java

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。