Exception seems to kill updating of a JList

Question

I'm using an ActionListener to update a JList whenever an item is selected.

jComboBox.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        JComboBox cb = (JComboBox) e.getSource();
        updateLocalFileList( cb.getSelectedItem().toString() );
    }
});

It is calling this method for the UI.

public void updateLocalFileList( String path ){
    DefaultListModel model = new DefaultListModel();
    for (String str : LocalFileSystem.getFileListFromDirectory( path )) {
        model.addElement( str );
    }
    getJList().setModel(model);
}

If getFileListFromDirectory() gives a NullPointerException, say when the letter of an empty dvd drive is selected, it seems to prevent the ActionListener from working as intended.

I'm not sure what's happening exactly but I suspect that passing a null value to the model is causing this issue.

Any ideas?

Edit

Here is the stacktrace as requested. As you can see, the method is obviously triggering a NullPointerException on inacessible drives. I don't precisely why it prevents the JList updating though since the rest of the application works fine.

java.lang.NullPointerException
    at mine.View.updateLocalFileList(View.java:274)
    at mine.View$1.actionPerformed(View.java:262)
    at javax.swing.JComboBox.fireActionEvent(Unknown Source)
    at javax.swing.JComboBox.setSelectedItem(Unknown Source)
    at javax.swing.JComboBox.setSelectedIndex(Unknown Source)
    at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(Unknown Source)
    at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at javax.swing.plaf.basic.BasicComboPopup$1.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(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)

Answer 1

The NullPointerException occurs on the EventDispatchThread (which is where all GUI update code normally occurs) so it interrupts the event listener itself. Hence your GUI doesn't get updated properly (anymore). To prevent that you must explicitly handle exceptions, or intercept the causes that would otherwise trigger them.

Answer 2

I would do two things to make this more robust.

1. Ensure that getFileListFromDirectory never returns null. If there are no items, return Collections.emptyList rather than null. If that is not possible, specifically check the return value before using the "for each" iteration. At it is, a null pointer will stop the model from being updated. (So clicking an empty drive will not clear the file list.)
2. assign cb.getSelectedItem() to a local variable and check for null before invoking the updateLocalFileList. If cb.getSelectedItem() is null, you might choose to clear the file list.

Answer 3

Your listener (the one that throws the exception) is not the only one that listens on the specific event. Core Swing classes (especially the UI delegates) register their own listener so that they can properly update the UI. If your listener fails, there is no guarantee that all (or any) of the other listeners will be notified with this specific event.

Answer 4

A couple of points:

What exactly is the LocalFileSystem class? Is that a custom class?

If it is expected in some cases that it will return null, then use a local variable for it and check if it is null before calling any method on it.

You can check if a given file or directory exists using the exists method on File. There is also an isDirectory method as well. I suggest you consider using it.