Question

Can anyone see why my toString() method isn’t working? It compiles just fine but when I run it there is a runtime error:

Here is my code:

import java.util.*;

public class ExamMarks {

    // fields
    private String moduleName;
    private int moduleCode;
    private int year;
    private int[] numbers;

    // methods
    public static int highestMark(int[] numbers) {
        int highest = numbers[numbers.length - 1];
        int i = numbers[0];
        for (i = 0; i < numbers.length - 1; i++) {
            if (numbers[i] > numbers[i + 1]) {
                highest = numbers[i];
            }
        }
        return highest;
    }

    public static double averageMark(int[] numbers) {
        int sum = 0;
        double average = 0.000;
        for (int i = 0; i < numbers.length; i++) {
            sum = numbers[i] + sum;
            average = sum / numbers.length;
        }
        return average;
    }

    // Constructor
    public ExamMarks() {
        moduleName = "Object Oriented Programming";
        moduleCode = 5015;
        year = 2013;
    }

    // toString
    @Override
    public String toString() {
        String result = ("The title of this module is : " + moduleName
                + "The code number of this module is: " + moduleCode
                + "The year of the Exam is: " + year + "Array average = "
                + averageMark(numbers) + "Highest grade = " + highestMark(numbers));
        return result;
    }

    // Tester
    public static void main(String[] args) {
        ExamMarks grade = new ExamMarks();
        Scanner percentages = new Scanner(System.in);
        int[] numbers = new int[20];
        double x = averageMark(numbers);

        System.out.println("Please enter 20 marks");

        for (int i = 0; i < numbers.length; i++) {
            numbers[i] = percentages.nextInt();

        }

        System.out.println(grade);
    }
}

And this is error I get

java.lang.NullPointerException at ExamMarks.averageMark(ExamMarks.java:26) 
    at ExamMarks.toString(ExamMarks.java:42) 
    at java.lang.String.valueOf(Unknown Source) 
    at java.io.PrintStream.println(Unknown Source) 
    at ExamMarks.main(ExamMarks.java:65) 
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
    at java.lang.reflect.Method.invoke(Unknown Source) 
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:27‌​2)

Answer 1

NullPointerException is here.
private int [] numbers;

numbers is null in your instance.

When you call grade.toString(), the method calls

 public static double averageMark(int [] numbers){
    int sum = 0;  
    double average = 0.000;
    for (int i = 0; i < numbers.length; i++){   // !!!! numbers is null