学步园

public class PolyTest2
{
                      public static void main(String[] args)
                                 {
                                       Fruit f=new Pear();
                                       Pear p = (Pear)f;
                                       p.sing();
                                  }
}
                   class Fruit
     {
                public void sing()
             {
                     System.out.println("fruit is singing");
             }
    }

                      class Pear extends Fruit
                 {
                             public void sing()
                       {
                            System.out.println("pear is singing");
                      }
               }