This gist is a solution to the comment Image below.

main.dart

import 'dart:math' as math;

void main() {
  final Circle c1 = Circle.all(color: 'blue', radius: 2.0);
  c1.getRadius();
  c1.getColor();
  c1.getArea();

  final Circle c2 = Circle.radius(radius: 2.0);
  c2.getRadius();
  c2.getColor();
  c2.getArea();

  final Circle c3 = Circle();
  c3.getRadius();
  c3.getColor();
  c3.getArea();
}

class Circle {
  // Multiple constructor can't have same name
  /// No radius and color is required
  Circle([this.radius = 1.0, this.color = 'red']);

  /// Radius is required
  Circle.radius({required this.radius, this.color = 'red'});

  /// Both radius and color is required
  Circle.all({required this.color, required this.radius});

  final double radius;
  final String color;

  /// Method to get the radius
  double getRadius() {
    print('The radius of circle is: $radius');
    return radius;
  }

  /// Method to get the color
  String getColor() {
    print('The color of circle is: $color');
    return color;
  }

  /// Method to get the area of the circle
  /// Since area of a circle is πr2
  /// We will define our pie with a dart maths const of π = 3.142
  double getArea() {
    /// making the object [this.radius] a square
    final radiusSquare = math.pow(radius, 2);
    final area = math.pi * radiusSquare;

    print('The area of the circle is: ${area.toStringAsFixed(2)}');
    return area;
  }
}

The implementation of this image can be see above.

Click HERE to run solution on Dartpad online