Java 3级嵌套列表到3级嵌套映射

Question

我有一个列表列表。如何转换为地图中的地图？

Public class Country{
private String name;
private List<State> states;
}
Public class State{
private String name;
private List<City> cities
}
Public class City{
private String name;
private List<Town> towns
}
public class Town{
private String name;
private Integer population;
}

我想要一张以下格式的城镇地图：

1. 使用整个对象作为关键点：第一级：状态->城市。二级：城市->镇

2. 使用名称作为关键字：第一级：州名称->城市名称。二级：城市名称->城镇

请帮忙，我一直试图通过使用流来实现这一点，但到目前为止还无法实现。

Answer 1

您可以执行以下操作：

Map<State, Map<City, List<Town>>> stateToCitiesToTowns =
country.getStates().stream()
.collect(Collectors.toMap(state -> state, state -> state.getCities().stream()
.collect(Collectors.toMap(city -> city, City::getTowns))));
System.out.println(stateToCitiesToTowns);
Map<String, Map<String, List<String>>> stateToCitiesToTownNames =
country.getStates().stream()
.collect(Collectors.toMap(State::getName, state -> state.getCities().stream()
.collect(Collectors.toMap(City::getName, city -> city.getTowns().stream()
.map(Town::getName)
.collect(Collectors.toList())))));
System.out.println(stateToCitiesToTownNames);

输入：

Country country = new Country("CountryA", Arrays.asList(
new State("StateA", Arrays.asList(
new City("cityA", Arrays.asList(
new Town("townA", 4),
new Town("townB", 5),
new Town("townC", 42),
new Town("townD", 67)
)),
new City("cityB", Arrays.asList(
new Town("townE", 65),
new Town("townF", 14),
new Town("townG", 24),
new Town("townH", 33)
)))),
new State("StateB", Arrays.asList(
new City("cityC", Arrays.asList(
new Town("townI", 9),
new Town("townJ", 5),
new Town("townK", 12),
new Town("townL", 4)
)),
new City("cityD", Arrays.asList(
new Town("townM", 5),
new Town("townN", 7),
new Town("townO", 24),
new Town("townP", 6)
))))));

输出(键和值都有相同的列表，这已经很长时间了(：

{State(name=StateB, cities=[City(name=cityC, towns=[Town(name=townI, population=9), Town(name=townJ, population=5), Town(name=townK, population=12), Town(name=townL, population=4)]), City(name=cityD, towns=[Town(name=townM, population=5), Town(name=townN, population=7), Town(name=townO, population=24), Town(name=townP, population=6)])])={City(name=cityC, towns=[Town(name=townI, population=9), Town(name=townJ, population=5), Town(name=townK, population=12), Town(name=townL, population=4)])=[Town(name=townI, population=9), Town(name=townJ, population=5), Town(name=townK, population=12), Town(name=townL, population=4)], City(name=cityD, towns=[Town(name=townM, population=5), Town(name=townN, population=7), Town(name=townO, population=24), Town(name=townP, population=6)])=[Town(name=townM, population=5), Town(name=townN, population=7), Town(name=townO, population=24), Town(name=townP, population=6)]}, State(name=StateA, cities=[City(name=cityA, towns=[Town(name=townA, population=4), Town(name=townB, population=5), Town(name=townC, population=42), Town(name=townD, population=67)]), City(name=cityB, towns=[Town(name=townE, population=65), Town(name=townF, population=14), Town(name=townG, population=24), Town(name=townH, population=33)])])={City(name=cityB, towns=[Town(name=townE, population=65), Town(name=townF, population=14), Town(name=townG, population=24), Town(name=townH, population=33)])=[Town(name=townE, population=65), Town(name=townF, population=14), Town(name=townG, population=24), Town(name=townH, population=33)], City(name=cityA, towns=[Town(name=townA, population=4), Town(name=townB, population=5), Town(name=townC, population=42), Town(name=townD, population=67)])=[Town(name=townA, population=4), Town(name=townB, population=5), Town(name=townC, population=42), Town(name=townD, population=67)]}}

输出：

{
StateA=
{
cityA=[townA, townB, townC, townD],
cityB=[townE, townF, townG, townH]
}, 
StateB={
cityC=[townI, townJ, townK, townL], 
cityD=[townM, townN, townO, townP]
}
}

Answer 2

不要忘记将getter、setter和构造函数添加到类中

Map<String, Map<String, Map<String, Map<String, Integer>>>> map = countries.stream()
.collect(Collectors.toMap(Country::getName, country -> country.getStates().stream()
.collect(Collectors.toMap(State::getName, state -> state.getCities().stream()
.collect(Collectors.toMap(City::getName, city -> city.getTowns().stream()
.collect(Collectors.toMap(Town::getName, Town::getPopulation))))))));

所以，我使用了@BuildSlayer的答案的输入值

class Solution {
public static void main(String[] args) {
List<Country> countries = Collections.singletonList(new Country("CountryA", Arrays.asList(
new State("StateA", Arrays.asList(
new City("cityA", Arrays.asList(
new Town("townA", 4),
new Town("townB", 5),
new Town("townC", 42),
new Town("townD", 67)
)),
new City("cityB", Arrays.asList(
new Town("townE", 65),
new Town("townF", 14),
new Town("townG", 24),
new Town("townH", 33)
)))),
new State("StateB", Arrays.asList(
new City("cityC", Arrays.asList(
new Town("townI", 9),
new Town("townJ", 5),
new Town("townK", 12),
new Town("townL", 4)
)),
new City("cityD", Arrays.asList(
new Town("townM", 5),
new Town("townN", 7),
new Town("townO", 24),
new Town("townP", 6)
)))))));
Map<String, Map<String, Map<String, Map<String, Integer>>>> map = countries.stream().collect(Collectors.toMap(Country::getName, country -> country.getStates().stream().collect(Collectors.toMap(State::getName, state -> state.getCities().stream().collect(Collectors.toMap(City::getName, city -> city.getTowns().stream().collect(Collectors.toMap(Town::getName, Town::getPopulation))))))));
System.out.println(map);
}
}
class Country {
String name;
List<State> states;
public Country(String name, List<State> states) {
this.name = name;
this.states = states;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<State> getStates() {
return states;
}
public void setStates(List<State> states) {
this.states = states;
}
}
class State {
String name;
List<City> cities;
public State(String name, List<City> cities) {
this.name = name;
this.cities = cities;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<City> getCities() {
return cities;
}
public void setCities(List<City> cities) {
this.cities = cities;
}
}
class City {
String name;
List<Town> towns;
public City(String name, List<Town> towns) {
this.name = name;
this.towns = towns;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Town> getTowns() {
return towns;
}
public void setTowns(List<Town> towns) {
this.towns = towns;
}
}
class Town {
private String name;
private Integer population;
public Town(String name, Integer population) {
this.name = name;
this.population = population;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getPopulation() {
return population;
}
public void setPopulation(Integer population) {
this.population = population;
}
}

输出为：

{CountryA={StateA={cityA={townD=67, townB=5, townC=42, townA=4}, cityB={townH=33, townF=14, townG=24, townE=65}}, StateB={cityC={townI=9, townL=4, townJ=5, townK=12}, cityD={townP=6, townN=7, townO=24, townM=5}}}}