Making sense of exception

Question

Hi there,
I'm working on a project where I'm trying to save to file all the objects
contained in a class that extends JPanel and implements Serializable. I attempt the save using the following code:
FileOutputStream fStream = null;
ObjectOutputStream stream = null;
JFileChooser chooser = new JFileChooser();
chooser.setCurrentDirectory(new File("."));
int result = chooser.showSaveDialog(test.this);
if(result != JFileChooser.APPROVE_OPTION)return;
File chosenFile = chooser.getSelectedFile();
FileOpen = chosenFile.getAbsolutePath();
try
    fStream = new FileOutputStream(chosenFile);
    stream = new ObjectOutputStream(fStream);
    stream.writeObject(docPanel);
    stream.close();
    fStream.close();
catch(Exception ex)
    ex.printStackTrace();
    try{stream.close();fStream.close();}catch(IOException ioe){}
    JOptionPane.showMessageDialog(test.this, "Error: " + ex.toString(),
        "Warning", JOptionPane.WARNING_MESSAGE);
[/code
The save process appears successful since I'm able to open the file and look at it. However, immediately after the save I get the following error at run-time:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javax.swing.plaf.basic.BasicScrollPaneUI.paint(Unknown Source)
        at javax.swing.plaf.ComponentUI.update(Unknown Source)
        at javax.swing.JComponent.paintComponent(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JViewport.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintWithOffscreenBuffer(Unknown Source)
        at javax.swing.JComponent.paintDoubleBuffered(Unknown Source)
        at javax.swing.JComponent._paintImmediately(Unknown Source)
        at javax.swing.JComponent.paintImmediately(Unknown Source)
        at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
        at
javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(Unknow
n Source)
        at java.awt.event.InvocationEvent.dispatch(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown
Source)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown
Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javax.swing.plaf.basic.BasicScrollPaneUI.paint(Unknown Source)
        at javax.swing.plaf.ComponentUI.update(Unknown Source)
        at javax.swing.JComponent.paintComponent(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintWithOffscreenBuffer(Unknown Source)
        at javax.swing.JComponent.paintDoubleBuffered(Unknown Source)
        at javax.swing.JComponent._paintImmediately(Unknown Source)
        at javax.swing.JComponent.paintImmediately(Unknown Source)
        at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
        at
javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(Unknow
n Source)
        at java.awt.event.InvocationEvent.dispatch(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown
Source)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown
Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.run(Unknown Source)
There's no reference to any code that I've written in my project, so how am
I suppose to know where to go to fix the problem?
The only thing I understand is somewhere there's a NullPointerException.
But from where does it originate?
Alan                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

Answer 1

Hi there,
I believe you're right on the money. But damned if I know how to fix it! My project is not that complicated. My main class that extends JFrame declares an instance of JScrollPane at the top, then loads the JScrollPane in an actionPerformed method like so:
public class test extends JFrame implements ActionListener
  JScrollPane sp2 = new JScrollPane();
  DocumentPanel docPanel = null;
  JMenuItem New = new JMenuItem("New");
  public void actionPerformed(ActionEvent e)
     if(e.getSource() == New)
        Thread newThread = new Thread()
            public void run()
               //Display a new DocumentPanel
               docPanel = new DocumentPanel();
               sp2.getViewport().add(docPanel);
               container.add(sp2,BorderLayout.CENTER);
               container.validate();
       newThread.start();
}docPanel is an object I created that extends JPanel and contains many other objects.
In the same actionPerformed() method is the code for saving the docPanel from the earlier post. So, any further ideas as to how to prevent the NullPointerException I've been getting with the JScrollPane?
Alan