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If I understand correctly, you want the numbering to change so that the items are numbered in increasing order without skipping any numbers when an item is removed. I assume that your "list box" is either a JList or a List. In either case, I think you should create a class that implements ListModel. This class can be responsible for ensuring that the number in front of the rest of the label is correct.
If you are using JComboBox instead, you should implement ComboBoxModel. In either case, the API provides an abstract model that implements many of the details that are often the same between different models. Check out the Java Tutorial for more details on how this works.