How to initialize final variables in Dart

There are four ways (That I know of) to initialize final variables in Dart.

1. Initializing formal parameters
2. Initialize final fields at a declaration
3. Initialize in an initializer list
4. Lazy initialization with late keyword

Initializing formal parameters

Initializing formal parameters is a goto method to initialize fields in Dart. We can initialize each parameter by listing them as constructor arguments, Point(this.x, this.y).

class Point {
  final double x;
  final double y;

  Point(this.x, this.y);
}

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Initialize final fields at a declaration

This might not be very useful, but if your fields don't need user input, you can initialize them at a declaration.

class Log {
  final DateTime createdAt = DateTime.now();
  final String message;

  Log(this.message);
}

Initialize in an initializer list

Dart has a unique syntax to initialize variables in a constructor.

You initialize any variable in the Initializer list, which is a special place before a constructor body.

It has the following syntax:

1. You put a colon (:) before a constructor body.
2. You assign each instance variable after the colon.
3. Separate each expression with a comma (,).

class Square {
  final double width;
  final double height;

  Square(double dimension)
      : width = dimension,
        height = dimension;
}

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Lazy initialization with late keyword

The late keyword delays the initialization to a later phrase.

This allows us to skip all the initialization constraints and do it later in a constructor's body.

class Point {
  late final double x;
  late final double y;

  Point(double x, double y) {
    this.x = x;
    this.y = y;
  }
}