I have two classes: Country and City.

Country has the following attributes: Countrycode, Countryname, capital, population, Continent and a list of type City.

City has countrycode, name and population attributes.

I am trying to find the highest populated city of each Country.

I want to use the Stream API.

import java.util.ArrayList;
import java.util.Comparator;
import java.util.NoSuchElementException;

public class Main13 {

    public static void main(String[] args) {
        
        ArrayList<Country> Countries=new ArrayList<Country>();
        
        //Countrycode,Countryname,capital,population,Continent
        
        Country coun1=new Country(1,"Japan","Tokyo",4000000,"Asia");
        Country coun2=new Country(2,"USA","DC",400000000,"America");
        
        
        City c1=new City(1,"Tokyo",100000);
        City c2=new City(1,"Osaka",10000);
        City c3=new City(1,"Nagoya",20000);
        
        City n1=new City(2,"NYC",4000000);
        City n2=new City(2,"LA",1000000);
        
        
        coun1.Cities.add(c1);
        coun1.Cities.add(c2);
        coun1.Cities.add(c3);
        
        coun2.Cities.add(n1);
        coun2.Cities.add(n2);
        
        Countries.add(coun1);
        Countries.add(coun2);
        
        Country Max2=Countries.stream().max(Comparator.comparingInt(Country::getpop)).orElseThrow(NoSuchElementException::new);
        
        System.out.println(Max2.Countryname);
    }

}

////////////////////////////////////////////
import java.util.ArrayList;
import java.util.List;

public class Country {
    int countrycode;
    String Countryname; 
    String Capital; 
    int Population;
    String Continent; 
    ArrayList<City> Cities=new ArrayList<City>();

    public Country(int code,String n,String c,int p,String con) {
        countrycode=code;
        Countryname=n; 
        Capital=c; 
        Population=p;
        Continent=con;
    }

    public int getpop() {
        return Population;
    }

}
////////////////////////

public class City {

    int CountryCode;
    String name;
    int Population;
    
    
    
    public City(int code,String n,int pop) {
        CountryCode=code;
        name=n;
        Population=pop;
    }
}

CodePudding user response：

As you want to do something for each country, you need to loop on the coutries, then apply the logic

for (Country c : countries) {
    City max2 = c.cities.stream().max(Comparator.comparingInt(City::getPopulation))
            .orElseThrow(NoSuchElementException::new);
    System.out.println(c.getName()   " "   max2.getName()   " "   max2.getPopulation());
}

Japan Tokyo 100000
USA NYC 4000000

Note : java variable naming convention is lowerCamelCase

class Country {
    int code;
    String name;
    String capital;
    int population;
    String continent;
    List<City> cities;
}

CodePudding user response：

I am trying to find the highest populated city of each Country.

That's how you can create a list of the most populated cities in each country:

List<City> largestCities = countries.stream()
    .map(country -> country.getCities().stream().max(Comparator.comparingInt(City::getPopulation)))
    .map(Optional::orElseThrow)
    .collect(Collectors.toList());
    
largestCities.forEach(System.out::println);

Output:

City{CountryCode=1, name='Tokyo', population=100000}
City{CountryCode=2, name='NYC', population=4000000}

A link to the Online Demo

Important notes:

• Providing NoSuchElementException an argument of orElseThrow() doesn't make sense because parameterless version of this method throw NoSuchElementException is a case of empty optional.

• Adhere to the Java Naming Conventions. Names of variables and methods should be written in so-called camel-case (mixed case) and always start with a lower case letter: cities, getCities(), countryCode, population, etc.

• Write your code against interfaces not against concrete implementations, use List instead of ArrayList. See What does it mean to "program to an interface"?

• Use access modifiers to encapsulate class-members withing the class. In order to be able to change a state of a particular field you need to introduce a method, avoid accessing the field directly from outside the class (don't access the field directly - it's not a good practice in Java):

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

public void addCity(City city) {
    cities.add(city);
}