Constructor

In Dart, constructors are special methods used to initialize objects of a class. //constructor same name as class

They allow you to set initial values to an object's properties when it is created. 

calling with class itself

Dart provides several types of constructors:

void main()

{

 var student1=Student();

 student1.id=1;

 student1.name='Sandeep';

 student1.study();   //calling behavour/method

}

//In Class- Define states(Properties) and behaviour of a student

class Student

{

 int? id;        //property of Student

 String? name;        //property of Student

 Student(){  //constructor same name as class

   print('This is constructor i.e calling with class itself');

 }

 void study() //Behaviour of Student

 {

   print('${this.name} is now studing');

 }

}

Default Constructor

The default constructor is automatically created by Dart if no other constructor is defined (using class name by default)

It has no parameters and simply creates an instance of the class.

class Person {

 String? name;

 int? age;

 Person(){

   print('This is also a proof of constructor/Method'); //called itself firstly

 }

}

void main() {

 var person = Person(); // Calls the default constructor (class name/Method)

 print("Name: ${person.name}, Age: ${person.age}");

}

2. Named Constructor

Dart allows you to create multiple constructors for a class by using named constructors. These are helpful when you need multiple ways to initialize an object.

class Person {

 String? name;

 int? age;

 Person.named(this.name, this.age);  // Named constructor

 Person.withDefault()         // Another named constructor

{        

   name = "John Doe";

   age = 30;

 }

}

void main() {

 var person1 = Person.named("Alice", 25);

 print("Name: ${person1.name}, Age: ${person1.age}");

 var person2 = Person.withDefault();

 print("Name: ${person2.name}, Age: ${person2.age}");

}

3. Parameterized Constructor

//A parameterized constructor allows you to pass parameters to initialize an object.

class Person {

 String name;

 int age;

 // Parameterized constructor

 Person(this.name, this.age);

}

void main() {

 var person = Person("Bob", 40);

 print("Name: ${person.name}, Age: ${person.age}");

}

4. Named Optional or Default Values Constructor

You can use named or optional parameters in constructors to make initialization more flexible.

class Person {

  String name;

  int age;

  // Constructor with named optional parameters

  Person({this.name = "Unknown", this.age = 0});

}

void main() {

  var person1 = Person(name: "Charlie");

  print("Name: ${person1.name}, Age: ${person1.age}");

  var person2 = Person(age: 35);

  print("Name: ${person2.name}, Age: ${person2.age}");

}

5. Redirecting Constructor

A constructor can redirect to another constructor in the same class to reuse initialization logic.

class Person {

  String name;

  int age;

  // Main constructor

  Person(this.name, this.age);

  // Redirecting constructor

  Person.defaultPerson() : this("Default Name", 18);

}

void main() {

  var person = Person.defaultPerson();

  print("Name: ${person.name}, Age: ${person.age}");

}

6. Factory Constructor

A factory constructor is used when a constructor needs to return a new instance or an existing instance (e.g., singleton patterns).

class Singleton {

  static final Singleton _instance = Singleton._internal();

  // Private named constructor

  Singleton._internal();

  // Factory constructor

  factory Singleton() {

    return _instance;

  }

}

void main() {

  var obj1 = Singleton();

  var obj2 = Singleton();

  print(identical(obj1, obj2)); // true (same instance)

}

Dart constructors provide great flexibility, enabling you to create robust and customizable object initialization for your classes. Let me know if you'd like examples for a specific use case!