Abstract Classes trong Dart

abstract class Shape {
  // Abstract method - không có body
  double area();
  double perimeter();
  
  // Concrete method - có implementation
  void describe() {
    print('This is a shape');
  }
}

abstract class Shape {
  double area();
  double perimeter();
}
 
class Circle extends Shape {
  double radius;
  
  Circle(this.radius);
  
  @override
  double area() => 3.14159 * radius * radius;
  
  @override
  double perimeter() => 2 * 3.14159 * radius;
}
 
class Rectangle extends Shape {
  double width, height;
  
  Rectangle(this.width, this.height);
  
  @override
  double area() => width * height;
  
  @override
  double perimeter() => 2 * (width + height);
}

abstract class Animal {
  String name;
  
  Animal(this.name);
  
  // Abstract method
  void makeSound();
  
  // Concrete method
  void eat() {
    print('$name is eating');
  }
}
 
class Dog extends Animal {
  Dog(String name) : super(name);
  
  @override
  void makeSound() {
    print('$name barks');
  }
}

abstract class Drawable {
  // Abstract getter
  String get color;
  
  // Abstract setter
  set color(String value);
  
  void draw();
}
 
class Circle extends Drawable {
  String _color = 'red';
  
  @override
  String get color => _color;
  
  @override
  set color(String value) => _color = value;
  
  @override
  void draw() => print('Drawing $color circle');
}

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

abstract class Database {
  void connect();
  void query(String sql);
  
  factory Database(String type) {
    switch (type) {
      case 'mysql':
        return MySQLDatabase();
      case 'postgres':
        return PostgresDatabase();
      default:
        throw ArgumentError('Unknown database type');
    }
  }
}
 
class MySQLDatabase implements Database {
  @override
  void connect() => print('Connecting to MySQL');
  
  @override
  void query(String sql) => print('MySQL: $sql');
}
 
class PostgresDatabase implements Database {
  @override
  void connect() => print('Connecting to PostgreSQL');
  
  @override
  void query(String sql) => print('Postgres: $sql');
}

// Abstract class - có thể có implementation
abstract class Animal {
  void eat() => print('Eating');  // Có body
  void speak();  // Abstract
}
 
// Interface (implicit) - chỉ định nghĩa contract
class Flyable {
  void fly() {}
}
 
class Bird extends Animal implements Flyable {
  @override
  void speak() => print('Chirp');
  
  @override
  void fly() => print('Flying');
}