Constructor types

Learn named and default constructors.

What is a constructor?

A constructor is a special method called when a new instance is created. In Dart the constructor shares the class name; no return type is written.

Example: class User { String name; User(this.name); }. Here User(this.name) is the default constructor; the this.name shortcut assigns the parameter directly to the field. The constructor is called by the class name without new: User('Alex'). A Dart class can have at most one default constructor; for more, use a named constructor.

Default and named

Dart has two main constructor kinds. A default constructor shares the class name and assigns parameters directly to fields: User(this.name). Only one is allowed per class.

When a second constructor is needed, use a named constructor: User.guest() : name = 'Guest';. The .guest suffix gives the constructor a name. A class can hold one default and many named constructors side by side; this is Dart's replacement for overloading.

Exercise Define the User class

Define a class named User. Add a String name field. Write a default constructor of the form User(this.name). In main, create a User('Alex') instance and print its name field with print.

Show example solution

// Define the User class
class User {
  String name;
  User(this.name);
}

void main() {
  var u = User('Alex');
  print(u.name);
}

Test yourself

Which is a valid Dart constructor declaration?
- User(this.name);
- function User(this.name);
- constructor User(this.name);
- new User(this.name);
Show answer

User(this.name); — A constructor is declared as a method with the same name as the class; no extra keyword is needed.

Complete the constructor body

class User {
  String name;
  User(____);
}

Show answer

class User {
  String name;
  User(this.name);
}

Match the constructor parts

User

Class and constructor name
this.name

Field assignment shortcut
User('Ayşe')

Instance creation call

Next lesson: Named constructors

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