# Beginner question regarding splitWhen/groupBy followed by fold

**URL:** <https://discuss.akka.io/t/beginner-question-regarding-splitwhen-groupby-followed-by-fold/804>\
**Category:** Akka Streams & Alpakka\
**Tags:** java\
**Created:** [April 24, 2018, 9:21am UTC](https://discuss.akka.io/t/beginner-question-regarding-splitwhen-groupby-followed-by-fold/804 "2018-04-24T09:21:31Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Beginner](https://avatars.discourse-cdn.com/v4/letter/b/e495f1/32.png) [@Beginner](https://discuss.akka.io/u/Beginner)\
**Post date:** [April 24, 2018, 9:21am UTC](https://discuss.akka.io/t/beginner-question-regarding-splitwhen-groupby-followed-by-fold/804/1 "2018-04-24T09:21:31Z")

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Hey guys,

I’m new to Akka Streams and I’m encountering unexpected behaviour. My question is probably quite silly - but I really don’t understand what I’m seeing right now…

I’m trying to group a couple of simple data objects by one of their members. A very simple example would be grouping a range of numbers in even and uneven numbers.

So for example for range (1 … 10) I’d expect the two lists/collections: [1, 3, 5, 7, 9] and [2, 4, 6, 8, 10].

I’m trying to implement it like this:

```auto
Source.from(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
    .groupBy(Integer.MAX_VALUE, i -> i % 2 == 0)
    .fold(new ArrayList<Integer>(), (e, f) -> {e.add(f); return f;})
    .mergeSubstreams()
    .runWith(Sink.seq(), mat);

```

Instead of the expected two distinct lists, I get the two lists that contain all elements: [2, 1, 3, 4, 5, 6, 7, 8, 9, 10] and [2, 1, 3, 4, 5, 6, 7, 8, 9, 10]

My question at this point is: Why do both lists contain all elements?  
Bonus question: Why are the first two elements of the lists out of order?

One more detail: At first I suspected that I misunderstood groupBy or splitWhen. However when I use  
`reduce( (e, f) -> e + f)`  
instead of fold I get the expected result [25, 30] where reduce was run once for even numbers and once for uneven numbers. Why does fold seemingly behave so differently then?

Any help is greatly appreciated!

Edit: I have found another clue: In fold I create a mutable list as “zero”. Apparently both SubSources use the same list instance which explains the odd behaviour. To fix that I would have to provide ArrayList::new rather than creating the instance myself. Unfortunately there is no method for that.

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**Author:** ![johanandren](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.akka.io/johanandren/32/85_2.png) [@johanandren](https://discuss.akka.io/u/johanandren)\
**Post date:** [April 24, 2018, 11:44am UTC](https://discuss.akka.io/t/beginner-question-regarding-splitwhen-groupby-followed-by-fold/804/2 "2018-04-24T11:44:00Z")

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Your observation is correct, two `Fold` stages are created, but they share the same mutable collection.

On the Scala side of things we pretty much always use immutable collections, which is a bit harder in Java as there are no such collections in the standard library. Even in Scala a factory based zero could be useful though.

I have created an issue to discuss if we should add support for this or at least clarify docs: [https://github.com/akka/akka/issues/24971](https://github.com/akka/akka/issues/24971)

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**Author:** ![Beginner](https://avatars.discourse-cdn.com/v4/letter/b/e495f1/32.png) [@Beginner](https://discuss.akka.io/u/Beginner)\
**Post date:** [April 24, 2018, 12:18pm UTC](https://discuss.akka.io/t/beginner-question-regarding-splitwhen-groupby-followed-by-fold/804/3 "2018-04-24T12:18:16Z")

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Thanks Johan :)

I’m curious how the discussion will go!