Check if value from one list is present in another list using stream

Question

I have List<RegionCountry> regionCountries of:

class RegionCountry {
    Region region;
    List<Country> countries;
   }

And Region and Country looks like:

class Region {
    Long regionId;
    String regionName;
}

class Country {
    Long countryId;
    String countryName;
}

Each Region have multiple countries within them. I am getting List<Long> countryIds and List<Long> regionIds from the UI which I want to compare against this regionCountries . So I did something like:

List<RegionCountry> filteredList = regionCountries
                            .stream()
                            .filter(regionCountry -> regionIds.contains(regionCountry.getRegion().getRegionId()))
                            .filter(regionCountry -> regionCountry.getCountries()
                                                    .stream()
                                                    .allMatch(country -> countryIds.contains(country.getCountryId())))
                            .collect(Collectors.toList());
                        

But it is not returning anything. I am trying to figure out how to compare List<Long> countryIds with List<Country> countries of RegionCountry . Is there any better approach to do it in Java 8 or 8+ using stream API ? Or can it be simplified by using any other approach?

Answer 1

The presented code seems to be fine and working, so the actual datasets need to be checked if they meet the provided condition: regionIds contain rc.regionId AND countryIds contain IDs of all countries.

Online demo (using record instead of classes to avoid boilerplate code) :

non-filtered
RegionCountry[region=Region[regionId=1, regionName=Europe], countries=[Country[countryId=1, countryName=Albania], Country[countryId=2, countryName=Andorra], Country[countryId=6, countryName=Germany]]]
RegionCountry[region=Region[regionId=3, regionName=America], countries=[Country[countryId=15, countryName=Canada], Country[countryId=16, countryName=US]]]
region ids: [3, 1]
country ids: [1, 2, 3, 15, 6]
filtered
RegionCountry[region=Region[regionId=1, regionName=Europe], countries=[Country[countryId=1, countryName=Albania], Country[countryId=2, countryName=Andorra], Country[countryId=6, countryName=Germany]]]

A minor optimization would be to use Set instead of List for regionIds , countryIds to make faster checks with Set::contains .