java - 在 Java 中操作二维数组

Question

因此，对于家庭作业，我必须编写一个程序，打印出默认情况下用 - 表示的航空公司座位表(即，以表明它们是“开放的”并且有资格预订)。随后，应该提示用户选择他们想要购买的座位，并将数组中该特定位置的值更改为 X。

这就是我的代码现在的样子 - 它可以很好地打印出表格，但是当我尝试更改位置时它会给我一个错误:

import java.util.*;

public class AirlineSeeting {

    public static void main(String[]args) {
        int row = 0;
        int colum = 0;
        System.out.println("Enter n:");
        Scanner scanner = new Scanner(System.in);

        int n = scanner.nextInt();
        String[][] SeetingChart = new String[n][4];

        for (int i = 1; i <= 4; i++) {
            System.out.printf("\t%d\t", i);
        }

        System.out.println("");

        for (int j = 1; j <= n; j++) {
            System.out.printf("%d", j);
            for (int k = 1; k <= 4; k++) {
                System.out.print("\t-\t");
            }
            System.out.println("");
        } 
        for( int i = 0 ; i < SeetingChart.length; i++){
            System.out.println("What row would you like to sit in");
            row = scanner.nextInt();
            System.out.println("What colum would you like to sit in");
            colum = scanner.nextInt();
            if (SeetingChart[row][colum].equals("X")){
                System.out.println("Please pick a seat that is avalable");
            }
            else if (SeetingChart[row][colum].equals("-")){
                SeetingChart[row][colum] = "X";
            }
            for (int j = 1; j <= 4; j++) {
                System.out.printf("\t%d", j);
            }

            System.out.println("");

            for (int j = 1; j <= n; j++) {
                System.out.printf("%d", j);
                for (int k = 1; k <= 4; k++) {
                    System.out.print("\t-");
                }
                System.out.println("");
            } 
        }
    } 
}

这是执行上述代码时得到的输出:

Enter n:
9
    1       2       3       4   
1   -       -       -       -   
2   -       -       -       -   
3   -       -       -       -   
4   -       -       -       -   
5   -       -       -       -   
6   -       -       -       -   
7   -       -       -       -   
8   -       -       -       -   
9   -       -       -       -   
What row would you like to sit in
2
What colum would you like to sit in
2
Exception in thread "main" java.lang.NullPointerException
    at AirlineSeeting.main(AirlineSeeting.java:30)

如有任何帮助，我们将不胜感激!谢谢!

Answer 1

SeetingChart 是一个对象数组，即 String。因此，所有条目都使用 null 进行初始化。您的程序崩溃于

            if (SeetingChart[row][colum].equals("X")){

因为您尝试在 null 上调用方法(即 equals)。这就是为什么你应该改变声明的原因:

            if ("X".equals(SeetingChart[row][colum])){

这可以防止 NullPointerException。