从Java中读取文件和空指针异常(Reading From File in Java and Null Pointer Exception)

 我是java的新手，我遇到了一个我自己无法解决的问题。 我用Google搜索了异常，但问题太具体了，据我所知，所以我发现自己在这里。 这是我的问题。 
 
 我有一个名为Student的类，它有一个不合格的数据成员及其get / set方法： 
 
private String studentNumber;
private String firstName;
private String lastName;
private int age;
private String gender;
private String country;
 
 然后我创建了一个实例数组： 
 
Student studentList[] = new Student(10);
 
 我有一个数据库（文本文件）作为休息 
 
081935 Cengiz rrrrr Male 21 Turkey
082935 Ayşe aaaaa Female 22 England
083935 Onur bbbbb Male 23 Germany
084935 Fatma ccccc Female 24 Cyprus
085935 Ali dddd Male 21 China
086935 Zehra eeee Female 22 Denmark
087935 Murat ffff Male 25 France
088935 Selin ggggg Female 26 Japan
086935 Cengiz hhhh Male 20 Korea
080935 Damla qqqqqq Female 19 Iran
 
 我想要做的是将所有这些信息传递给我的类实例，并尝试实现这一目标： 
 
import java.io.*;
import java.util.Scanner;

public class StudentTracker  {

    private static int counter = 0;
    private static Student studentList[];

    public static void readFromFile() throws FileNotFoundException {
        File file = new File("Database.txt");
        Scanner scanner = new Scanner(new FileReader(file));
        try {
              while (scanner.hasNextLine()){
                processLine(scanner.nextLine());
              }
        } finally {
            scanner.close();
        }
    }


    public static void processLine(String line) {
        Scanner scanner = new Scanner(line);
        scanner.useDelimiter(" ");
        if (scanner.hasNext()) {
          studentList[counter].setStudentNumber(scanner.next());
          studentList[counter].setFirstName(scanner.next());
          studentList[counter].setLastName(scanner.next());
          studentList[counter].setGender(scanner.next());
          studentList[counter].setAge(Integer.parseInt(scanner.next()));
          studentList[counter].setCountry(scanner.next());      
          counter++;
        }
        else {
            System.out.println("Empty or invalid line. Unable to process.");
        }
      }

    public static void main(String[] args) throws FileNotFoundException {

        studentList = new Student[10];
        readFromFile();
        for(int i = 0; i < 10; i++) {
            System.out.printf(studentList[i].getStudentNumber(), " ",
                              studentList[i].getFirstName());

        }

    }

}
 
 但它会导致以下错误： 
 
Exception in thread "main" java.lang.NullPointerException
    at StudentTracker.processLine(StudentTracker.java:28)
    at StudentTracker.readFromFile(StudentTracker.java:16)
    at StudentTracker.main(StudentTracker.java:68)
 
 顺便说一句找不到像C-scanf这样的函数，它获取输入直到第一个空格，所以我找到另一种解析字符串的方法 
 
 readFromFile（）和processLine函数，但我不确定它们是否按预期工作。 
 
 先谢谢你 

I'm pretty new to java and I've got a problem that I couldn't solve by myself. I googled the exception but problem is too specific as far as I can understand so I find myself here. Here is my problem.
 
I have a class called Student which has fallowing data members and their get/set methods:
 
private String studentNumber;
private String firstName;
private String lastName;
private int age;
private String gender;
private String country;
 
and I created an array of instances as fallowing:
 
Student studentList[] = new Student(10);
 
I got a database(a text file) as fallowing
 
081935 Cengiz rrrrr Male 21 Turkey
082935 Ayşe aaaaa Female 22 England
083935 Onur bbbbb Male 23 Germany
084935 Fatma ccccc Female 24 Cyprus
085935 Ali dddd Male 21 China
086935 Zehra eeee Female 22 Denmark
087935 Murat ffff Male 25 France
088935 Selin ggggg Female 26 Japan
086935 Cengiz hhhh Male 20 Korea
080935 Damla qqqqqq Female 19 Iran
 
What I'm trying to do is getting all these informations to my class instances and I try to achieve this is fallowing:
 
import java.io.*;
import java.util.Scanner;

public class StudentTracker  {

    private static int counter = 0;
    private static Student studentList[];

    public static void readFromFile() throws FileNotFoundException {
        File file = new File("Database.txt");
        Scanner scanner = new Scanner(new FileReader(file));
        try {
              while (scanner.hasNextLine()){
                processLine(scanner.nextLine());
              }
        } finally {
            scanner.close();
        }
    }


    public static void processLine(String line) {
        Scanner scanner = new Scanner(line);
        scanner.useDelimiter(" ");
        if (scanner.hasNext()) {
          studentList[counter].setStudentNumber(scanner.next());
          studentList[counter].setFirstName(scanner.next());
          studentList[counter].setLastName(scanner.next());
          studentList[counter].setGender(scanner.next());
          studentList[counter].setAge(Integer.parseInt(scanner.next()));
          studentList[counter].setCountry(scanner.next());      
          counter++;
        }
        else {
            System.out.println("Empty or invalid line. Unable to process.");
        }
      }

    public static void main(String[] args) throws FileNotFoundException {

        studentList = new Student[10];
        readFromFile();
        for(int i = 0; i < 10; i++) {
            System.out.printf(studentList[i].getStudentNumber(), " ",
                              studentList[i].getFirstName());

        }

    }

}
 
but it gives fallowing error:
 
Exception in thread "main" java.lang.NullPointerException
    at StudentTracker.processLine(StudentTracker.java:28)
    at StudentTracker.readFromFile(StudentTracker.java:16)
    at StudentTracker.main(StudentTracker.java:68)
 
by the way couldn't find a function like C-scanf which gets the input until first white space so I find another way to parse the strings from line with
 
readFromFile() and processLine functions but I'm not sure if they're workig as intended.
 
Thank you in advance

原文：https://stackoverflow.com/questions/9791837