计算Java中按字段分组的对象

Question

I have a class Flight and class for Airport which holds collection with several flights.

public class Flight {

String flightNumber;            
String departureCity;
String departureCountry;
String departure;
String arrivalCity;
String arrivalCountry;
String arrival;
//etc...}

public class Airport {

String name;
String city;
String country;
LinkedList <Flight> flights;
//etc...}

I have a task in university to count the number of flights in one airport for each departureCountry field of Flight class. Is there any algorithm to do that? I thought to sort the collection somehow by this parameter(departureCountry) but couldn't come up with some clear decision. I will be grateful for your help.

Answer 1

I'm not going to write the whole code here, but just the steps.

• for each airport
  • keep a map of departureCountry to count (ie a map<String, Integer>)
  • for each flight in the airport
    • increment the count for the departureCountry in the map // that's it, you have the counts now

Sorting is not needed at all given that who just need the count, ie essentially grouping for which a map is sufficient as explained in the steps above.

Answer 2

There is a neat way using Java 8 streams:

import java.util.*;
import static java.util.stream.Collectors.*;

public class Main {
    public static void main(String[] args) {
        // I made constructor for departureCountry only to keep it simple
        List<Flight> flights = Arrays.asList(
          new Flight("USA"),
          new Flight("UK"),
          new Flight("CAN"),
          new Flight("USA"),
          new Flight("UK")
        );

        Map<String, Long> flightsPerCountry =
          flights.stream()
                 .collect(groupingBy(Flight::getDepartureCountry, counting()));


        System.out.println(flightsPerCountry);
        // {CAN=1, USA=2, UK=2}
    }
}