问Dart 2类的构造函数与其他语言的多态非常相似。

EN

import 'dart:math';

class Photo {
  final double area;

  // This constructor is library-private. So no other code can extend
  // from this class.
  Photo._(this.area);

  // These factories aren't needed – but might be nice
  factory Photo.rect(double width, double height) => new RectPhoto(width, height);
  factory Photo.circle(double radius) => new CirclePhoto(radius);
}

class CirclePhoto extends Photo {
  final double radius;

  CirclePhoto(this.radius) : super._(pi * pow(radius, 2));
}

class RectPhoto extends Photo {
  final double width, height;

  RectPhoto(this.width, this.height): super._(width * height);
}

import 'dart:math';

abstract class Photo {
  double area();
}

class CircularPhoto extends Photo {
  double radius;

  CircularPhoto(this.radius);

  @override
  double area() {
    return pi * pow(this.radius, 2);
  }
}

class RectPhoto extends Photo {
  double length;
  double height;

  RectPhoto(this.length, this.height);

  @override
  double area() {
    return this.length * this.height;
  }
}

void main(){
  Photo p = new CircularPhoto(5);
  print(p.area());
}