class Student                                      
    {        
         void printStuMessage ()                           
         {
           System.out.println ("a student ");     
         }
         
    }//end Student
    public class GraduateStudent extends Student
    {         
         void printStuMessage ()                  
         {
            System.out.println ("a graduate student "); 
         }
         void printStuMessage (String name)                  
         {
            System.out.println ("a graduate student : " + name); 
         }    
         public static void main(String[] args)         
         {  
            GraduateStudent g = null;
            Student s = new Student();                
            s.printStuMessage();
            s = new GraduateStudent();
            s.printStuMessage();
            //s.printStuMessage("Tom");                      //
            if( s instanceof GraduateStudent)            
            {
               g = (GraduateStudent)s;
               g.printStuMessage();
               g.printStuMessage("Tom");
}
         }         
     }//end GraduateStudent
