Hello! Does `.forEach{it->}` guarantee the original sequence during execution?

# getting-started

[毛小川]

Hello! Does .forEach{it->} guarantee the original sequence during execution?

[gildor]

yes

[gildor]

Though not all collections preserve order

[毛小川]

Thanks! This seems to be different from the stream().forEach() of java

[毛小川]

https://kotlinlang.slack.com/archives/C0B8MA7FA/p1646125840005099?thread_ts=1646125736.314299&cid=C0B8MA7FA
This refers to a collection that is itself unordered, right?🤔

[gildor]

right

[gildor]

This seems to be different from the stream().forEach() of java

I believe it’s not different except if you use parallel stream

[gildor]

It’s not different on practice, but order is not specified (and parallel stream is an example where forEach will not executed in the same order)_

[gildor]

So I would say that you should first be sure about what kind forEach you are talking about, Collection.forEach ( either Java and Kotlin extension) just use iterator order and order of forEach will be the same as order of this collection iterator implementation

[Vampire]

It's actually exactly the same as for stream().forEach() in Java.
If the Stream is ordered and sequential, forEach will preserve the order. If the stream is unordered or parallel, order might not be preserved.

Same for forEach in Kotlin. If the collection you run it on preserves order, forEach preserves order too, if no, then not.

[gildor]

Parallel stream is still quite a different case

[Vampire]

Ah, well, actually forEach does have no guarantee even for ordered sequential stream, but there the order usually is preserved.
Forgot about forEachOrdered which adds this guarantee for ordered streams, no matter whether sequential or parallel.