Learning Dart - 4 Create factory via factory constructor

main.dart

import 'dart:math' as math;

abstract class Shape {
  num get area;
  factory Shape(String type) {
    if (type == 'circle') return Circle(2);
    if (type == 'square') return Square(2);
    throw 'Can\'t create $type';
  }
}

class Circle implements Shape {
  final num radius;
  num get area => math.pi * math.pow(radius, 2);

  Circle(this.radius);
}

class Square implements Shape {
  final num side;
  num get area => math.pow(side, 2);

  Square(this.side);
}

main() {
  final circle = Shape('circle');
  final square = Shape('square');

  print(circle.area);
  print(square.area);
}