Question

我创建了一个HashMap，其中每个键都包含一个ArrayList作为值。我无法理解如何获取与键关联的ArrayList并获取存储在其中的所有值。请参阅下面运行程序时出现的错误。

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Scanner;

public class Flights {

private HashMap<String, ArrayList<String>> flights = new HashMap<String, ArrayList<String>>();

private void readFlights(String filename) {
    try {
        BufferedReader bf = new BufferedReader(new FileReader(filename));

        while (true) {
            String line = bf.readLine();
            if (line == null) {
                break;
            }
            if (!line.isEmpty()) {
                String fromCity = line.substring(0, line.indexOf("-"));
                String toCity = line.substring(line.indexOf(">") + 2);
                ArrayList<String> city = flights.get(fromCity);
                if (city != null) {
                    city.add(toCity);
                } else {
                    ArrayList<String> destinations = new ArrayList<String>();
                    destinations.add(toCity);
                    flights.put(fromCity, destinations);
                }
            }

        }
        bf.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

private void printCities() {
    Iterator<String> fi = flights.keySet().iterator();
    while (fi.hasNext()) {
        String next = fi.next();
        System.out.println(next + "-> "+ flights.get(next));
    }
}

@SuppressWarnings("resource")
private void printWelcome() {
    System.out
            .println("Welcome to the flight planner.\nHere is a list of all of our cities:");
    printCities();
    System.out.print("Enter the starting city.");
    Scanner in = new Scanner(System.in);
    String input = in.nextLine();
    System.out.println("from " + input + " you can fly directly to:");
    printAvailableFlights(input);
}

private void printAvailableFlights(String city) {
    ArrayList<String> origin = flights.get(city);
    for (String cities: origin) {
        System.out.println(cities);
    }
}

public static void main(String[] args) {
    Flights f = new Flights();
    f.readFlights("flights.txt");
    f.printWelcome();
}

}

这是flights.txt文件：

San Jose -> San Francisco
San Jose -> Anchorage

New York -> Anchorage
New York -> San Jose
New York -> San Francisco
New York -> Honolulu

Anchorage -> New York
Anchorage -> San Jose

Honolulu -> New York
Honolulu -> San Francisco

Denver -> San Jose

San Francisco -> New York
San Francisco -> Honolulu
San Francisco -> Denver

以下是我在运行程序时在控制台中看到的内容：

Welcome to the flight planner.
Here is a list of all of our cities:
Honolulu -> [New York, San Francisco]
Denver -> [San Jose]
Anchorage -> [New York, San Jose]
San Francisco -> [New York, Honolulu, Denver]
New York -> [Anchorage, San Jose, San Francisco, Honolulu]
San Jose -> [San Francisco, Anchorage]
Enter the starting city.Denver
from Denver you can fly directly to:
Exception in thread "main" java.lang.NullPointerException
at Flights.printAvailableFlights(Flights.java:64)
at Flights.printWelcome(Flights.java:59)
at Flights.main(Flights.java:72)

Answer 1

这一行

String fromCity = line.substring(0, line.indexOf("-"));

在起始城市之后留下一个空格，因为这是您的文本文件的格式。您的密钥看起来像"Denver "和"San Jose "。您应该将其更改为：

String fromCity = line.substring(0, line.indexOf("-") - 1);

此外，使用整个分隔符" -> "更有意义，因为有些城市中有短划线，例如Wilkes-Barre。

抛出

NullPointerException，因为如果密钥不存在，Map将返回null。您应该在printAvailableFlights。

中考虑到这一点

HashMap - 获取与键关联的所有值

1 个答案: