Class và Object trong Dart

class Person {
  String name;
  int age;
  
  Person(this.name, this.age);
  
  void greet() {
    print("Hello, I'm $name");
  }
}
 
void main() {
  var person = Person("Alice", 25);
  person.greet();
}

class User {
  final String id;
  String name;
  int _age = 0;  // Private (underscore)
  
  User(this.id, this.name);
  
  int get age => _age;
  set age(int value) {
    if (value >= 0) _age = value;
  }
}

class Point {
  final double x;
  final double y;
  final double distance;
  
  Point(double x, double y)
      : x = x,
        y = y,
        distance = sqrt(x * x + y * y);
}

class UserModel {
  final String id;
  final String name;
  final String? avatar;
  
  const UserModel({
    required this.id,
    required this.name,
    this.avatar,
  });
}