private void checkEnabled() {
   if (selection.hasSelection()) {
     List<? extends DbObject> objects = source.getSelectedObjects();
     for (DbObject dbo : objects) {
       if (!(dbo instanceof TableIdentifier)) {
         setEnabled(false);
         return;
       }
     }
     setEnabled(true);
   } else {
     setEnabled(false);
   }
 }