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I have this snippet of code. How can I transform it? to functional style? In fact, I have List<X>. each X contains List<V>. Each V in this List has List<M> as a parameter. And I need to build Map<X,Y>, where is Y is an amount of all M objects that are stored in all V objects aggregated in object X.

HashMap<Country, Integer> modelsPerCountryMap = new HashMap<>();
int count;
for (Country country : CountryDataSingleton.getCountryDataCollection()) {
    count = 0;
    for (CarMaker cm : country.getListOfMakers()) {
        count += cm.getModels().size();
    }
    modelsPerCountryMap.put(country, count);
}

I'd stream the countries, and then collect them to a map, where the key is the country and the value can be the summed stream of maker sizes:

Map<Country, Integer> modelsPerCountryMap =
     CountryDataSingleton.getCountryDataCollection()
                         .stream()
                         .collect(
                             Collectors.toMap(
                                 Function.identity(),
                                 c -> c.getListOfMakers()
                                       .stream()
                                       .mapToInt(cm -> cm.getModels().size())
                                       .sum()
                              )
                         );

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What about something like this:

public class CountriesFunctionalTest {

private class CarMaker {

    private final String name;
    private final List<String> models;

    public CarMaker(String name, List<String> models) {
        this.name = name;
        this.models = models;
    }

    public String getName() {
        return name;
    }

    public List<String> getModels() {
        return models;
    }
}

private class Country {

    private final String name;
    private final List<CarMaker> makers;

    public Country(String name, List<CarMaker> makers) {
        this.name = name;
        this.makers = makers;
    }

    public String getName() {
        return name;
    }

    public List<CarMaker> getMakers() {
        return makers;
    }
}

@Test
public void test() throws DeliveryException {
    List<Country> countries = Arrays.asList(new Country[]{
            new Country("Country A", Arrays.asList(new CarMaker[]{ new CarMaker("Maker A", Arrays.asList(new String [] {"model a", "model a1", "model a2"})) })),
            new Country("Country B", Arrays.asList(new CarMaker[]{ new CarMaker("Maker B", Arrays.asList(new String [] {"model b", "model b1"})) })),
            new Country("Country C", Arrays.asList(new CarMaker[]{ new CarMaker("Maker C", Arrays.asList(new String [] {"model c", "model c1", "model c2", "model c3"})) }))
    });

    Map<Country, Integer> conuntriesModels = IntStream.range(0, countries.size()).mapToObj(i -> new AbstractMap.SimpleEntry<Country, Integer>(countries.get(i),
            IntStream.range(0, countries.get(i).getMakers().size())
                .map(ix -> countries.get(i).getMakers().get(ix).getModels().size()).sum()
                )
            ).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

    for (Country next : conuntriesModels.keySet()) {
        System.out.println(next.getName() + " models -> " + conuntriesModels.get(next));
    }
}

}

Output:

Country B models -> 2

Country A models -> 3

Country C models -> 4

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If you intend to go functional in java, consider using vavr.io. Vavr provides you with the tools you need to apply the functional paradigm in a convenient and consistent way. In this case immutable data structures. Utilizing io.vavr.collection.Stream and io.vavr.collection.HashMap, collecting your data into a Map<Country, Integer> could look something like this:

Stream.ofAll(CountryDataSingleton.getCountryDataCollection())
        .map(country -> Tuple.of(country, country.getListOfMakers().foldLeft(0, (acc, cm) -> acc + cm.getModels().size())))
        .foldLeft(HashMap.<Country, Integer>empty(), (acc, entry) -> acc.put(entry));

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Comments
  • You could replace your code with streams, but I honestly don't think you should. This is plenty clear and streams have overhead.
  • Also, your descriptor for what the list is is unclear. Is it List<List<Pojo>> where Pojo has a field List<Model>?
  • Function.identity() instead of indentity() ?
  • @Timir yes, that was a typo on my side - thanks for noticing! Edited and fixed.
  • Utterly terrifying.