How to collect list from lambda expression

Question

I am writing a method which will return a list of regiondata, i am doing in following manner but getting error

@Override
    public List<RegionData> getAllRegionsForDeliveryCountries()
    {
        final List<RegionData> regionData = new ArrayList<>();
        final List<String> countriesIso = getCountryService().getDeliveryCountriesIso();
        regionData = countriesIso.stream().map(c->i18nFacade.getRegionsForCountryIso(c)).collect(Collectors.toList());
        return regionData;
    }

I am getting error on

type mismatch: cannot convert from List<List<RegionData>> to List<RegionData>

on line regionData = countriesIso.stream().map(c->i18nFacade.getRegionsForCountryIso(c)).collect(Collectors.toList());

The function i18nFacade.getRegionsForCountryIso(c) is returning a list of region data, I want to combine these lists into single list. I tried with lambda but unable to do so.

[A note on styling.](https://www.youtube.com/watch?v=1OpAgZvYXLQ&t=4632) — shmosel, Dec 14 '16 at 05:46

score 10 · Accepted Answer · edited Jun 02 '23 at 17:23

10

You need to use flatMap with stream.

regionData = countriesIso.stream()
    .flatMap(c -> i18nFacade.getRegionsForCountryIso(c).stream())
    .collect(Collectors.toList());

edited Jun 02 '23 at 17:23

MC Emperor

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answered Dec 14 '16 at 05:50

Raghav

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Or `regionData = countriesIso.stream().map(i18nFacade::getRegionsForCountryIso).flatMap(List::stream).toList()`. – MC Emperor Jun 02 '23 at 17:26

thegauravmahawar · Answer 2 · 2016-12-14T06:02:31.733

5

Use flatMap:

Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

regionData = countriesIso
               .stream()
               .flatMap(c -> i18nFacade.getRegionsForCountryIso(c)
               .stream())
               .collect(Collectors.toList());

edited Dec 14 '16 at 06:02

answered Dec 14 '16 at 05:53

thegauravmahawar

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score 2 · Answer 3 · edited Jun 02 '23 at 16:40

2

You want to be using Stream#flatMap instead of Stream#map.

edited Jun 02 '23 at 16:40

TuGordoBello

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answered Dec 14 '16 at 05:45

Jacob G.

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This is basically correct but not very helpful. Please elaborate. – shmosel Dec 14 '16 at 05:51

How to collect list from lambda expression

3 Answers3