Named Constructor in Dart

Named Constructor In Dart

In most programming languages like java, c++, c#, etc., we can create multiple constructors with the same name. But in Dart, this is not possible. Well, there is a way. We can create multiple constructors with the same name using named constructors.

Example 1: Named Constructor In Dart

In this example below, there is a class Student with three properties: name, age, and rollNumber. The class has two constructors. The first constructor is a default constructor. The second constructor is a named constructor. The named constructor is used to initialize the values of the three properties. We also have an object of the class Student called student.

class Student {
  String? name;
  int? age;
  int? rollNumber;

  // Default Constructor
  Student() {
    print("This is a default constructor");
  }

  // Named Constructor
  Student.namedConstructor(String name, int age, int rollNumber) {
    this.name = name;
    this.age = age;
    this.rollNumber = rollNumber;
  }
}

void main() {
  // Here student is object of class Student.
  Student student = Student.namedConstructor("John", 20, 1);
  print("Name: ${student.name}");
  print("Age: ${student.age}");
  print("Roll Number: ${student.rollNumber}");
}

This is a default constructor
Name: John
Age: 20
Roll Number: 1

Example 2: Named Constructor In Dart

In this example below, there is class Mobile with three properties name, color, and price. The class has one method display which prints out the values of the three properties. We also have an object of the class Mobile called mobile. There is also constructor Mobile which takes all the three properties as parameters. Named constructor Mobile.namedConstructor is used to create an object of the class Mobile with name, color and optional price. The default value of the price is 0. If the price is not passed, then the default value is used.

class Mobile {
  String? name;
  String? color;
  int? price;

  Mobile(this.name, this.color, this.price);
  // here Mobile() is a named constructor
  Mobile.namedConstructor(this.name, this.color, [this.price = 0]);

  void displayMobileDetails() {
    print("Mobile name: $name.");
    print("Mobile color: $color.");
    print("Mobile price: $price");
  }
}

void main() {
  var mobile1 = Mobile("Samsung", "Black", 20000);
  mobile1.displayMobileDetails();
  var mobile2 = Mobile.namedConstructor("Apple", "White");
  mobile2.displayMobileDetails();
}

Mobile name: Samsung.
Mobile color: Black.
Mobile price: 20000
Mobile name: Apple.
Mobile color: White.
Mobile price: 0

Example 3: Named Constructor In Dart

In this example below, there is a class Animal with two properties name and age. The class has three constructors. The first constructor is a default constructor. The second and third constructors are named constructors. The second constructor is used to initialize the values of name and age, and the third constructor is used to initialize the value of name only. We also have an object of the class Animal called animal.

class Animal {
  String? name;
  int? age;

  // Default Constructor
  Animal() {
    print("This is a default constructor");
  }

  // Named Constructor
  Animal.namedConstructor(String name, int age) {
    this.name = name;
    this.age = age;
  }

  // Named Constructor
  Animal.namedConstructor2(String name) {
    this.name = name;
  }
}
void main(){
  // Here animal is object of class Animal.
  Animal animal = Animal.namedConstructor("Dog", 5);
  print("Name: ${animal.name}");
  print("Age: ${animal.age}");

  Animal animal2 = Animal.namedConstructor2("Cat");
  print("Name: ${animal2.name}");
}

Name: Dog
Age: 5
Name: Cat

Example 4: Real Life Example Of Named Constructor In Dart

In this example below, there is a class Person with two properties name and age. The class has three constructors. The first is a parameterized constructor which takes two parameters name and age. The second and third constructors are named constructors. Second constructor fromJson is used to create an object of the class Person from a JSON. The third fromJsonString is used to create an object of the class Person from a JSON string. We also have an object of the class Person called person.

import 'dart:convert';

class Person {
  String? name;
  int? age;

  Person(this.name, this.age);

  Person.fromJson(Map<String, dynamic> json) {
    name = json['name'];
    age = json['age'];
  }

  Person.fromJsonString(String jsonString) {
    Map<String, dynamic> json = jsonDecode(jsonString);
    name = json['name'];
    age = json['age'];
  }
}

void main() {
// Here person is object of class Person.
  String jsonString1 = '{"name": "Bishworaj", "age": 25}';
  String jsonString2 = '{"name": "John", "age": 30}';

  Person p1 = Person.fromJsonString(jsonString1);
  print("Person 1 name: ${p1.name}");
  print("Person 1 age: ${p1.age}");

  Person p2 = Person.fromJsonString(jsonString2);
  print("Person 2 name: ${p2.name}");
  print("Person 2 age: ${p2.age}");
}

Person 1 name: Bishworaj
Person 1 age: 25
Person 2 name: John
Person 2 age: 30

Challenge

Try to create a class Car with three properties name, color, and price and one method display which prints out the values of the three properties. Create a constructor, which takes all 3 parameters. Create a named constructor which takes two parameters name and color. Create an object of the class from both the constructors and call the method display.

Video

Watch our video on default constructor in Dart.