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

MC Emperor · Accepted Answer · 2022-08-06 01:03:40Z

3

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();

answered Aug 6, 2022 at 1:03

MC Emperor

23.3k16 gold badges90 silver badges139 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

rgettman · Accepted Answer · 2022-08-06 00:34:49Z

2

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]

answered Aug 6, 2022 at 0:34

rgettman

179k30 gold badges282 silver badges365 bronze badges

1 Comment

currykasparov Over a year ago

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

Collectives™ on Stack Overflow

Java 8: Mapping multiple properties of class in same stream

2 Answers 2

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related