54 PM

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

elizarov

11/03/2018, 12:57 PM

It is called companion object.

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

karelpeeters

11/03/2018, 1:01 PM

Huh might be misremembering then, wasn't there something special about

[Job]

ckchen

11/03/2018, 1:04 PM

I see this

public companion object Key : CoroutineContext.Key<Job>

Job

. So, why not

coroutineContext[Job.Key]

elizarov

11/03/2018, 1:05 PM

You can use

Job.Key

. But since

Key

is a companion object to

Job

, simply using

Job

works, too.

ckchen

11/03/2018, 1:06 PM

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

karelpeeters

11/03/2018, 1:08 PM

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

ckchen

11/03/2018, 1:11 PM

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

11/03/2018, 3:31 PM

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

ckchen

11/04/2018, 12:52 AM

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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