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JFrame.processWindowEvent() contains the following 'switch' operator:

if (e.getID() == WindowEvent.WINDOW_CLOSING) {
    switch(defaultCloseOperation) {
      case HIDE_ON_CLOSE:
         setVisible(false);
         break;
      case DISPOSE_ON_CLOSE:
         dispose();
         break;
      case DO_NOTHING_ON_CLOSE:
         default:
         break;
      case EXIT_ON_CLOSE:
          // This needs to match the checkExit call in
          // setDefaultCloseOperation
         System.exit(0);
         break;
    }
}

Notice that the code is intricate. The 'default:' label is not the last one, but it is just after 'DO_NOTHING_ON_CLOSE:' label. Despite the code works properly, it should be rewritten in the following way:

if (e.getID() == WindowEvent.WINDOW_CLOSING) {
    switch(defaultCloseOperation) {
      case HIDE_ON_CLOSE:
         setVisible(false);
         break;
      case DISPOSE_ON_CLOSE:
         dispose();
         break;
      case DO_NOTHING_ON_CLOSE:
         break;
      case EXIT_ON_CLOSE:
          // This needs to match the checkExit call in
          // setDefaultCloseOperation
         System.exit(0);
         break;
      default:
         break;
    }
}