為什么我在數組中得到了空指針異常？

Question

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Scanner;

public class Test {
private static ArrayList<Countries> arr =null;
static String country;
static String capital;
static String cities;
static String answer;

public static void main(String args[]) throws IOException{
    Scanner keybord = new Scanner(System.in);
    ArrayList<Countries> d = read("data.txt");
    String res = "";
    for(int i = 0; i < d.size(); i++){
             res += d.get(i).toString();
        answer = keybord.nextLine();
        if(answer.equalsIgnoreCase(d.get(i).getCapital()))
            res+= "The answer is true!";
        else
            res += "The answer is not true" + "The answer is " + d.get(i).getCapital();
        System.out.println(res);
    }
}

public static ArrayList<Countries> read(String filename) throws IOException{
    Scanner scan = new Scanner(new File(filename));
    while(scan.hasNext()){
        country = scan.nextLine(); //System.out.println(country);
        String cities1 = scan.nextLine(); //System.out.println(cities1);
        String cities2 = scan.nextLine(); //System.out.println(cities2);
        String cities3 = scan.nextLine(); //System.out.println(cities3);
        String cities4 = scan.nextLine(); //System.out.println(cities4);
        String capital = scan.nextLine(); //System.out.println(capital);
        Countries c = new Countries(cities1, cities2, cities3, cities4, capital); 
        arr.add(c); // where i get the exception
        scan.nextLine();
    }
    return arr;

}

我無法理解為什么在嘗試將國家/地區添加到數組列表時出現空指針異常？ 創建它之前，我不應該將其實例化為null嗎？

Answer 1

您尚未初始化arr因此默認情況下為null

通過初始化

private static ArrayList<Countries> arr = new ArrayList<Countries>();

還有 ，

您檢查下一個元素是否存在

 while(scan.hasNext()){

然后您正在閱讀下7行

country = scan.nextLine(); //System.out.println(country);
String cities1 = scan.nextLine(); //System.out.println(cities1);
String cities2 = scan.nextLine(); //System.out.println(cities2);
String cities3 = scan.nextLine(); //System.out.println(cities3);
String cities4 = scan.nextLine(); //System.out.println(cities4);
String capital = scan.nextLine(); //System.out.println(capital);

scan.nextLine();

哪個可能失敗

Answer 2

您需要初始化arr：

private static ArrayList<Countries> arr = new ArrayList<Countries>();

如果省略new ArrayList<Countries>(); 部分，默認值將為null，然后在調用arr.add()時得到NullPointerException 。

Answer 3

arr變量未初始化，替換

private static ArrayList<Countries> arr

與

private static ArrayList<Countries> arr = new ArrayList<Countries>();

Answer 4

您從未初始化過arr 。

添加arr = new ArrayList<Countries>(); 到你的main 。