Programming Forums > Application Development > Java

Jarod Silverstar · Dec 1st, 2006, 9:50 PM

Hello all.
New programming novice here that has run into a weird issue.

Trying to create a program to input student data from a text file into an array, but getting an error when I run it. I am probably doing something wrong in a very simple way, but just not getting it.

I can run the sample program one of two ways, as input into a single class, or into an array. It works and returns the data correctly as a single class, but when I try to do the array part, it compiles okay, but throws an error on running: NullPointerException

(Toggle Plain Text)

import java.io.*;
import javax.swing.*;

public class GradeReader
{
	final int NUM_PERSON = 4;
	Grades[] data = new Grades[NUM_PERSON];
	// Grades data = new Grades();
	
	String firstName, lastName, classNum, grade;
	
	public static void main(String[] args) throws IOException
	{
		new GradeReader();
	}		
	
	public GradeReader() throws IOException  
	{
		getFile(); 		
		JOptionPane.showMessageDialog(null, "Reading data from File. Please hit 'OK' to continue.");	
		showResults();	
	}	
	
	private void getFile() throws IOException
	{
		String filename = "Grades.txt";
		FileReader freader = new FileReader(filename);
		BufferedReader inputFile = new BufferedReader(freader);

		for (int index = 0; index<3; index++)
			{
				firstName = inputFile.readLine();
				lastName = inputFile.readLine();
				classNum = inputFile.readLine();
				grade = inputFile.readLine();
				
				data[index].setGrades(firstName, lastName, classNum, grade);
				// data.setGrades(firstName, lastName, classNum, grade);
				
			}
		inputFile.close();
	}
	
	private void showResults()
		{
			JTextArea outArea;
			JScrollPane scroller;
			outArea = new JTextArea(30,35);
			outArea.setText(data.toString());
			scroller = new JScrollPane(outArea);
			JOptionPane.showMessageDialog(null,scroller);
			System.exit(0);
		}	

}

Let me know if you want to see any of the other code, or the txt list that the input is coming from.

titaniumdecoy · Dec 1st, 2006, 10:58 PM

(Toggle Plain Text)

Grades[] data = new Grades[NUM_PERSON];

This code creates an array capable of storing NUM_PERSON Grades objects. By default, each index is initialized to null.

(Toggle Plain Text)

data[index].setGrades(firstName, lastName, classNum, grade);

You are attempting to call the method setGrades on null, hence the NullPointerException.

One solution is to initialize each index of data to a new instance of Grades in the GradeReader class' constructor, which will be called when a new instance of GradeReader is created:

(Toggle Plain Text)

public class GradeReader {

      public GradeReader() {
            for (int i = 0; i < NUM_PERSON; i++)
                  data[i] = new Grades();
      }

...

Jarod Silverstar · Dec 2nd, 2006, 4:23 AM

Okay, that got me past the NullPointer error. But I am not getting my output now.

Here is the output from my Grades class:

(Toggle Plain Text)

	
public String toString()
		{
			String studentStr = "Student name: " + getFirstName() 
			+ " " + getLastName()
			+ "\nClass: " + getClassNum() 
			+ "\nGrade: " + getGrade();
			
			return studentStr;
		}

And here is where I am trying to call it:

(Toggle Plain Text)

private void showResults()
		{
			JTextArea outArea;
			JScrollPane scroller;
			outArea = new JTextArea(30,35);
			outArea.setText(data.toString());
			scroller = new JScrollPane(outArea);
			JOptionPane.showMessageDialog(null,scroller);
			System.exit(0);
		}

Again, I got the output doing a single object, but now that I do have mutliple objects in the array, I get junk returned.

I assume I need to do something with this line:
outArea.setText(data.toString());
But so far I have not been able to come up with the right option.

Thank you.

Jarod Silverstar · Dec 2nd, 2006, 5:01 AM

Well, I partly figured it out.

When I do this:

(Toggle Plain Text)

outArea.setText(data[0].toString() + data[1].toString());

I get my output. But it seems to be that there must be a much better way of doing this.

titaniumdecoy · Dec 2nd, 2006, 3:19 PM

If you want to print every item in the list, use a loop. Consider cocatenating each item into a temporary String variable, or using JTextArea's append method.

