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javatreejava-streamflatten

Flatmaping a n-order tree structure : Java Streams

Suppose I have a non-binary tree structure (each node may have n child nodes). In that sense, what could be a good method for flattening the whole tree into a single list using java streams?

rootNode.getChildren().stream()
    .flatMap(node -> node.getChildren().stream())
    .collect(Collectors.toList());

This example is what I would do for a single level flattening, and still such would not include the root node (which is desired).

Solution

  • Lets call your class Node. In that class, add the method (assuming getChildren() never return null) :

    public Stream<Node> streamAll(){
    return Stream.concat(Stream.of(this), getChildren().stream().flatMap(Node::streamAll));
}

    Then to get a list, you can simply call

    rootNode.streamAll().collect(Collectors.toList());

    or since java 16

    rootNode.streamAll().toList();