java - 如何修复代码中的空指针异常？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 6 年前。

我正在尝试读取一个文件并将文件中的每一行设置为我创建的对象 OS_Process 的参数，然后将这些进程放入链接列表队列中。但是，我不断收到空指针异常。数据文件如下所示。每个新的过程数据都在一个新行上。

3 //counter
1 3 6 //process 1 data
3 2 6 //process 2 data
4 3 7 //process 3 data

这是我的代码

    import java.io.*;
    import java.util.*;

    public class OS_Scheduler
    {
    public static void main(String[] args)
    {
        Queue<OS_Process> jobs = new LinkedList<OS_Process>();

        try
        {
            System.out.print("Enter the file name: ");
            Scanner file = new Scanner(System.in);
            File filename = new File(file.nextLine());

            OS_Process proc = null;
            String s = null;
            int a = 0, p = 0, b = 0;
            BufferedReader input = new BufferedReader(new FileReader(filename));
            StringTokenizer st = new StringTokenizer(s);
            int count = Integer.parseInt(st.nextToken());
            while ((s = input.readLine()) != null)
            {
                st = new StringTokenizer(s);
                a = Integer.parseInt(st.nextToken());
                p = Integer.parseInt(st.nextToken());
                b = Integer.parseInt(st.nextToken());
                proc = new OS_Process(a, p, b, 0);
                jobs.add(proc);
            }
            input.close();
        }
        catch (Exception ex)
        {
            ex.printStackTrace();
        }

    }
}

Answer 1

您遇到了 NullpointerException，因为您设置了 String s = null;，然后调用了 StringTokenizer stz = new StringTokenizer(s); 等于 StringTokenizer stz = new StringTokenizer(null); 并且这会获取 Nullpointer。

您不需要知道 count 行，因为 while 循环会迭代文件中的所有行，并且如果到达文件末尾就会停止因此请按如下方式更新您的代码:

String s = input.readLine();//read first line to get rid of it
if(s == null){
    //File is empty -> abort
    System.out.println("The file is empty");
    System.exit(0);
}
int a = 0, p = 0, b = 0;
StringTokenizer st;
while ((s = input.readLine()) != null)
{...}

或者如果您想使用计数，您可以这样做:

String s = input.readLine();
checkReadLineNotNull(s);
int a = 0, p = 0, b = 0;
StringTokenizer st = new StringTokenizer(s);
int count = Integer.parseInt(st.nextToken());

for (int i = 0; i < count; i++) {
    s = input.readLine();
    checkReadLineNotNull(s);
    st = new StringTokenizer(s);
    //...
}

//Checks if s != null otherwise kills the programm
private static void checkReadLineNotNull(String s) {
    if(s == null){
        //File is empty abort
        System.out.println("The file is empty");
        System.exit(0);
    }
}