How to create Multiple Constructors in Dart

20 March 2023
Dart

Constructor overloading is the ability to create multiple constructors with different parameters.

For example, we have a Color class with a constructor that initialize four instance variable, red, green, blue, and alpha.

class Color {
  final double red;
  final double green;
  final double blue;
  final double alpha;

  Color(
      {required this.red,
      required this.green,
      required this.blue,
      required this.alpha});
}

Let's say we want to create an overload initializer to initialize only the red value. We might come up with a new constructor like this.

Color(this.red)
    : green = 0,
      blue = 0,
      alpha = 1;

Unfortunately, in Dart, you will get the following error.

The unnamed constructor is already defined.
Try giving one of the constructors a name.

The unnamed constructor is already defined #

In Dart, we can have one unnamed constructor, but we can have multiple named constructor.

Unnamed constructor, a constructor with the same name as its class.
Named constructors, a constructor with a name to provide extra clarity. This is a Dart way to implement multiple constructors for a class.

This is the reason why we get the error when trying to implement an overload initializer in the previous section.

How to create Multiple Constructors in Dart #

So, we can't have constructor overloading, but we can create as many named constructors as we want.

We can create a named constructor by appending a dot (.) and a name of your choice to an unnamed constructor.

Here is an example of a named constructor that initializes only the red value of the Color class.

Color.red(this.red)
    : green = 0,
      blue = 0,
      alpha = 1;

Then, we can use it like this.

Color.red(0.5);

We can have as many named constructors as we want. Here is an example where I created two more named constructors, Color.blue(this.blue) and Color.green(this.green).

class Color {
  final double red;
  final double green;
  final double blue;
  final double alpha;

  Color(
      {required this.red,
      required this.green,
      required this.blue,
      required this.alpha});

  Color.red(this.red)
      : green = 0,
        blue = 0,
        alpha = 1;

  Color.blue(this.blue)
      : green = 0,
        red = 0,
        alpha = 1;

  Color.green(this.green)
      : blue = 0,
        red = 0,
        alpha = 1;
}