public static void main(String [ ] args){
   Lunch lunch = new Lunch(); // receiver
   Command lunchCommand = new LunchCommand(lunch); // concrete command
   Dinner dinner = new Dinner(); // receiver
   Command dinnerCommand = new DinnerCommand(dinner); // concrete command
   MealInvoker mealInvoker = new MealInvoker(lunchCommand); // invoker
   mealInvoker.invoke();
   mealInvoker.setCommand(dinnerCommand);
   mealInvoker.invoke();
  }