Kế thừa (Inheritance) trong Dart

class Animal {
  String name;
  
  Animal(this.name);
  
  void speak() {
    print('$name makes a sound');
  }
}
 
class Dog extends Animal {
  Dog(String name) : super(name);
  
  @override
  void speak() {
    print('$name barks');
  }
}
 
void main() {
  var dog = Dog('Buddy');
  dog.speak();  // Buddy barks
}

class Vehicle {
  String brand;
  int year;
  
  Vehicle(this.brand, this.year);
  
  void info() {
    print('$brand - $year');
  }
}
 
class Car extends Vehicle {
  int doors;
  
  Car(String brand, int year, this.doors) : super(brand, year);
  
  @override
  void info() {
    super.info();  // Gọi method cha
    print('Doors: $doors');
  }
}

class Shape {
  double area() => 0;
}
 
class Circle extends Shape {
  double radius;
  
  Circle(this.radius);
  
  @override
  double area() => 3.14159 * radius * radius;
}
 
class Rectangle extends Shape {
  double width, height;
  
  Rectangle(this.width, this.height);
  
  @override
  double area() => width * height;
}

void main() {
  List<Shape> shapes = [
    Circle(5),
    Rectangle(4, 3),
  ];
  
  for (var shape in shapes) {
    print('Area: ${shape.area()}');
  }
}

class Parent {
  String _secret = 'hidden';  // Private trong library
  
  void showSecret() {
    print(_secret);
  }
}
 
class Child extends Parent {
  void reveal() {
    // Có thể truy cập _secret nếu cùng file
    print(_secret);
  }
}

class Person {
  String name;
  int age;
  
  Person(this.name, this.age);
  
  Person.guest() : name = 'Guest', age = 0;
}
 
class Student extends Person {
  String school;
  
  Student(String name, int age, this.school) : super(name, age);
  
  Student.guest(this.school) : super.guest();
}

class Animal {
  void chase(Animal other) {
    print('Chasing animal');
  }
}
 
class Dog extends Animal {
  @override
  void chase(covariant Dog other) {
    print('Chasing dog');
  }
}