Java 8: Mapping multiple properties of class in same stream

Question

Hello my question is very similar to Java8: Stream map two properties in the same stream but for some reason I cannot get my code to work.

So like that question, say I have a class Model

class Model {
    private List<Integer> listA;
    private List<Integer> listB;
    private List<Integer> listC;
    public List<Integer> getListA() {
        return listA;
    }

    public List<Integer> getListB() {
        return listB;
    }
    public List<Integer> getListC() {
        return listC;
    }
}

So I want to combine these lists from a List<Model> myList using a stream and my code looks like this:

myList.stream()
      .flatmap(i -> Stream.of(i.getListA(), i.getListB(), i.getListC()))
      .collect(Collectors.toList())
      .get(0)

But this approach ends up returning an empty list. I would appreaciate any suggestions

score 3 · Answer 1 · answered Aug 06 '22 at 01:03

Well, the difference with the linked question is that you have three levels: a sequence of Models, each model having multiple attributes. Those attributes are Lists, so they as well may contain multiple elements.

Now you can use flatMap twice instead of once.

myList.stream()
    .flatMap(model -> Stream.of(model.getListA(), model.getListB(), model.getListC())
        .flatMap(List::stream))
    .toList();

score 2 · Accepted Answer · answered Aug 06 '22 at 00:34

You must concatenate the 3 lists with Stream.concat. Once you've concatenated the streams (don't cross them!), flat map them to a Stream<Integer>, then collect to a List.

myList.stream()
      .flatMap(m -> Stream.concat(Stream.concat(m.getListA().stream(),
                                  m.getListB().stream()),
                    m.getListC().stream()))
      .collect(Collectors.toList());

With

List<Model> models = List.of(
    new Model(Arrays.asList(1, 2, 3), Arrays.asList(4, 5, 6), Arrays.asList(7, 8, 9)),
    new Model(Arrays.asList(11, 12, 13), Arrays.asList(14, 15, 16), Arrays.asList(17, 18, 19)),
    new Model(Arrays.asList(21, 22, 23), Arrays.asList(24, 25, 26), Arrays.asList(27, 28, 29))
);

The output is:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29]

Thanks I really appreciate the help! Since I think Stream.concat only allows for 2 parameters, what I did was followed the example that they gave in the Stream.concat documentation for concatening multiple streams: Streams.of(e.getListA().stream(), .... ).flatMap(s->s).collect(...) and that fixed it — currykasparov, Aug 06 '22 at 00:47

Java 8: Mapping multiple properties of class in same stream

2 Answers2