Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

From:

Warren Tang <warren.tang@nospam.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 08 Jun 2009 22:56:55 +0800

Message-ID:

<h0j8t8$h73$1@aioe.org>

Hello,

I see nothing wrong within my beginner's program, yet it throws an
NullPointerException. Could you look through the code and tell where I
was doing wrong please? I am using Fedora Linux / Eclipse JDT if that
matters.

Here is the stack trace, followed by the source code.
----------------------------------
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at tang.warren.LookFrame.makeButton(LookTest.java:40)
    at tang.warren.LookFrame.<init>(LookTest.java:31)
    at tang.warren.LookTest$1.run(LookTest.java:13)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:227)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:603)
    at
java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:276)
    at
java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:201)
    at
java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:191)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:186)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:178)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:139)

----------------------------------
package tang.warren;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.UIManager.LookAndFeelInfo;

public class LookTest {
    public static void main(String[] args){
        EventQueue.invokeLater(new Runnable(){
            @Override
            public void run() {
                JFrame f = new LookFrame();
                f.setVisible(true);
            }
        });
    }
}

class LookFrame extends JFrame {
    private JPanel lookPanel;

    public LookFrame(){
        this.setTitle("Look and Feel");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(Toolkit.getDefaultToolkit().getScreenSize());

        for(LookAndFeelInfo laf : UIManager.getInstalledLookAndFeels())
        {
         //if(laf != null)
         makeButton(laf);
        }

        lookPanel = new JPanel();
        this.add(lookPanel);
    }

    private void makeButton(final LookAndFeelInfo laf) {
        JButton lookButton = new JButton(laf.getName());
        lookPanel.add(lookButton);

        lookButton.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent arg) {
                try {
                    UIManager.setLookAndFeel(laf.getClassName());
                    SwingUtilities.updateComponentTreeUI(LookFrame.this);
                }catch(Exception e){
                    e.printStackTrace();
                }
            }
        });
    }
}