Null pointer with 2d arrays and objects

Question

Hello I am making a project of multiple classes that creates a progress report. However I am testing out methods and am not yet complete with the project and came across a null pointer exception. Take a look at the code and see if you can help me out please. Keep in mind all methods aren't finished just trying to focus on my problem first. I also have a separate driver file that i do not find relevant to post, unless needed otherwise.

Student class:

public class Student {
private String name;
private char grade;
private double average;
private int[] scores = new int[5];

// Constructor
public Student() {
    this.name = name;
    this.grade = grade;
    this.average = average;
    this.scores = scores;
}


// Get the Name.
public String getName() {
    return name;
}

// Set the Name.
public void setName(String name) {
    this.name = name;
}

// Get the Grade.
public char getGrade() {
    return grade;
}

// Set the Grade.
public void setGrade(char grade) {
    this.grade = grade;
}

// Get the Average.
public double getAverage() {
    return average;
}

// Set the Average.
public void setAverage(double average) {
    this.average = average;
}

// Get the Scores.
public int[] getScores() {
    return scores;
}

// Set the Scores.
public void setScores(int[] scores) {
    this.scores = scores;
}

// Determine the average of the five test scores for each student
public void calculateAverage(){

}

public void calculateGrade(){

}
}

ProgressReport class (where im getting the null pointer exception):

public class ProgressReport {
// Create array to hold sections and students.
Student[][] sectionArray = new Student[2][];

// Constructor.
public ProgressReport() {

}

// Get sectionArray.
public Student[][] getSectionArray() {
    return sectionArray;
}

// Set sectionArray.
public void setSectionArray(Student[][] sectionArray) {
    this.sectionArray = sectionArray;
} 

// Read the input file.
public void readInputFile() throws FileNotFoundException{
    String line;
    int studentNo;
    // Open file
    File inFile = new File("file.in");
    // Create scanner for reading.
    Scanner scanner = new Scanner(inFile);
    // While inFile has more lines.
    while(scanner.hasNext()){
        // Read the next line.
        line = scanner.nextLine();
        // Trim line.
        line = line.trim();
        //Parse line into int.
        studentNo = Integer.parseInt(line);
        // For the number of students in section 1 extract data.
        for(int i = 0; i<= studentNo; i++){
            //Create new student.
            sectionArray[0][i] = new Student(); **THIS IS WHERE I GET NULL POINTER EXCEPTION**
            // Read next line.
            line = scanner.nextLine();
            // Create String Tokenizer using a space as the delimiter.
            StringTokenizer strTokenizer = new StringTokenizer(line," ");
            // While the String Tokeizer has more tokens get data.
            while(strTokenizer.hasMoreTokens()){
                // Extract name
                String name = strTokenizer.nextToken();
                // Set name
                sectionArray[0][i].setName(name);

                int[] scores = new int[5];
                // Extract scores.
                int score1 = Integer.parseInt(strTokenizer.nextToken());
                int score2 = Integer.parseInt(strTokenizer.nextToken());
                int score3 = Integer.parseInt(strTokenizer.nextToken());
                int score4 = Integer.parseInt(strTokenizer.nextToken());
                int score5 = Integer.parseInt(strTokenizer.nextToken());
                //Put scores in scores array.
                scores[0] = score1;
                scores[1] = score2;
                scores[2] = score3;
                scores[3] = score4;
                scores[4] = score5;
                // Set scores.
                sectionArray[0][i].setScores(scores);   

            }
        }
    }
}

// Generate a report.
public void generateReport(){
    System.out.println("Progress Report\n");
    System.out.println("Section 1");
    System.out.println(sectionArray[0][0].getName());
}

// Sort by name.
public void sortByName(){

}

// Binary search.
public Student binarySearch(int section, String searchName){
    return null;

}

}

I'm not asking anyone to finish my work, just explain why I am getting a null pointer exception please.

Ravi K Thapliyal · Accepted Answer · 2013-10-13 19:03:45Z

3

You need to initialize the second dimension once you know your number of Students as

    studentNo = Integer.parseInt(line);

    // initialize the Array
    sectionArray[0] = new Student[studentNo];

    // For the number of students in section 1 extract data.
    for(int i = 0; i<= studentNo; i++){

You've always used your sectionArray as sectionArray[0][*]. I'm not sure if you actually need the array to be two-dimensional. Initializing it as new Student[2][]; suggests that you would be using sectionArray[1][*] as well at some point of time.

If you do that later on; you would need to initialize sectionArray[1] as well.

edited Oct 13, 2013 at 19:03

answered Oct 13, 2013 at 18:58

Ravi K Thapliyal

51.9k9 gold badges80 silver badges90 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

edward Over a year ago

Thanks I understand the concept now.

Ravi K Thapliyal Over a year ago

@edward Glad to know this helped.

Pshemo · Accepted Answer · 2013-10-13 19:06:52Z

1

If you do something like this

String[][] array = new String[2][];

it would create one array that will have two null elements, so it is the same as

String[][] array = {null,null};

and since you are invoking sectionArray[0][i] on such array it is the same as invoking null[i] which throws NPE.

answered Oct 13, 2013 at 19:06

Pshemo

125k26 gold badges194 silver badges280 bronze badges

1 Comment

Ravi K Thapliyal Over a year ago

+1 This gives a good picture to OP on what's causing the NPE.

APerson · Accepted Answer · 2013-10-13 19:06:16Z

0

You need to specify both dimensions like: Student[][] sectionArray = new Student[2][2]; or initialize the second dimension like this: sectionArray[0] = new Student[students]; and sectionArray[1] = new Student[students];.

answered Oct 13, 2013 at 19:06

APerson

7227 silver badges10 bronze badges

Comments

Tarang · Accepted Answer · 2013-10-13 19:35:41Z

0

Well You are using array of array here for Student Class.

For each array of (array of array) you need to initiate each array with its required number of elements .

Here : Before this line... where you are getting Null pointer Exception,

sectionArray[0][i] = new Student();

you need to initiate the array sectionArray[0] with new keyword like this.

sectionArray[0]= new Student[studentNo];             // add this line to you code

then the code you have used will come.

answered Oct 13, 2013 at 19:35

Tarang

3181 silver badge10 bronze badges

Collectives™ on Stack Overflow

Null pointer with 2d arrays and objects

4 Answers 4

2 Comments

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related