How to convert nested ArrayList in Array?

Kr Neki :

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?

Jackkobec :

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);