Need help with Exception in thread "main" java.lang.NullPointerException

[JGeorge337]

Hello everybody im new to the board. I recently started tring to learn Java programing. I have a problem with my code giving me this error when I try to run it can anyone help me out.This is the exception that I am getting.

Exception in thread "main" java.lang.NullPointerException
at calc.Calc.main(Calc.java:57)
Java Result: 1
BUILD SUCCESSFUL (total time: 1 second)

Thanks in advance.

package calc;
 
/**
 *
 * @author
 */
 
   import javax.swing.*;
   import javax.swing.event.*;
   import java.awt.*;
   import java.awt.event.*;
 
public class Calc  extends JFrame {
        public static JPanel panel;
        public static JFrame frame;
        public static JTextField input;
        public static JTextField output;
        public static JButton calculate;
        public static JButton clear;
        public static JButton quit;
 
        public Calc(){
        setLayout(new FlowLayout ());
        calculate = new JButton("Calculate");
        clear = new JButton("Clear");
        quit = new JButton("Quit");
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();
        frame.add(panel);
        panel.add(output);
        frame.setVisible(true);
        panel.setVisible(true);
        quitevent quitevent = new quitevent();
        clearevent clearevent = new clearevent();
        ooop ooop = new ooop();
 
        quit.addActionListener(quitevent);
        clear.addActionListener(clearevent);
        calculate.addActionListener(ooop);
 
}
public class quitevent implements ActionListener {
    public void actionPerformed(ActionEvent quitevent){
 
    }
 
}
public class clearevent implements ActionListener{
    public void actionPerformed(ActionEvent clearevent){
}
}
public class ooop implements ActionListener{
    public void actionPerformed(ActionEvent ooop){
}}
    public static void main(String args[]){
 
       frame.add(panel);
}       
}

[copeg]

For future reference, please wrap your code in the code tags (see the getting help link in my signature for more information). I have done this for you.

Please post the full exception - it contains valuable information indicating where the exception is being thrown

[JGeorge337]

Thanks copeg!!

[copeg]

You are welcome.

A comment about the code. First, there is no need to declare the variables of Calc static - they should be instance variables and part of the Calc class. See the following for a better description than I can provide at the moment: Understanding Instance and Class Members (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

Regarding the Exception, it indicates something is Null, or has not been instantiated (eg using the new keyword). Glance through the code, use println's, or do whatever you are more comfortable with to find out what has not been instantiated (that's all I can output at the moment, and I've said the main point - hint, hint). For what its worth, to avoid this, I often instantiate variables that do not depend on runtime values when they are declared (eg private JPanel myPanel = new JPanel() ).

[daniel.j2ee]

The exception is thrown at this line: frame.add(panel);

Remove the JFrame and JPanel from the constructor. Basically what you do there, is to redeclare them locally, and the actual frame/panel which you intended to use, are never initialized.

application context

[Mr.777]

Read copeg's post again and carefully. You will find answer.