How do I initialize a final class property in a constructor?

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You cannot instantiate final fields in the constructor body. There is a special syntax for that:

class Point {
  final num x;
  final num y;
  final num distanceFromOrigin;

  // Old syntax
  // Point(x, y) :
  //   x = x,
  //   y = y,
  //   distanceFromOrigin = sqrt(pow(x, 2) + pow(y, 2));

  // New syntax
  Point(this.x, this.y) :
    distanceFromOrigin = sqrt(pow(x, 2) + pow(y, 2));
}

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