JComboBox behavior

Question

I'm trying to verify my understanding of the JComboBox behavior. Specifically the events which are generated and when. I have a JComboBox that I reload based on the contents of an "upstream" field. I reload the JComboBox using the focusLost() method of this upstream field. The behavior I see exhibited is that when I add the first item I get an actionPerformed event and an itemStateChanged event (or two) both of which show that the first item in the JComboBox is the selected item. Now, at this time the user has not chosen an item and, actually, I don't even want an item in the combo box portion of the control. After I have loaded the data into the JComboBox I invoke setSelectedIndex(-1) to clear out the JComboBox which again generates an actionPerformed evemt and an itemStateChanged event. Frankly, I'm not interested in reacting to any of the above events during this process. I'm looking for the standard way that people use to avoid reacting to these events. The approach I have taken is to call setEnabled(false) before I manipulate the JComboBox data and call setEnabled(true) when I'm done. Then, in the methods which handle these two events I call isEnabled() and simple return if the JComboBox isn't enabled (obviously, to me at least, if the JComboBox was disabled when this event was generated, it [the event]must have been generated under program control and should be ignored).
Any comments as to the appropriateness of this approach would be appreciated or if I'm missing an elegant solution.
Thank you for your attention to this matter!
Peter

Answer 1

There's not really an elegant solution to your problem.
However, your solution is reasonable but does have some minor flaws.
It assumes that the portion of code responsible for reloading the data can disable the combo box (or combo boxes if the data is shared). Ideally it would simply be the model that gets manipulated; access to the view should be ring-fenced.
The combo box might be disabled for a real reason (like the user can't alter it directly) but its selected item may be changed in another way (eg, the user selects something in another combo box that alters the disabled combo box).
You can simply set a flag somewhere else in your application that indicates whether listeners should take action:
public void itemStateChanged(...)
  if(DataManager.isLoading())
    return;
  /* ... your code here ... */
}This then doesn't hijack the view code to shoehorn in something that's not view-related.
It's certainly not elegant but does provide a clear contract for indicating to your listeners that there's something going on that might influence the action they take.
Another option is to use a ComboBoxModel subclass that is told that it's about to reload and suspends event dispatch until it's told that loading's finished. This is arguably better but does require that you write your own models:
public class MyComboBoxModel implements ComboBoxModel
  private boolean eventsActive;
  public boolean setEventsActive(boolean eventsActive)
    this.eventsActive = eventsActive;
    // If events are reactivated then just pretend that the entire model's changed (for example)!!
    if(eventsActive)
      fireContentsChanged();
      fireSelectedItemChanged();
  protected void fireSelectedItemChanged()
    if(!eventsActive)
      return;
    // loop over listeners, fire item change event
}You get the idea...
Hope this helps.