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

From:

dimka <dooman87@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 8 Jun 2009 09:22:30 -0700 (PDT)

Message-ID:

<e36875ae-20bb-4fe5-8127-01428dd8cef3@v4g2000vba.googlegroups.com>

On 8 =C9=C0=CE, 18:56, Warren Tang <warren.t...@nospam.com> wrote:

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:2=

27)

at java.awt.EventQueue.dispatchEvent(EventQueue.java:603)
at
java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.j=

av-a:276)

at
java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java=

:2-01)

at
java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.j=

av-a:191)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.ja=

va:186)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.ja=

va: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.getC=

lassName());

SwingUtilities.updateComponentTre=

eUI(LookFrame.this);

                                }catch(Exception e){
                                        e.printStackTrace();
                                }
                        }
                });
        }

}

Hello!
You have NPE in method makeButton in line:
--->lookPanel.add(lookButton);

You create lookPanel only after cycle, where you create buttons.
Modify you code like this:
        private JPanel lookPanel;

        public LookFrame(){
                this.setTitle("Look and Feel");
                this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                this.setSize(Toolkit.getDefaultToolkit().getScreenSize
());
                lookPanel = new JPanel();
            for(LookAndFeelInfo laf :
UIManager.getInstalledLookAndFeels())
            {
                //if(laf != null)
                makeButton(laf);
            }
                this.add(lookPanel);
        }