getting NullPointerException

[Rupesh Pradhan]

Hi!
I sincerely hope that this post fits in the beginners category because I *am* a beginner, whether or not the code looks like beginners stuff or not!

I am not sure if what I am saying is right, so correct me. "Can I use an array element as a handle for an object?"

I thinks the above sums up what I am trying to achieve. The code given below may be pretty long but it is well commented. Please help me out.

Thanks
Rupesh

/**
 * To generate examination result
 * The Result class has variables to store information pertaining to a student
 * 
 * The inputData() method gets data for one student
 * 
 * I have tried to create an array for students, so that each element of the array student represents
 * one students. In this way I can get the data for all students by putting it in a loop.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
 
// Declare packages to be imported
import java.io.*;
 
public class Result
{
    // instance variables 
    int rollNumber;  // to store roll number 
    String name;    // to store name
    int marks[];    // to store marks, index of array will represent a particular subject
                    // 1 - English, 2 - Mathematics, 3 - Science, 4 - Arts
    int totalMarks; // to store total marks
    float aggPercentage; // to store percentage calculated
    
    // to get users input
    InputStreamReader isr = new InputStreamReader(System.in);
    BufferedReader br = new BufferedReader(isr);
 
    int i;
    
    /**
     * Constructor for objects of class Result
     */
    public Result()
    {
        // initialise instance variables
        rollNumber = 0;
        name = "";
        marks = new int[5];
        totalMarks = 0;
        aggPercentage = 0.0F;
        
    }
 
    /**
     * Method to input data
     * 
     * @param  nothing
     * @return     nothing
     */
    public void inputData()
    {
    try
    {
    
        // for reading users input  
        System.out.println("Enter the information requested:");
        System.out.println("--------------------------------");
        
        // get roll number
        System.out.print("Roll Number : ");
        rollNumber = Integer.parseInt(br.readLine()); 
     
        // get name
        System.out.print("Name        : ");
        name = br.readLine();
 
        // get markss
        for (i = 1; i < 5; i++)
        {
            System.out.print("Subject  "+(i)+" : ");
            marks[i] = Integer.parseInt(br.readLine());        
        } // end of for loop
        
    }  // end of try
    
    catch (IOException ioe)
    {
        System.out.println("Error is : "+ioe.toString());
    } // end of catch
    
    } // end of inputData() method
 

    // main method    
    public static void main (String args[])
    {
        // create object of class Result to store info for 10 students
        Result Class8C[] = new Result[10];
        
        // get data for first student
        Class8C[1].inputData(); // I GET NullPointerException HERE
        
        // when I run the line below, it get output as 'NULL'. Could this be the reason for the error.
        // To get this, the second line in the main() has to be uncommented also.
        //System.out.println (Class8C[1]);
        
        // the code works if I do the following
        //Result Class8C = new Result();
        //Class8C.inputData();
        // but it does not server the purpose as I get data for only one student
 
    } // end main
    
} // end of class

[Mark Patrick]

Does declaration of an object array automatically initialize it's elements?

Result Class8C[] = new Result[10];  

This line does not mean that Class8C is an array filled with new Result objects. It means that Class8C is a new array object that can hold Result objects, however, each element is null. You have to assign each element an actual Result object.

[Rupesh Pradhan]

Can I request you to please help me out by providing a few lines of code on how to "assign each element an actual Result object." in this program.

Rupesh

[Ernest Friedman-Hill]

Here you (perhaps incorrectly -- more in a minute) assign a number to each element of an array:

        for (i = 1; i < 5; i++)
        {
            System.out.print("Subject  "+(i)+" : ");
            marks[i] = Integer.parseInt(br.readLine());        
        } // end of for lo

You just have to do the same thing, but using "new" to create a Result object for each element; you've already shown in a comment how to create a single Result, you just have to do it the same way and assign them to the array elements. I don't want to simply write the code, as puzzling it out yourself has some benefits, and you're already quite close.

As for the "perhaps" incorrect part: note that the first element of a Java array is element 0, but this loop only assigns values to elements 1, 2, 3, 4 (because 4 is less than 5.) You actually want the lower limit of your loop to be at 0, not 1, so that all 5 values get assigned.

[Joyce Lee]

Originally posted by Rupesh Pradhan:
Can I request you to please help me out by providing a few lines of code on how to "assign each element an actual Result object." in this program.

Rupesh

  
Result[] Class8C = new Result[10];
for (int i = 0; i < Class8C.length; i++) {
    Class8C[i] = new Result();
}

The index of the first element in an array is 0. Check out the Sun Tutorial: Arrays for more info.

[Rupesh Pradhan]

That does the trick.

Thanks a lot.