Initializer list

// Initializer list sets instance variables before the constructor body runs. 
Point.fromJson(Map<String, num> json) 
   : x = json['x'], 
   y = json['y'] { 
      print('In Point.fromJson(): ($x, $y)'); 
}

Besides invoking a superclass constructor, you can also initialize instance variables before the constructor body runs.

During development, you can validate inputs by using assert in the initializer list.

Point.withAssert(this.x, this.y) : assert(x >= 0) { 
   print('In Point.withAssert(): ($x, $y)'); 
}

1. Initializer list.
2. Superclass’s no-arg constructor.
3. Main class’s no-arg constructor.