Dart Lesson 41 of 102 3 min read

Constructors in Dart: Default, Named, Initializer Lists and Redirecting

Every kind of generative constructor in Dart: default, initializing formals, named parameters, named constructors and initializer lists.

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A constructor creates an object and makes sure its fields have values. Dart offers several styles, and the short ones cover most needs.

The default constructor #

If you write no constructor, Dart gives you one with no parameters. Fields then need default values.

class Light {
  bool isOn = false;
  int brightness = 50;
}

void main() {
  var light = Light();
  print('${light.isOn} ${light.brightness}');
}

false 50

Initializing formals #

Writing this.name in the parameter list assigns the argument straight to the field. This is the form you will write most.

class Student {
  final String name;
  final int grade;

  Student(this.name, this.grade);
}

void main() {
  var s = Student('Sabina', 10);
  print('${s.name} is in grade ${s.grade}');
}

Sabina is in grade 10

It replaces the long version found in other languages:

Student(String name, int grade) {
  this.name = name;
  this.grade = grade;
}

Named parameters in constructors #

For classes with more than two or three fields, named parameters make the call readable. Flutter widgets are written this way.

class Product {
  final String name;
  final double price;
  final int stock;
  final String? description;

  Product({
    required this.name,
    required this.price,
    this.stock = 0,
    this.description,
  });
}

void main() {
  var p = Product(name: 'Notebook', price: 120);
  print('${p.name}: Rs. ${p.price}, ${p.stock} in stock');
}

Notebook: Rs. 120.0, 0 in stock

Named constructors #

A class can have several constructors as long as each has a different name. Use them to offer clear alternative ways of creating an object.

class Point {
  final double x;
  final double y;

  Point(this.x, this.y);
  Point.origin() : x = 0, y = 0;
  Point.onXAxis(this.x) : y = 0;
  Point.fromList(List<double> values) : x = values[0], y = values[1];

  @override
  String toString() => '($x, $y)';
}

void main() {
  print(Point(2, 3));
  print(Point.origin());
  print(Point.onXAxis(5));
  print(Point.fromList([7, 8]));
}

(2.0, 3.0)
(0.0, 0.0)
(5.0, 0.0)
(7.0, 8.0)

The initializer list #

The part after the colon runs before the constructor body. It is the only place you can set final fields from calculated values, and you can also put asserts there.

class Temperature {
  final double celsius;
  final double fahrenheit;

  Temperature(this.celsius)
      : assert(celsius >= -273.15, 'Below absolute zero'),
        fahrenheit = celsius * 9 / 5 + 32;
}

void main() {
  var t = Temperature(100);
  print(t.fahrenheit);
}

212.0

Redirecting constructors #

One constructor can hand its work to another with : this(...).

class Circle {
  final double radius;

  Circle(this.radius);
  Circle.unit() : this(1);
  Circle.fromDiameter(double diameter) : this(diameter / 2);
}

void main() {
  print(Circle.unit().radius);
  print(Circle.fromDiameter(10).radius);
}

1.0
5.0

A constructor body #

Add a body in braces when you need to run statements after the fields are set.

class Order {
  final List<double> prices;
  double total = 0;

  Order(this.prices) {
    for (final p in prices) {
      total += p;
    }
  }
}

void main() {
  print(Order([10, 20.5]).total);
}

30.5

Private constructors #

A constructor whose name starts with _ can only be called inside its own file. Combined with a static field, this makes a class with exactly one instance.

class Settings {
  Settings._();
  static final Settings instance = Settings._();

  String theme = 'light';
}

void main() {
  Settings.instance.theme = 'dark';
  print(Settings.instance.theme);
}

dark

Constructors that decide which object to return are called factory constructors, and are covered next.

Try it yourself #

Write a Book class with title, author and pages. Give it a main constructor using named parameters, and a named constructor Book.untitled(String author) that redirects to the main one with the title 'Untitled' and 0 pages.