the event originally is sent by model, handled by tabbed pane UI and
after that transferred to you.
So you can include your additional code into the model before it fires
a change event:

// DefaultSingleSelectionModel.java
public void setSelectedIndex(int index) {
 // show a dialog here
 // or create and fire your own additional event
 // or do something else you want
 // or return without switching pages
 if (this.index != index) {
   this.index = index;
   fireStateChanged();
 }
}