Hi all can someone help me

Question

can someone tell me what this message means, I wrote a program to receive input from a user and output it as uppercase and lowercase, but I am getting this message, I have also included my program
thanks
c:\j2sdk1.4.0_01\bin>java Cases
java.lang.NullPointerException
     at java.awt.Container.addImpl(Container.java:584)
     at java.awt.Container.add(Container.java:298)
     at Cases.<init>(Cases.java:45)
     at Cases.main(Cases.java:58)
Exception in thread "main"
here is the program
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Cases extends JFrame {
private JLabel prompt1;
private JLabel prompt2;
private JTextField input;
private JTextField output, result;
private JTextArea outputArea;
private JLabel resultLabel;
// Constructor     
public Cases()
     super("Converting Text");
prompt1 =
new JLabel( "Enter text to convert" );
resultLabel = new JLabel ("Result");
     result = new JTextField(22);
     result.setEditable(false);
     input = new JTextField( 20 );
input.addActionListener(
new ActionListener() {
public void actionPerformed( ActionEvent e )
String words;
words= (input.getText());
result.setText(words.toUpperCase() + ' ' + words.toLowerCase());
     Container c = getContentPane();
c.setLayout( new FlowLayout() );
     c.add (prompt1);
     c.add (prompt2);
     c.add (input);
c.add(resultLabel);
     c.add (result);
     outputArea = new JTextArea( 40, 50 );
c.add (outputArea );
setSize( 500, 400 ); // set the window size
show(); // show the window
public static void main(String args[])
     Cases app = new Cases();
     app.addWindowListener (
     new WindowAdapter() {
     public void windowClosing(WindowEvent e)
     System.exit(0);

Answer 1

I forgot to thank you manalar, that is exactly what was wrong with my program !!