1 package day03; 2  3 public class UpTest { 4     public static void main(){ 5         Aoo o = new Aoo(); 6         o.a = 3; 7         o.b = 3;  //错误,不能访问子类成员 8          9         Boo x = new Boo();10         x.b = 3;11         x.a = 3;12         x.say();13         14         Aoo y = new Boo();  //向上造型，父类引用指向子类对象15         y.a = 3;16         y.b = 3;17         18     }19 }20 21 class Aoo{22     int a;23     void show(){24     }25 }26 27 class Boo extends Aoo{28     int b;29     void say(){}30 }

父类、子类 ：继承、向上造型。