Open-closed principle example

main.dart

sealed class Shape {
  double calcSquare();
}

class Circle extends Shape {
  Circle(this.radius);

  final double radius;
  
  @override
  double calcSquare() => 3.14 * radius * radius;
}

class Rectangle extends Shape {
  Rectangle(this.width, this.height);
    
  final double width;
  final double height;
  
  @override
  double calcSquare() => width * height;
}

class SquareCalculator {
  double calcSquare(Shape shape) => shape.calcSquare();
}

void main() {
 final calculator = SquareCalculator();
    
    final circle = Circle(25);
    
    // prints 1962,5
    print(calculator.calcSquare(circle)); 
    
    final rectangle = Rectangle(10, 20);
    
    /// prints 200
    print(calculator.calcSquare(rectangle));
}