How to collect list from lambda expression [duplicate]

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.

shmosel
– shmosel

2016-12-14 05:46:17 +00:00
Commented Dec 14, 2016 at 5:46 — shmosel
– shmosel, Commented Dec 14, 2016 at 5:46

MC Emperor · Accepted Answer · 2023-06-02 17:23:40Z

10

You need to use flatMap with stream.

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

edited Jun 2, 2023 at 17:23

MC Emperor

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answered Dec 14, 2016 at 5:50

Raghav

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1 Comment

MC Emperor Over a year ago

Or regionData = countriesIso.stream().map(i18nFacade::getRegionsForCountryIso).flatMap(List::stream).toList().

thegauravmahawar · Accepted Answer · 2016-12-14 06:02:31Z

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, 2016 at 6:02

answered Dec 14, 2016 at 5:53

thegauravmahawar

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Comments

TuGordoBello · Accepted Answer · 2023-06-02 16:40:04Z

2

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

edited Jun 2, 2023 at 16:40

TuGordoBello

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answered Dec 14, 2016 at 5:45

Jacob G.

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1 Comment

shmosel Over a year ago

This is basically correct but not very helpful. Please elaborate.

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