Constructor

You can create an object using a constructor.
Constructor names can be either ClassName or ClassName.identifier.

var p1 = Point(2, 2); var p2 = Point.fromJson({'x': 1, 'y': 2});

Code creates Point objects using the Point() and Point.fromJson() constructors.

The new keyword became optional in Dart 2.
Some classes provide constant constructors.

var p = const ImmutablePoint(2, 2);

To create a compile-time constant using a constant constructor, put the const keyword before the constructor name.

Constructing two identical compile-time constants results in a single, canonical instance.

var a = const ImmutablePoint(1, 1); var b = const ImmutablePoint(1, 1); assert(identical(a, b)); // They are the same instance!

The most common form of constructor, the generative constructor, creates a new instance of a class.

class Point { num x, y; // Syntactic sugar for setting x and y before the constructor body runs. Point(this.x, this.y); }

class Point { num x, y; Point(num x, num y) { // There's a better way to do this. this.x = x; this.y = y; } }

Constructors aren’t inherited.

Subclasses don’t inherit constructors from their superclass.

A subclass that declares no constructors has only the default (no argument, no name) constructor.

class Person { 
   String firstName; 

   Person.fromJson(Map data) { 
   print('in Person'); 
   } 
} 

class Employee extends Person { 
   // Person does not have a default constructor; you must call super.fromJson(data). 
   Employee.fromJson(Map data) : super.fromJson(data) { 
   print('in Employee'); 
   } 
} 

main() { 
   var emp = new Employee.fromJson({}); 

   // Prints: 
   // in Person 
   // in Employee 
   if (emp is Person) { 
      // Type check 
      emp.firstName = 'Bob'; 
   } 
   (emp as Person).firstName = 'Bob'; 
}

Sometimes a constructor’s only purpose is to redirect to another constructor in the same class.

A redirecting constructor’s body is empty, with the constructor call appearing after a colon (:).

class Point { num x, y; // The main constructor for this class. Point(this.x, this.y); // Delegates to the main constructor. Point.alongXAxis(num x) : this(x, 0); }

Invoking a non-default superclass constructor

Redirecting constructors

Related concepts

Constructor — Structure map

Constructor — Related pages:

Classes

Generics

Metadata

dart:core

dart:math

Functions