Intro to Dart for Java Developers: Create a factory

shape.dart

import 'dart:math';

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

  num get area;
}

class Circle implements Shape {
  final num radius;
  Circle(this.radius);

  @override
  num get area => PI * pow(radius, 2);
}

class Square implements Shape {
  final num side;
  Square(this.side);

  @override
  num get area => pow(side, 2);
}

main() {
  try {
    print(new Shape('circle').area);
    print(new Shape('square').area);
    print(new Shape('wrong').area); // throw and exception
  } catch (error) {
    print(error);
  }
}