Beginning Java (Working with Arrays; Class Assignment)

I am to the point where I feel as if I correctly wrote the code for this homework assignment. We were given a skeleton and 2 classes that we had to import (FileIOHelper and Student).

 /*
 * Created: *** put the date here ***
 * 
 * Author: *** put your name here ***
 * 
 * The program will read information about students and their
 * scores from a file, and output the name of each student with
 * all his/her scores and the total score, plus the average score
 * of the class, and the name and total score of the students with
 * the highest and lowest total score.
 */

//
import java.util.Scanner;
import java.io.*;

// C:\Users\Adam\info.txt

    public class Lab6
{
    public static void main(String[] args) throws IOException
    {

    // Fill in the body acc开发者_Go百科ording to the following comments
 Scanner key

boardFile = new Scanner(System.in);


     // Input file name
     String filename = getFileName(keyboardFile);

     //Open the file

     // Input number of students
      int numStudents = FileIOHelper.getNumberOfStudents(filename);
      Student students[] = new Student[numStudents];

     // Input all student records and create Student array and
     // integer array for total scores

      int totalScore[] = new int[students.length];
      for (int i = 0; i < students.length; i++){
       for(int j = 1; j < 4; j++){
                 totalScore[i] = totalScore[i] + students[i].getScore(j);
         }

      }



     // Compute total scores and find students with lowest and
     // highest total score

      int maxScore = 0;
      int minScore = 0;
      for(int i = 0; i < students.length; i++){
             if(totalScore[i] >= totalScore[maxScore]){
                     maxScore = i;
             }
             else if(totalScore[i] <= totalScore[minScore]){
                     minScore = i;
             }
     }

     // Compute average total score

      int allScores = 0;
      int average = 0;
      for (int i = 0; i < totalScore.length; i++){
       allScores = allScores + totalScore[i];
      }

      average = allScores / totalScore.length;


     // Output results
      outputResults(students, totalScore, maxScore, minScore, average);
    }

    // Given a Scanner in, this method prompts the user to enter
    // a file name, inputs it, and returns it.
    private static String getFileName(Scanner in)
    {
        // Fill in the body
     System.out.print("Enter the name of a file: ");
     String filename = in.next();
     return filename;

        // Do not declare the Scanner variable in this method.
        // You must use the value this method receives in the
        // argument (in).
   }


    // Given the number of students records n to input, this
    // method creates an array of Student of the appropriate size,
    // reads n student records using the FileIOHelper, and stores
    // them in the array, and finally returns the Student array.
    private static Student[] getStudents(int n)
    {
     Student[] myStudents = new Student[n];
     for(int i = 0; i <= n; i++){
      myStudents[i] = FileIOHelper.getNextStudent();
     }
  return myStudents;
    }

    // Given an array of Student records, an array with the total scores,
    // the indices in the arrays of the students with the highest and
    // lowest total scores, and the average total score for the class,
    // this method outputs a table of all the students appropriately
    // formatted, plus the total number of students, the average score
    // of the class, and the name and total score of the students with
    // the highest and lowest total score.
    private static void outputResults(
            Student[] students, int[] totalScores,
            int maxIndex, int minIndex, int average
        )
    {
        // Fill in the body
     System.out.println("\nName \t\tScore1 \tScore2 \tScore3 \tTotal");
        System.out.println("--------------------------------------------------------");
        for(int i = 0; i < students.length; i++){
                outputStudent(students[i], totalScores[i], average);
                System.out.println();
        }
        System.out.println("--------------------------------------------------------");
        outputNumberOfStudents(students.length);
        outputAverage(average);
        outputMaxStudent(students[maxIndex], totalScores[maxIndex]);
        outputMinStudent(students[minIndex], totalScores[minIndex]);
        System.out.println("--------------------------------------------------------");

    }

    // Given a Student record, the total score for the student,
    // and the average total score for all the students, this method
    // outputs one line in the result table appropriately formatted.
    private static void outputStudent(Student s, int total, int avg)
    {
            System.out.print(s.getName() + "\t");
            for(int i = 1; i < 4; i++){
                    System.out.print(s.getScore(i) + "\t");
            }
            System.out.print(total + "\t");
            if(total < avg){
                    System.out.print("-");
            }else if(total > avg){
                    System.out.print("+");
            }else{
                    System.out.print("=");
            }
    }

    // Given the number of students, this method outputs a message
    // stating what the total number of students in the class is.
    private static void outputNumberOfStudents(int n)
    {
            System.out.println("The total number of students in this class is: \t" + n);
    }

    // Given the average total score of all students, this method
    // outputs a message stating what the average total score of
    // the class is.
    private static void outputAverage(int average)
    {
            System.out.println("The average total score of the class is: \t" + average);
    }

    // Given the Student with highest total score and the student's
    // total score, this method outputs a message stating the name
    // of the student and the highest score.
    private static void outputMaxStudent(
                    Student student,
                    int score
    )
    {
            System.out.println(student.getName() + " got the maximum total score of: \t" + score);
    }

    // Given the Student with lowest total score and the student's
    // total score, this method outputs a message stating the name
    // of the student and the lowest score.
    private static void outputMinStudent(
                    Student student,
                    int score
    )
    {
            System.out.println(student.getName() + " got the minimum total score of: \t" + score);
    }
}

But now I get an error at the line totalScore[i] = totalScore[i] + students[i].getScore(j);

Exception in thread "main" java.lang.NullPointerException

at Lab6.main(Lab6.java:42)

---

Your Array of students isn't initialized.

student[0] up to student[student.length] are null

According to your code you should use

Student student[] = getStudents(numStudents); 

instead of

Student student[] = new Student[numStudents]; 

---

First of, Nettogrof probably has the right answer. Just trying to teach you to fish rather than giving you the fish...

A nullpointer exception means you are dereferencing from something that is null (mostly it has never been initialized). Dereferencing in java is done by the '.' operator.

So in this case, if there is a nullpointer exception in

totalScore[i] = totalScore[i] + students[i].getScore(j);

then you should immediately start to investigate students[i], as this is the only object from which is dereferenced in this line. Maybe students[i] is null, or the whole array is null due to some error in initialization probably.