Class fields in Dart can be final. Marking a field as final tells Dart that this variable can never be re-assigned. Importantly, this doesn't mean that the variable must be declared and assigned at the same time. Often, you'll use the final keyword for class properties, and assign them a value in the constructor.

class Rectangle {
  // these are assigned in the constructor,
  // and can never be changed.
  final int width;
  final int height;
  Rectangle(this.width, this.height);
}

Side note: This square is said to be immutable. All of it's field are final, so it cannot be changed once created.

Initializer Lists

An initializer list allows you to assign properties to a new instance variables before the constructor body runs, but after creation. This is handy when you want to set a final variables value, but the value isn't a compile-time constant. For instance:

main() {
  final rectangle = Rectangle(2, 5);
}
class Rectangle {
  final int width;
  final int height;
  final int area;
  The area of the rectangle uses the `width` and `height`
  variables, but cannot be done outside of the constructor
  because the class doesn't know what the width and height
  are equal to until an instance is created.
  Rectangle(this.width, this.height) 
    : area = width * height {
      print(area);
    }
}