How to convert nested ArrayList in Array?

Question

I have to convert an ArrayList of ArrayList in to Array.

List<List<TockaXY>> clustersPorazdeljeni = new ArrayList<>(centers.size());

I know that is possible to convert a single ArrayList to Array like

 TockaXY[] arrayOfClusters = clustersPorazdeljeni.toArray(new TockaXY[centers.size()]);

But these does not convert the nested part. As i understand is now array of ArrayLists. So is it possible to get an Array out of nested ArrayLists?

Please read: Why is “Is it possible to…” a poorly worded question?. To answer your question: yes, it is possible. — Turing85
– Turing85, Commented Sep 7, 2019 at 11:53

Trein · Accepted Answer · 2019-09-07 11:59:45Z

3

You can use flatMap:

List<List<TockaXY>> clustersPorazdeljeni = new ArrayList<>();
TockaXY[] flattened = clustersPorazdeljeni.stream()
        .flatMap(x -> x.stream())
        .toArray(x -> new TockaXY[0]);

answered Sep 7, 2019 at 11:59

Trein

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Comments

Michał Krzywański · Accepted Answer · 2019-09-07 12:00:49Z

2

It is possible using Java Stream and Stream::flatMap :

List<List<TockaXY>> clustersPorazdeljeni = new ArrayList<>();

TockaXY[] strings = clustersPorazdeljeni.stream()
    .flatMap(Collection::stream)
    .toArray(TockaXY[]::new);

answered Sep 7, 2019 at 12:00

Michał Krzywański

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Comments

Jackkobec · Accepted Answer · 2019-09-07 12:23:26Z

0

There is linear alternative for flatMap():

    List<List<TockaXY>> clustersPorazdeljeni = new ArrayList<>();

    List<TockaXY> flatArray = new ArrayList<>();
    clustersPorazdeljeni.forEach(flatArray::addAll);
    TockaXY[] arrayOfClusters = flatArray.toArray(new TockaXY[0]);

Or with flatMap:

TockaXY[] arrayOfClusters = clustersPorazdeljeni.stream().flatMap(Collection::stream).toArray(TockaXY[]::new);

edited Sep 7, 2019 at 12:23

answered Sep 7, 2019 at 12:06

Jackkobec

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