从列表中分离一个字段并创建一个新列表

Question

我有这个：

 List<City> cities = new ArrayList<>();
    cities.add(new City("SF","USA"));
    cities.add(new City("Agra","India"));
    cities.add(new City("Mumbai","India"));
    cities.add(new City("NY","USA"));

所需输出：

List<"Some thing"> combined = new ArrayList();
combined is like this :
USA
SF
NY
India
Mumbai 
Agra 

我使用了这种方法：

public class ListBreak {
public static void main(String[] args) {

    List<City> cities = new ArrayList<>();
    cities.add(new City("SF","USA"));
    cities.add(new City("Agra","India"));
    cities.add(new City("Mumbai","India"));
    cities.add(new City("NY","USA"));

    List<ABC> abcs = new ArrayList<>();
    HashMap<Country,List<City>> countryListHashMap= new HashMap<>();
    // way one
    for (City city:cities) {
         Country coutry= new Country(city.countryName);
        if(!countryListHashMap.containsKey(coutry))
        {
            countryListHashMap.put(coutry,new ArrayList<>());
        }
        List<City> list = countryListHashMap.get(coutry);
        list.add(city);
        countryListHashMap.put(coutry,list);
    }
    for (Map.Entry<Country,List<City>> entrySet:countryListHashMap.entrySet()) {
          abcs.add(entrySet.getKey());
          abcs.addAll(entrySet.getValue());
    }
    System.out.println(abcs);
}
static class City implements ABC{
    String cityName;
    String countryName;

    public City(String cityName, String countryName) {
        this.cityName = cityName;
        this.countryName = countryName;
    }
}
static class Country implements ABC{
    String countryName;

    public Country(String countryName) {
        this.countryName = countryName;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Country)) return false;
        Country country = (Country) o;
        return countryName.equals(country.countryName);
    }

    @Override
    public int hashCode() {
        return Objects.hash(countryName);
    }
}

}

任何更好、更清洁、更快的方法

Answer 1

package clean.code;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collector;
import java.util.stream.Collectors;

public class ListBreak {

    public static class City {

        private String cityName;
        private String countryName;

        public City(String cityName, String countryName) {
            this.setCityName(cityName);
            this.setCountryName(countryName);
        }

        public String getCityName() {
            return cityName;
        }

        public void setCityName(String cityName) {
            this.cityName = cityName;
        }

        public String getCountryName() {
            return countryName;
        }

        public void setCountryName(String countryName) {
            this.countryName = countryName;
        }

    }

    public static void main(String[] args) {

        List<City> cities = new ArrayList<>();

        cities.add(new City("SF", "USA"));
        cities.add(new City("Agra", "India"));
        cities.add(new City("Mumbai", "India"));
        cities.add(new City("NY", "USA"));

        cities.stream()
                .collect(Collectors.groupingBy(City::getCountryName, collectCityNames()))
                .forEach((countryName, cityNames) -> printCityNamesByCountryName(countryName, cityNames));
    }

    private static Collector<City, ?, List<String>> collectCityNames() {
        return Collectors.mapping(City::getCityName, Collectors.toList());
    }

    private static void printCityNamesByCountryName(String countryName, List<String> cityNames) {
        System.out.println(countryName);
        cityNames.forEach(System.out::println);
    }
}

Answer 2

class City {
    String cityName;
    String countryName;

//Getter,Setter and Constructors
}

public class CandidateCode {
    public static void main(String[] args) {
        List<City> cities = new ArrayList<>();
        //stuffs
        final Map<String, List<String>> collect1 = cities.stream()
                .collect(Collectors.groupingBy(City::getCountryName, Collectors.mapping(City::getCityName, Collectors.toList())));
        for (Map.Entry str : collect1.entrySet()) {
            System.out.println(str.getKey());
            for (String s : collect1.get(str.getKey())) {
                System.out.println(s);
            }

        }

    }
}

输出：

USA
SF
NY
India
Agra
Mumbai

Answer 3

    class City{

        String cityName;
        String countryName;
        public String getCityName() {
            return cityName;
        }
        public void setCityName(String cityName) {
            this.cityName = cityName;
        }
        public String getCountryName() {
            return countryName;
        }
        public void setCountryName(String countryName) {
            this.countryName = countryName;
        }

        public City(String string, String string2) {
            this.cityName = string;
            this.countryName=string2;

        }

        //getter setter ns constructor stuffs

     }
    public class Test  {
        public static void main(String[] args) {
            List<City> cities = new ArrayList<>();
            cities.add(new City("SF","USA"));
            cities.add(new City("Agra","India"));
            cities.add(new City("Mumbai","India"));
            cities.add(new City("NY","USA"));
            List<String> result = new ArrayList<>();

            Map<String, List<City>> res = cities.stream().collect(Collectors.groupingBy(City::getCountryName));
            for(HashMap.Entry<String, List<City>> valuepair:res.entrySet())
            {
                List<City> value = valuepair.getValue();
                City countryName = value.get(0);
                String cname = countryName.getCountryName();
                result.add(cname);
                for (City count : value) {
                    result.add(count.getCityName());

                }

            }
            for (String string : result) {
                System.out.println(string);
            }





    }

}

输出

USA
SF
NY
India
Agra
Mumbai

Answer 4

请试试这个。

Map<String, List<String>> countryListHashMap =
                cities.stream().collect(Collectors.groupingBy(City::getCountryName, Collectors.mapping(City::getCityName, Collectors.toList())));

countryListHashMap.forEach((k,v)-> {System.out.println(k);
            v.forEach(System.out::println);});

---

输出：

USA
SF
NY
India
Agra
Mumbai