【好程序员训练营】----子类实例化函数复写对象转型

public class Student extends Person{
    /*
     * 继承父类成员和函数，不能继承构造函数
     * 在子类构造函数中，须调用父类的构造函数
     */
    Student(){
        super();//用于调用父类构造函数；若写则在第一条语句，若不写，编译器自动添加；可以调用父类有参构造函数super(..,..);
        System.out.println("student无参数构造函数");
    }
    public static void main(String agrc[]){
        Student s = new Student();
    }
}
class Person{
    int age;
    String name;
    Person(){
        
    }
    Person(int age, String name){
        this.age = age;
        this.name = name;
    }
}

public class Student extends Person{
    int grade;
    void eat(){
        System.out.println("son");
    }
    public static void main(String agrc[]){
        /**************************************/
        //向上转型,也可以Person p1 = new Student();(学生是人)
        Student s1 = new Student();
        Person p1 = s1;
        //p1.grade = 5;出错，p能调用的成员根据p的类型Person中的成员来确定
        p1.eat();//输出son
        /**************************************/
        //向下转型,先向上转型再转回来;(学生是人,人再是学生)
        Person p2 = new Student();
        Student s2 = (Student)p2;
        //错误的转型(人是学生？？！！)
        //Person p2 = new Person();
        //Student s2 = (Student)p2;
    }
}
class Person{
    int age;
    String name;
    void eat(){
        System.out.println("father");
    }
}