NullPointerException

[tyler naughton]

This code should create an array of Student objects where each Student object contains fields consisting of the first name, last name and average score for the 3 exams. It will then iterate through that array and print out the values of those fields. I can't change anything in the "main" method because it's part of an assignment. I'm getting a NullPointerException on line 59. I can't figure out what why I'm getting this error.

Exception in thread "main" java.lang.NullPointerException
at ProgramIII.populateStudentArray(ProgramIII.java:59)
at ProgramIII.main(ProgramIII.java:33)

01 import java.io.File;
02 import java.io.FileNotFoundException;
03 import java.util.Scanner;
04
05 public class ProgramIII {
06 public static void main(String[] args) {
07 Student[] studentArray = null;
08 Scanner sc = new Scanner(System.in);
09 Scanner readFile = null;
10 String fileName = "";
11 boolean done = false;
12 int tries = 1;
13 File file = null;
14 while (!done) {
15 try {
16 System.out.println("Please enter the name of the file: ");
17 fileName = sc.nextLine();
18 file = new File(fileName);
19 readFile = new Scanner(file);
20 done = true;
21 }
22 catch (FileNotFoundException fnfe) {
23 System.out.println("File not found");
24 if (tries == 3) {
25 System.out.println("Maximum tries exceeded.");
26 System.exit(1);
27 }
28 tries++;
29 }
30 }
31 studentArray = createStudentArray(readFile);
32 populateStudentArray(studentArray, readFile);
33
34 readFile.close();
35 printArray(studentArray);
36 }
37
38 //creates and returns an uninitialized array of Student instances
39 //the returned array will be of the right size but each "slot" will
40 //have null values.
41 //needs to read the first line of the input file. Hence a scanner
42 //needs to be passed to it as a parameter
43 public static Student[] createStudentArray(Scanner sc) {
44 String firstLine = sc.next();
45 int number = Integer.parseInt(firstLine);
46 Student[] studentArray = new Student[number];
47 return studentArray;
48 }
49
50 //reads the rest of the file and fills up studentArray parameter with Student instance
51 //where each instance contains the first and last names and the average is caluclated from
52 //the 3 test scores.
53 //Note that the last dataset in the file may or may not have a blank line following it
54 public static void populateStudentArray(Student[] studentArray, Scanner sc) {
55 for(int i = 0; i < studentArray.length; i++){
56 sc.nextLine();
57 double ave;
58 studentArray[i].setFirstName(sc.nextLine());
59 studentArray[i].setLastName(sc.nextLine());
60 ave = Double.valueOf(sc.nextLine().trim()).doubleValue();
61 ave += Double.valueOf(sc.nextLine().trim()).doubleValue();
62 ave += Double.valueOf(sc.nextLine().trim()).doubleValue();
63 studentArray[i].setAverage(ave/3.0);
64 }
65 System.out.print(studentArray);
66 }
67
68 //prints out the contents of studentArray by iterating over this array
69 //and calling System.out.println() on each Student instance in the array
70 public static void printArray(Student[] studentArray) {
71 for(int i = 0; i < studentArray.length; i++){
72 System.out.println(studentArray.toString());
73 }
74 }
75 }

Here is the Student class...

01 public class Student {
02 private String firstName;
03 private String lastName;
04 private double average;
05
06 public Student(String firstName, String lastName, double average) {
07 this.firstName = firstName;
08 this.lastName = lastName;
09 this.average = average;
10 }
11
12
13 public String getFirstName() {
14 return firstName;
15 }
16 public String getLastName() {
17 return lastName;
18 }
19
20 public double getAverage() {
21 return average;
22 }
23
24 public void setFirstName(String firstName) {
25 this.firstName = firstName;
26 }
27
28 public void setLastName(String lastName) {
29 this.lastName = lastName;
30 }
31
32 public void setAverage(double average) {
33 this.average = average;
34 }
35
36 public String toString() {
37 return firstName + "\n" + lastName + "\n" + average;
38 }
39 }

This is the format of the file I'm supposed to read from. It will not change.

2

Joe
Student
80
90
100

Jill
Student
85
95
100

If anybody could help me out that would be great. Thank you!

[Rob Spoor]

Welcome to the Ranch!

Next time, please don't add your own line numbers. They will be added automatically once you add code tags, and by adding them yourself you made it quite hard for us to simply copy-paste-compile-run your code.

However, I think I already found the problem:

 public static Student[] createStudentArray(Scanner sc) {
String firstLine = sc.next();
int number = Integer.parseInt(firstLine);
Student[] studentArray = new Student[number];
return studentArray;
}

studentArray is an array with of size "number" with all null elements. You need to initialize them first, either in createStudentArray or just before using an array element in populateStudentArray. Let's do it in createStudentArray:

Student[] studentArray = new Student[number];
for (int i = 0; i < number; i++) {
studentArray[i] = new Student();
}
return studentArray;

[Campbell Ritchie]

Welcome to the Ranch

Please don't post line numbers; the code tags add them automatically. Also please keep exactly the same formatting; I ran your code and got the NullPointerException at line 58! Insert this line before the line which sufferes the Exception

System.err.printf("StudentArray exists: %b%n", studentArray != null);//test

. . . and see its output.

[Campbell Ritchie]

. . . and as Rob said, you find the array exists, but the individual members are still null.

[tyler naughton]

Sorry about that, I won't do it again. I took your suggestion and tried running my code with
that line implemented and it worked. Thank you for the help!

[Campbell Ritchie]

You're welcome