NullPoniter Exception while Adding in JComboBox

Question

Hi All, My application is a Applet - EJB based and Server is sending Serialized Objects to add in to the JCOMBOBOX object.
I have the following Exception coming up after a few iteratons ( 200 ) of opening up the screen and closing it which I am doing at a normal speed.
Code......
List lstSeverities = (List)e.getReturnValue();
if(lstSeverities!=null){
Iterator oIterator = lstSeverities.iterator();
while (oIterator.hasNext())
CaseSeverity oCaseSeverity = (CaseSeverity)oIterator.next();
if(oCaseSeverity != null && oCaseSeverity.getSeverityName() != null)
m_cbxSeverity.addItem(oCaseSeverity); /* This Statement is throwing exception.*/}
StackTrace.......
java.lang.NullPointerException
at javax.swing.plaf.basic.BasicComboBoxUI$ListDataHandler.contentsChanged(Unknown Source)
at javax.swing.plaf.basic.BasicComboBoxUI$ListDataHandler.intervalAdded(Unknown Source)
at javax.swing.AbstractListModel.fireIntervalAdded(Unknown Source)
at javax.swing.DefaultComboBoxModel.addElement(Unknown Source)
at javax.swing.JComboBox.addItem(Unknown Source)
at sync.client.contactmgmt.activity.CaseGeneral.handleResponse(CaseGeneral.java:1382)
at sync.client.gateway.ServerProxy.handleResponse(ServerProxy.java:96)
at sync.client.gateway.UniversalGateway$1.run(UniversalGateway.java:142)
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)
Note - 1. This Exception is coming only once in some 200 repetitions.
2. The issue is with only this ComboBox , I have another 10 ComboBox.
3. This ComboBox is populated with Serialized objects.
Please help to identify the cause and to resolve this.
Thanks.

Answer 1

I have looked in Sun sources:
Regarding the stack trace u post:
from AbstractListModel source
protected void fireIntervalAdded(Object source, int index0, int index1)
     Object[] listeners = listenerList.getListenerList();
     ListDataEvent e = null;
     for (int i = listeners.length - 2; i >= 0; i -= 2) {
     if (listeners[i] == ListDataListener.class) {
          if (e == null) {
          e = new ListDataEvent(source, ListDataEvent.INTERVAL_ADDED, index0, index1);
          ((ListDataListener)listeners[i+1]).intervalAdded(e);
it fires an event that is hanled in nested class ListDataHanlder of
BasicComboBoxIUI:
public void intervalAdded( ListDataEvent e ) {
     isDisplaySizeDirty = true;
     contentsChanged( e );
it invokes the contentChanged mothod of the same nested class
public void contentsChanged( ListDataEvent e ) {
     if ( !(e.getIndex0() == -1 && e.getIndex1() == -1) ) {
          isMinimumSizeDirty = true;
          comboBox.revalidate();
Now:
The 2 objects that might be null are
1) e: the event
Improbably, is the event dispatched from the JComboxBox add method
2) the comboBox Object
Most probably.
I imagine that, when the event is disptached, the JComboBox is valid
(not equals to null)
But when the events reaches contentChange method, the comboBox
pointer in BasicComboBoxUI is just null.
This pointer is setted by installUI method and is forced to null
in uninstallUI method.
So, it semmes to be not a Swing bug, but, most probably,
somothing in your code cause the call of unistallUI method or,
simply, destroys the JComboBox after that the event is shipped.
Take a look.