NullPointerException

**bbr201** · July 29th, 2010, 06:17 PM

hi all,

I'm getting the following due to the setText call in the change event handler. I must be missing something obvious but I don't see it right now!

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javaapplication4.Main.stateChanged(Main.java:49)
at javax.swing.AbstractButton.fireStateChanged(Abstra ctButton.java:1889)
at javax.swing.AbstractButton$Handler.stateChanged(Ab stractButton.java:2310)
at javax.swing.DefaultButtonModel.fireStateChanged(De faultButtonModel.java:333)
at javax.swing.DefaultButtonModel.setMnemonic(Default ButtonModel.java:274)
at javax.swing.AbstractButton.setMnemonic(AbstractBut ton.java:1548)
at javax.swing.AbstractButton.setMnemonic(AbstractBut ton.java:1569)
at javaapplication4.Main.<init>(Main.java:29)
at javaapplication4.Main$1.run(Main.java:42)
at java.awt.event.InvocationEvent.dispatch(Invocation Event.java:209)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 597)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:122)

public class Main implements ChangeListener {
 
    JButton jbtnFirst;
    JLabel jlab;
 
    Main() {
        JFrame jfrm = new JFrame("Testing Buttons!");
        jfrm.setLayout(new FlowLayout());
        jfrm.setSize(350, 350);
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
        jbtnFirst = new JButton("First");
        jbtnFirst.addChangeListener(this);
        jbtnFirst.setMnemonic('F');
        jfrm.add(jbtnFirst);
 
        jlab = new JLabel();
        jfrm.add(jlab);
 
        jfrm.setVisible(true);
    }
 
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
 
            public void run() {
                new Main();
            }
        });
 
    }
 
    public void stateChanged(ChangeEvent ce) {
        jlab.setText("aa"); //What is wrong with this?
    }
}

**copeg** · July 29th, 2010, 06:37 PM

Your change listener is fired before you create the jlab variable...it is fired when the component is changed in some way (eg setMnemonic). see code below:

public class Main implements ChangeListener {
 
    JButton jbtnFirst;
    JLabel jlab;
 
    Main() {
        JFrame jfrm = new JFrame("Testing Buttons!");
        jfrm.setLayout(new FlowLayout());
        jfrm.setSize(350, 350);
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
        jbtnFirst = new JButton("First");
        jbtnFirst.addChangeListener(this);
        jbtnFirst.setMnemonic('F');
        jfrm.add(jbtnFirst);
        System.out.println("Creating Label");
        jlab = new JLabel();
        jfrm.add(jlab);
 
        jfrm.setVisible(true);
    }
 
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
 
            public void run() {
                new Main();
            }
        });
 
    }
 
    public void stateChanged(ChangeEvent ce) {
      System.out.println("Changed");//  jlab.setText("aa"); //What is wrong with this?
    }
}

Any reason you are not using an ActionListener as opposed to a ChangeListener?

**bbr201** · July 29th, 2010, 06:53 PM

Ahh, I see. I didn't know using setMnemonic qualified as a change event. Thanks!

**bbr201** · July 29th, 2010, 07:06 PM

Well, I just got to the ChangeListener section in the book. So I was just trying it out to see how it worked.

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