java - 计算 Java 中按字段分组的对象

Question

我有一个 Flight 类和一个 Airport 类，它包含多个航类。

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...}

我在大学里有一项任务，要为 Flight 类的每个 departureCountry 字段计算一个机场的航类数量。有没有算法可以做到这一点？我想通过这个参数(departureCountry)以某种方式对集合进行排序，但无法做出明确的决定。我将感谢你的帮助。

Answer 1

有一种使用 Java 8 流的巧妙方法:

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}
    }
}