How to fix the NullPointerException in my code

Question

So this is the question:

Your program should be able to read from a file the above data into an array and sort the array by the student’s name in an ascending order. Selection sort algorithm is strongly recommended on the array of objects. And also the record office would like to have a sorted class list for each class, freshman, sophomore, junior, and senior.

Print out the following:

1. The sorted master list with the average GPA of the entire college.
2. The freshman list with the average GPA of the freshmen.
3. The sophomore list with the average GPA of the sophomores.
4. The junior list with the average GPA of the juniors.
5. The senior list with the average GPA of the seniors.

So far I'm not able to get my list to sort itself. Every time I try to compile, it gives me the exception:

Exception in thread "main" java.lang.NullPointerException
    at Project1.main(Project1.java:33)

Here is my code:

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;

public class Project1 {
    public static void main(String[] args) throws IOException {
        BufferedReader reader =
            new BufferedReader(new FileReader("/Users/Ayesha/Desktop/inputfile.txt"));
        ArrayList<String> list = new ArrayList<>();
        String data[] = new String[25];
        while (true) {
            String line = reader.readLine();
            for (int i = 0; i < 24; i++) {
                data[i] = line;
            }
            if (line == null) {
                break;
            }
            System.out.println(line);
            list.add(line);
        }

        // String data[]=list.toArray(new String[25]);

        reader.close();

        for (int i = 0; i < data.length - 1; ++i) {
            int minIndex = i;
            for (int j = i + 1; j < data.length; ++j) {
                if (data[j].compareTo(data[minIndex]) < 0) {
                    minIndex = j;
                }
            }
            String temp = data[i];
            data[i] = data[minIndex];
            data[minIndex] = temp;
        }
        System.out.println(data);
    }
}

Here is the source file:

NAME  CLASS   GPA
Williams, Leonard Freshman    1.85
Smith, Sheila Senior  2.99
Anderson, Andy    Sophomore   3.01
Wiser, Bud    Freshman    4.00
Robertson, Jully  Junior  2,78
Koran, Korn   Junior  3.50
Smith, Sam    Junior  2.14
Johnson, Jim  Junior  3.05
Johnson, Jane Junior  3.75
Potter, Pam   Senior  2.98
Brown, Bill   Sophomore   2.55
Crooks, Cathy Freshman    1.99
Gregg, Howard Senior  2.44
Nicholas, Judy    Senior  3.69
White, Bob    Sophomore   1.64
Walsh, Fred   Junior  4.00
Dennis, Susan Senior  2.06
Roberts, Rachel   Sophomore   4.00
Fredericks, Mary  Freshman    2.89
Holmes, Wendy Senior  2.56
Edwards, James    Sophomore   3.00
Green, Barbara    Sophomore   3.67
Brown, David  Freshman    2.00
Williamson, Walt  Sophomore   2.95
Carson, Jim   Sophomore   2.03

Answer 1

    while (true) {
        String line = reader.readLine();
        for(int i=0; i<24; i++){
            data[i]= line;
        }
        if (line == null) {
            break;
        }
        System.out.println(line);
        list.add(line);
    }   

in this code snippet, you're actually reading a line from your file and inserting that same line into all 24 of your 25 slots of String data[]= new String[25];. That might be a bug; you should be asking the for loop to run while it i is less than 25.

More importantly, you meant to read one line and insert that one line into one dataslot, then move on the the next. Because for a file that may not contain sufficient lines, let's say it only has 7 lines. When you're reading line 8, which your filereader translates as null, all of the Strings in data[] will be null.

Answer 2

This is improved code of your solution,

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;

public class Project1 {
    public static void main(String[] args) throws IOException {

        BufferedReader reader = new BufferedReader(new FileReader("/Users/Ayesha/Desktop/inputfile.txt"));
        ArrayList<String> list = new ArrayList<String>();
        while (true) {
            String line = reader.readLine();
            if (line == null) {
                break;
            }
            System.out.println(line);
            list.add(line);
        }

        String data[] = list.toArray(new String[list.size()]);

        reader.close();

        for (int i = 0; i < data.length - 1; ++i) {
            int minIndex = i;
            for (int j = i + 1; j < data.length; ++j) {
                if (data[j].compareTo(data[minIndex]) < 0) {
                    minIndex = j;
                }
            }
            String temp = data[i];
            data[i] = data[minIndex];
            data[minIndex] = temp;
        }

       // This is how you get each gpa for each line.   
       for (int i = 0; i < data.length; i++) {
          String[] words = data[i].split(" ");
          String gpa = words[3];
          System.out.println(gpa);
       }

    }
}