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Thread: NullPointerException

  1. #1
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    Default NullPointerException

    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?
        }
    }


  2. #2
    Administrator copeg's Avatar
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    Default Re: NullPointerException

    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?
    Last edited by copeg; July 29th, 2010 at 07:40 PM.

  3. The Following User Says Thank You to copeg For This Useful Post:

    bbr201 (July 29th, 2010)

  4. #3
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    Default Re: NullPointerException

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

  5. #4
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    Default Re: NullPointerException

    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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