subclass 这个词的意思和我想象中不太一样 

例子 We want to create a subclass of Person called CollegeKid, which represents someone who is attending college.

写出的代码是这样的

public class CollegeKid extends Person
{
   private double gpa;

  
1 2 3 4 5 6 public CollegeKid (String n, int a, double g) { super(n, a); /* the rest here */ gpa = g; }

   public double getGPA ()
   {
      return gpa;
   }

   public void setGPA (double g)
   {
      gpa = g;
   }

} // end class