Question

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

Answer 1

This line:

studentList = new Student[10];

creates an array of 10 elements. The value of each element isn’t a Student object; it’s a reference – and that reference will either be null, or a reference to a Student object (or a compatible type). Each element has a null value to start with.

That line doesn’t create any Student objects.

You need to create an instance of Student before you set its properties, e.g.

if (scanner.hasNext()) {
  studentList[counter] = new Student();
  studentList[counter].setStudentNumber(scanner.next());
  ...

Or:

if (scanner.hasNext()) {
  Student student = new Student();
  student.setStudentNumber(scanner.next());
  ... fill in the other properties ...
  studentList[counter] = student;
  ...

As a side note, this line:

private static Student studentList[];

… is not an idiomatic declaration. It’s valid, but most Java programmers would prefer to see:

private static Student[] studentList;

That way all the type information is kept in one place. (I’d also suggest using a List<Student> instead, and passing it into the method rather than using a static variable, but let’s not get ahead of ourselves…)