java - day10 - uptest

 package day03;

 public class UpTest {
     public static void main(){
         Aoo o = new Aoo();
         o.a = 3;
         o.b = 3;  //错误,不能访问子类成员

         Boo x = new Boo();
         x.b = 3;
         x.a = 3;
         x.say();

         Aoo y = new Boo();  //向上造型，父类引用指向子类对象
         y.a = 3;
         y.b = 3;

     }
 }

 class Aoo{
     int a;
     void show(){
     }
 }

 class Boo extends Aoo{
     int b;
     void say(){}
 }

父类、子类 ：继承、向上造型。