Java: replace forEach loop with stream

Question

In the following method I want to replace the forEach loop with a stream.

protected Set<String> getSelectedPaths(final CheckModel<TreeItem<String>> checkModel, final LazyTreeItem root) {

      final Set<String> ceckedList = new HashSet<>();
      final List<TreeItem<String>> children = root.getChildren();
      final List<LazyTreeItem> lazyChildren = children.stream().map((item -> (LazyTreeItem) item))
            .collect(Collectors.toList());

      final List<String> selectedChildren = new ArrayList<>();
      children.forEach(child -> selectedChildren.addAll(getSelectedPaths(checkModel, (LazyTreeItem) child)));

      /**
      * do sth with checkedList
      */
     return checkedList

the lines

      final List<String> selectedChildren = new ArrayList<>();
      children.forEach(child -> selectedChildren.addAll(getSelectedPaths(checkModel, (LazyTreeItem) child)));

should be replaced with a stream. I tried with

      final List<String> selectedChildren = children.stream().map(child->getSelectedPaths(checkModel, (LazyTreeItem) child)).collect(Collectors.toList());

but the compiler error "Type mismatch: cannot convert from List<Set<String>> to List<String>" is thrown. Is there an opportunity to replace this forEach loop with a stream?

Nicktar · Accepted Answer · 2020-01-22 07:28:53Z

2

You try to collect a Stream<Set<String>> into a List<String>. This is obviously a type mismatch. You need to flatten the stream into a Stream<String> prior to collecting it using .flatmap(Set::stream).

answered Jan 22, 2020 at 7:28

Nicktar

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Eran · Accepted Answer · 2020-01-22 07:33:34Z

1

Your map operation is producing a Stream<Set<String>> (since each element of the original Stream<TreeItem<String>> is mapped to a Set<String>), which results in a List<Set<String>>.

In order to get a List<String> you need to use flatMap:

final List<String> selectedChildren = 
    children.stream()
            .flatMap(child->getSelectedPaths(checkModel, (LazyTreeItem) child).stream())
            .collect(Collectors.toList());

edited Jan 22, 2020 at 7:33

answered Jan 22, 2020 at 7:27

Eran

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Comments

Sohan · Accepted Answer · 2020-01-22 07:36:30Z

1

Using .flatmap after Map ,

final List<String> selectedChildren = children.stream().map(child->getSelectedPaths(checkModel, (LazyTreeItem) child)).flatMap(child.stream()).distinct().collect(Collectors.toList());

answered Jan 22, 2020 at 7:36

Sohan

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