Expiring Today—Celebrate National IT Professionals Day with 3 months of free Premium Membership. Use Code ITDAY17

x
?
Solved

NullPointerException

Posted on 1998-11-18
2
Medium Priority
?
227 Views
Last Modified: 2010-03-30
This seems like a really dumb question
(but ill as it anyway)
The easiest way to describe it would just be to give you the
code:

import java.awt.*;
import java.awt.event.*;

public class WandGui extends Frame  {
     
     String wands[];
     Checkbox checkbox[];
     
     WandGui(String filename)  {

          super("Wand Selection");
         
          wands= getWandFile(filename);
         
         
          this.setLayout(new BorderLayout());
          this.setBackground(Color.lightGray);
          Font labelFont= new Font("SansSerif",Font.PLAIN,14);
         
          Panel p1= new Panel();
          p1.setLayout(new GridLayout(0,1,10,10));
         
          CheckboxGroup checkbox_group= new CheckboxGroup();
          Checkbox[] checkbox= new Checkbox[wands.length];            
         
          Label label = new Label("Choose a wand from the List below:");
          label.setFont(labelFont);
          p1.add(label);
         
          for (int i=0;i<wands.length;i++)  {
               
               checkbox[i]= new Checkbox("Wand " + wands[i],checkbox_group,false);
               checkbox[i].setFont(labelFont);
               p1.add(checkbox[i]);
          }
         
          Panel p2= new Panel();
          p2.setLayout(new FlowLayout(FlowLayout.CENTER,10,10));
         
          ButtonListener buttonlistener= new ButtonListener();
         
          Button okbutton= new Button("OK");
          okbutton.addActionListener(buttonlistener);
          okbutton.setActionCommand("ok");
          p2.add(okbutton);
          Button cancelbutton= new Button("Cancel");
          cancelbutton.addActionListener(buttonlistener);
          cancelbutton.setActionCommand("cancel");
          p2.add(cancelbutton);
         
         
          this.add(BorderLayout.WEST,new Label("  "));
          this.add(BorderLayout.EAST,new Label("  "));
          this.add(BorderLayout.CENTER,p1);
          this.add(BorderLayout.SOUTH,p2);
         
         
     }
     
     
     public static void main(String args[])  {
         
          WandGui wg= new WandGui("file.txt");
          wg.pack();
          wg.setVisible(true);
     }
     
     
     private String[] getWandFile(String filename)  {
         
          String wands[]={"AAA","BBB","CCC","DDD","EEE","FFF"};
         
          return wands;
     }
     
     class ButtonListener implements ActionListener  {
          public void actionPerformed(ActionEvent e)  {
               if (e.getActionCommand().equals("ok"))  {  
                    System.out.println(checkbox);    //comes up as null here
                    for (int i=0;i<checkbox.length;i++)  {
                         System.out.println("STATE: " + checkbox[i].getState());
                         if (checkbox[i].getState())
                              System.out.println("Picking " + wands[i]);
                    }
               }
               System.exit(0);
          }
     }
}

Notice in the event code I try to access the checkboxes that I initialized in the constructor...
why doesnt it work? and what do I need to do differently?

0
Comment
Question by:mbunkows
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
2 Comments
 
LVL 5

Accepted Solution

by:
msmolyak earned 200 total points
ID: 1227717
Look at the first and the last lines of the code below. The first line declares a instance variable while the last declares the local to constructor variable with the same name. The former one is never initialize. Remove the initializer from the last line.

Checkbox checkbox[];
         
         WandGui(String filename)  {

              super("Wand Selection");
               
              wands= getWandFile(filename);
               
               
              this.setLayout(new BorderLayout());
              this.setBackground(Color.lightGray);
              Font labelFont= new Font("SansSerif",Font.PLAIN,14);
               
              Panel p1= new Panel();
              p1.setLayout(new GridLayout(0,1,10,10));
               
              CheckboxGroup checkbox_group= new CheckboxGroup();
              Checkbox[] checkbox= new Checkbox[wands.length];      
0
 

Author Comment

by:mbunkows
ID: 1227718
ARRGGGHHHHH
someone slap me  :)

Thanks
0

Featured Post

VIDEO: THE CONCERTO CLOUD FOR HEALTHCARE

Modern healthcare requires a modern cloud. View this brief video to understand how the Concerto Cloud for Healthcare can help your organization.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Java contains several comparison operators (e.g., <, <=, >, >=, ==, !=) that allow you to compare primitive values. However, these operators cannot be used to compare the contents of objects. Interface Comparable is used to allow objects of a cl…
Java had always been an easily readable and understandable language.  Some relatively recent changes in the language seem to be changing this pretty fast, and anyone that had not seen any Java code for the last 5 years will possibly have issues unde…
Viewers learn about the scanner class in this video and are introduced to receiving user input for their programs. Additionally, objects, conditional statements, and loops are used to help reinforce the concepts. Introduce Scanner class: Importing…
This video teaches viewers about errors in exception handling.
Suggested Courses

719 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question