Question

I’m busy on a GUI application in Java in which I sometimes encounter IndexOutOfBoundsExceptions when a value is added to a jList.

The exception seems to occur when a value is selected, and then another is added. I have a listener for selection changes because something needs to happen when the user selects an index, but this event is fired when a new value is added as well. I use a custom ListModel that just extends AbstractListModel and overrides the necessary methods in a perfectly valid way.

1. Why does the selection change in the program when a value is added to the list? This is not visually represented.
2. Why does the jList allow selection of an index that is not really there?

I have used jList twice now (we’ve recently started doing GUI in school) and I’ve had the problem both times. First time I solved it by clearing selection before a value is added, but that’s not a really good solution. I don’t think this should be necessary.

I don’t know why this occurs, I have no strange code or anything. In pseudo-code, this is what happens:

listmodel.addValue(object);
listmodel.fireIntervalAdded();
//selection event occurs
selectedObject = listmodel.getValueAt(list.getSelectedIndex()); //indexoutofboundsexception
//index = 5, size = 3 (for example) when there are 2 objects in list and first is selected.

I’m not providing more code right now because I think it’s not really relevant. I think anyone that understands perfectly how a jList, its listmodel and its selectionmodel work, will understand what’s wrong. Any help on this is appreciated.

Answer 1

The problem is most likely that you’re calling fireIntervalAdded(this, 0, list.size()) when a single item is added to your list model. The signature is:

protected void fireIntervalAdded(Object source, int index0, int index1)

Note that index0 is the starting index of the added item and index1 is the ending index. Thus for a single item index0 should be the same as index1. When you call fireIntervalAdded with 0, list.size(), you are telling the JList that N items have been added, where N=list.size(). Thus the JList thinks there are more items than are in your list model.

The same goes for when you remove an item.