Dec 1st, 2006, 9:50 PM	#1
Jarod Silverstar Newbie Join Date: Dec 2006 Posts: 7 Rep Power: 0	Trouble inputing data to an Array Hello all. New programming novice here that has run into a weird issue. Trying to create a program to input student data from a text file into an array, but getting an error when I run it. I am probably doing something wrong in a very simple way, but just not getting it. I can run the sample program one of two ways, as input into a single class, or into an array. It works and returns the data correctly as a single class, but when I try to do the array part, it compiles okay, but throws an error on running: NullPointerException (Toggle Plain Text) import java.io.; import javax.swing.; public class GradeReader { final int NUM_PERSON = 4; Grades[] data = new Grades[NUM_PERSON]; // Grades data = new Grades(); String firstName, lastName, classNum, grade; public static void main(String[] args) throws IOException { new GradeReader(); } public GradeReader() throws IOException { getFile(); JOptionPane.showMessageDialog(null, "Reading data from File. Please hit 'OK' to continue."); showResults(); } private void getFile() throws IOException { String filename = "Grades.txt"; FileReader freader = new FileReader(filename); BufferedReader inputFile = new BufferedReader(freader); for (int index = 0; index<3; index++) { firstName = inputFile.readLine(); lastName = inputFile.readLine(); classNum = inputFile.readLine(); grade = inputFile.readLine(); data[index].setGrades(firstName, lastName, classNum, grade); // data.setGrades(firstName, lastName, classNum, grade); } inputFile.close(); } private void showResults() { JTextArea outArea; JScrollPane scroller; outArea = new JTextArea(30,35); outArea.setText(data.toString()); scroller = new JScrollPane(outArea); JOptionPane.showMessageDialog(null,scroller); System.exit(0); } } import java.io.; import javax.swing.; public class GradeReader { final int NUM_PERSON = 4; Grades[] data = new Grades[NUM_PERSON]; // Grades data = new Grades(); String firstName, lastName, classNum, grade; public static void main(String[] args) throws IOException { new GradeReader(); } public GradeReader() throws IOException { getFile(); JOptionPane.showMessageDialog(null, "Reading data from File. Please hit 'OK' to continue."); showResults(); } private void getFile() throws IOException { String filename = "Grades.txt"; FileReader freader = new FileReader(filename); BufferedReader inputFile = new BufferedReader(freader); for (int index = 0; index<3; index++) { firstName = inputFile.readLine(); lastName = inputFile.readLine(); classNum = inputFile.readLine(); grade = inputFile.readLine(); data[index].setGrades(firstName, lastName, classNum, grade); // data.setGrades(firstName, lastName, classNum, grade); } inputFile.close(); } private void showResults() { JTextArea outArea; JScrollPane scroller; outArea = new JTextArea(30,35); outArea.setText(data.toString()); scroller = new JScrollPane(outArea); JOptionPane.showMessageDialog(null,scroller); System.exit(0); } } Let me know if you want to see any of the other code, or the txt list that the input is coming from.

Dec 1st, 2006, 10:58 PM	#2
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 928 Rep Power: 4	(Toggle Plain Text) Grades[] data = new Grades[NUM_PERSON]; Grades[] data = new Grades[NUM_PERSON]; This code creates an array capable of storing NUM_PERSON Grades objects. By default, each index is initialized to null. (Toggle Plain Text) data[index].setGrades(firstName, lastName, classNum, grade); data[index].setGrades(firstName, lastName, classNum, grade); You are attempting to call the method setGrades on null, hence the NullPointerException. One solution is to initialize each index of data to a new instance of Grades in the GradeReader class' constructor, which will be called when a new instance of GradeReader is created: (Toggle Plain Text) public class GradeReader { public GradeReader() { for (int i = 0; i < NUM_PERSON; i++) data[i] = new Grades(); } ... public class GradeReader { public GradeReader() { for (int i = 0; i < NUM_PERSON; i++) data[i] = new Grades(); } ...

Dec 2nd, 2006, 4:23 AM	#3
Jarod Silverstar Newbie Join Date: Dec 2006 Posts: 7 Rep Power: 0	Okay, that got me past the NullPointer error. But I am not getting my output now. Here is the output from my Grades class: (Toggle Plain Text) public String toString() { String studentStr = "Student name: " + getFirstName() + " " + getLastName() + "\nClass: " + getClassNum() + "\nGrade: " + getGrade(); return studentStr; } public String toString() { String studentStr = "Student name: " + getFirstName() + " " + getLastName() + "\nClass: " + getClassNum() + "\nGrade: " + getGrade(); return studentStr; } And here is where I am trying to call it: (Toggle Plain Text) private void showResults() { JTextArea outArea; JScrollPane scroller; outArea = new JTextArea(30,35); outArea.setText(data.toString()); scroller = new JScrollPane(outArea); JOptionPane.showMessageDialog(null,scroller); System.exit(0); } private void showResults() { JTextArea outArea; JScrollPane scroller; outArea = new JTextArea(30,35); outArea.setText(data.toString()); scroller = new JScrollPane(outArea); JOptionPane.showMessageDialog(null,scroller); System.exit(0); } Again, I got the output doing a single object, but now that I do have mutliple objects in the array, I get junk returned. I assume I need to do something with this line: outArea.setText(data.toString()); But so far I have not been able to come up with the right option. Thank you.

Dec 2nd, 2006, 5:01 AM	#4
Jarod Silverstar Newbie Join Date: Dec 2006 Posts: 7 Rep Power: 0	Well, I partly figured it out. When I do this: (Toggle Plain Text) outArea.setText(data[0].toString() + data[1].toString()); outArea.setText(data[0].toString() + data[1].toString()); I get my output. But it seems to be that there must be a much better way of doing this.

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Dec 2nd, 2006, 3:19 PM	#5
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 928 Rep Power: 4	If you want to print every item in the list, use a loop. Consider cocatenating each item into a temporary String variable, or using JTextArea's append method.

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Programming Forums > Application Development > Java

Trouble inputing data to an Array