列表列表的列表到列表列表的扁平化地图

Question

我有一个列表（州）列表（城市）列表（城镇）。如何展平中间列表（城市）并转换为列表（城镇）的地图（城镇名称）的地图（州名称）？

这个问题可能看起来与另一个问题Java 3级嵌套列表到3级嵌套映射类似，但它们并不相同。这一个，我需要压平其中一个列表。

import lombok.AllArgsConstructor;
import lombok.Data;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

@Data
@AllArgsConstructor
class Country {
    private String name;
    private List<State> states;
}

@Data
@AllArgsConstructor
class State {
    private String name;
    private List<City> cities;
}

@Data
@AllArgsConstructor
class City {
    private String name;
    private List<Town> towns;
}

@Data
@AllArgsConstructor
class Town {
    private String name;
    private String address;
    private Integer population;
}

public static void main(String[]args) {
    List<Town> countryAStateACityATownsList = new ArrayList();
    countryAStateACityATownsList.add(new Town("CountryA-StateA-CityA-TownA", "addressOfCountryA-StateA-CityA-TownA", 100));
    countryAStateACityATownsList.add(new Town("CountryA-StateA-CityA-TownB", "addressOfCountryA-StateA-CityA-TownB", 100));
    countryAStateACityATownsList.add(new Town("CountryA-StateA-CityA-TownC", "addressOfCountryA-StateA-CityA-TownC", 100));

    List<Town> countryAStateACityBTownsList = new ArrayList();
    countryAStateACityBTownsList.add(new Town("CountryA-StateA-CityB-TownA", "addressOfCountryA-StateA-CityB-TownA", 100));
    countryAStateACityBTownsList.add(new Town("CountryA-StateA-CityB-TownB", "addressOfCountryA-StateA-CityB-TownB", 100));
    countryAStateACityBTownsList.add(new Town("CountryA-StateA-CityB-TownC", "addressOfCountryA-StateA-CityB-TownC", 100));

    List<Town> countryAStateACityCTownsList = new ArrayList();
    countryAStateACityCTownsList.add(new Town("CountryA-StateA-CityC-TownA", "addressOfCountryA-StateA-CityC-TownA", 100));
    countryAStateACityCTownsList.add(new Town("CountryA-StateA-CityC-TownB", "addressOfCountryA-StateA-CityC-TownB", 100));
    countryAStateACityCTownsList.add(new Town("CountryA-StateA-CityC-TownC", "addressOfCountryA-StateA-CityC-TownC", 100));

    City cityA = new City("cityA", countryAStateACityATownsList);
    City cityB = new City("cityB", countryAStateACityBTownsList);
    City cityC = new City("cityC", countryAStateACityCTownsList);

    List<City> countryAStateACitiesList = new ArrayList<>();
    countryAStateACitiesList.add(cityA);
    countryAStateACitiesList.add(cityB);
    countryAStateACitiesList.add(cityC);

        State stateA = new State("stateA",countryAStateACitiesList);

        Country countryA = new Country("countryA", Collections.singletonList(stateA));
   
    }

}

我想要一张各城镇的地图。 （因此城市列表被展平）： 州.名称 -> 城镇.名称 -> 城镇。 我想做一些像destinationMap.get("stateA").get("CountryA-StateA-CityA-TownA")这样的事情，这会给我名称为“CountryA-StateA-CityA-TownA”的城镇实例。

请帮忙，我一直在尝试使用流来完成此任务，但到目前为止还无法实现。

我试过这个：

        countryA.getStates()
               .stream()
               .collect(Collectors.toMap(State::getName,
                        st-> st.getCities()
                               .stream()
                               .collect(Collectors.toMap(City::getName,
                                                         City::getTowns))));

这将创建一个 state.name 到 city.name 到城镇的地图。但这不是我想要的。

我想要一张 state.name 到town.name 到同名城镇的地图。

Answer 1

我喜欢 Java 的功能部分，这就是我所做的，似乎按照你想要检索城镇的方式工作。

Map<String, Map<String, Town>> map =
  countryA.getStates()
    .stream()
    .collect(Collectors.toMap(State::getName,
      st -> st.getCities()
        .stream()
        .map(City::getTowns)
        .flatMap(List::stream)
        .collect(Collectors.toMap(Town::getName, Function.identity()))));
System.out.println(map.get("stateA").get("CountryA-StateA-CityA-TownA"));

提供输出

Town{name='CountryA-StateA-CityA-TownA', address='addressOfCountryA-StateA-CityA-TownA', population=100}

请注意，您的示例中有重复项，我认为不同城市中不应存在重复的城镇。让我知道这是否是您想要的！