class Person {
String name;
int age;

Person(this.name, this.age);

@override
String toString() {
return “name:$name,age:$age”;
}
}

class Student extends Person {
late String _school; // 通过下划线来标识私有字段（变量）
String? city;
String? country;
late String name;
Student(this._school, String name, int age, {required this.city, this.country = “China”}): name=‘$country.$city’, super(name, age);
Student.cover(Student stu):super(stu.name, stu.age);
}