Use stream() to convert nested list into list

Question

I have an arraylist 'response' with each element containing an object Meta and List(Values) i.e. a list which contains nested list and some meta data for each nested list. I would like to use stream() on this to create a new list. The new list will contain all the value objects and its corresponding meta data. How can i use response.stream() to accomplish this?

    //Input
    Meta meta1 = new Meta("name1");
    Meta meta2 = new Meta("name2");
    Meta meta3 = new Meta("name3");
    List<Values> values1 = new ArrayList<>();
    values1.add(new Values("1 Aug", 1.0F));
    values1.add(new Values("2 Aug", 2.0F));
    values1.add(new Values("3 Aug", 3.0F));
    List<Values> values2 = new ArrayList<>();
    values2.add(new Values("1 Aug", 4.0F));
    values2.add(new Values("2 Aug", 5.0F));
    values2.add(new Values("3 Aug", 6.0F));
    List<Values> values3 = new ArrayList<>();
    values3.add(new Values("1 Aug", 7.0F));
    values3.add(new Values("2 Aug", 8.0F));
    values3.add(new Values("3 Aug", 9.0F));
    Response response1 = new Response(meta1, values1);
    Response response2 = new Response(meta2, values2);
    Response response3 = new Response(meta3, values3);
    List<Response> response = new ArrayList<>();
    response.add(response1);
    response.add(response2);
    response.add(response3);

    //use response.stream()...

    //output
    List<MetaValue> metaValueList = new ArrayList<>();
    metaValueList.add(new MetaValue("name1", "1 Aug", 1.0F));
    metaValueList.add(new MetaValue("name1", "2 Aug", 2.0F));
    metaValueList.add(new MetaValue("name1", "3 Aug", 3.0F));
    metaValueList.add(new MetaValue("name2", "1 Aug", 4.0F));
    metaValueList.add(new MetaValue("name2", "2 Aug", 5.0F));
    metaValueList.add(new MetaValue("name2", "3 Aug", 6.0F));
    metaValueList.add(new MetaValue("name3", "1 Aug", 7.0F));
    metaValueList.add(new MetaValue("name3", "2 Aug", 8.0F));
    metaValueList.add(new MetaValue("name3", "3 Aug", 9.0F));

Are you sure you want a List? Sounds more like you want something akin to a Map. — Turing85
– Turing85, Commented Aug 13, 2021 at 18:22
You will need nested forEach loop and build the final data which you need. — Jignesh M. Khatri
– Jignesh M. Khatri, Commented Aug 13, 2021 at 18:26

shmosel · Accepted Answer · 2021-08-13 18:39:51Z

2

You need to stream the responses and flatMap the values. I'll take some liberty with the naming because you haven't provided class definitions, but this should give you an idea:

List<MetaValue> metaValueList = response.stream()
        .flatMap(r -> r.getValues()
                .stream()
                .map(v -> new MetaValue(r.getMeta().getName(), v.getDate(), v.getValue())))
        .collect(Collectors.toList());

answered Aug 13, 2021 at 18:39

shmosel

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