// combo boxes
 public void comboBoxActionPerformed(java.awt.event.ActionEvent ae) {
   if (ae.getSource() == modelComboBox) {
     if (modelComboBox.getSelectedItem() != null) {
       String model = (String) modelComboBox.getSelectedItem();
       // load the default hp and length for the model selected
       hpTextField.setText(engineModels.getModelHorsepower(model));
       weightTextField.setText(engineModels.getModelWeight(model));
       if (engineModels.getModelLength(model) != null
           && !engineModels.getModelLength(model).equals("")) {
         lengthComboBox.setSelectedItem(engineModels.getModelLength(model));
       }
       if (engineModels.getModelType(model) != null
           && !engineModels.getModelType(model).equals("")) {
         typeComboBox.setSelectedItem(engineModels.getModelType(model));
       }
     }
   }
   if (ae.getSource() == locationBox) {
     if (locationBox.getSelectedItem() == null) {
       trackLocationBox.removeAllItems();
     } else {
       log.debug("EnginesSetFrame sees location: " + locationBox.getSelectedItem());
       Location l = ((Location) locationBox.getSelectedItem());
       l.updateComboBox(trackLocationBox);
     }
   }
 }