program to create a constructor of class
constructor is a special member function of class that is called when an object of class is created

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

  // Constructor
  Student(String name, int age, int rollNumber) {
    print(
        "Constructor called"); // this is for checking the constructor is called or not.
    this.name = name;
    this.age = age;
    this.rollNumber = rollNumber;
  }
}

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

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Another example of creating a constructor

class Teacher {
  String? name;
  int? age;
  String? subject;
  double? salary;

  // Constructor
  Teacher(String name, int age, String subject, double salary) {
    this.name = name;
    this.age = age;
    this.subject = subject;
    this.salary = salary;
  }
  // Method
  void display() {
    print("Name: ${this.name}");
    print("Age: ${this.age}");
    print("Subject: ${this.subject}");
    print("Salary: ${this.salary}\n"); // \n is used for new line
  }
}

void main() {
  // Creating teacher1 object of class Teacher
  Teacher teacher1 = Teacher("John", 30, "Maths", 50000.0);
  teacher1.display();

  // Creating teacher2 object of class Teacher
  Teacher teacher2 = Teacher("Smith", 35, "Science", 60000.0);
  teacher2.display();
}

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Another example of creating a constructor

 class Car {
  String? name;
  double? prize;

  // Constructor
  Car(String name, double prize) {
    this.name = name;
    this.prize = prize;
  }

  // Method
  void display() {
    print("Name: ${this.name}");
    print("Prize: ${this.prize}");
  }
}

void main() {
  // Here car is object of class Car.
  Car car = Car("BMW", 500000.0);
  car.display();
}

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Another example of creating a constructor
In the following code phone2 data member will be set to null

 class Staff {
  String? name;
  int? phone1;
  int? phone2;
  String? subject;

  // Constructor
  Staff(String name, int phone1, String subject) {
    this.name = name;
    this.phone1 = phone1;
    this.subject = subject;
  }

  // Method
  void display() {
    print("Name: ${this.name}");
    print("Phone1: ${this.phone1}");
    print("Phone2: ${this.phone2}");
    print("Subject: ${this.subject}");
  }
}

void main() {
  // Here staff is object of class Staff.
  Staff staff = Staff("John", 1234567890, "Maths");
  staff.display();
}

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Program to create a single line constructor

class Person{
  String? name;
  int? age;
  String? subject;
  double? salary;

  // Constructor in short form
  Person(this.name, this.age, this.subject, this.salary);

  // display method
  void display(){
    print("Name: ${this.name}");
    print("Age: ${this.age}");
    print("Subject: ${this.subject}");
    print("Salary: ${this.salary}");
  }
}

void main(){
  Person person = Person("John", 30, "Maths", 50000.0);
  person.display();
}

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Program to create a constructor with optional parameters

class Employee {
  String? name;
  int? age;
  String? subject;
  double? salary;

  // Constructor
  Employee(this.name, this.age, [this.subject = "N/A", this.salary=0]);

  // Method
  void display() {
    print("Name: ${this.name}");
    print("Age: ${this.age}");
    print("Subject: ${this.subject}");
    print("Salary: ${this.salary}");
  }
}

void main(){
  Employee employee = Employee("John", 30);
  employee.display();
}

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constructor with default values

class Table {
  String? name;
  String? color;

  // Constructor
  Table({this.name = "Table1", this.color = "White"});

  // Method
  void display() {
    print("Name: ${this.name}");
    print("Color: ${this.color}");
  }
}

void main(){
  Table table = Table();
  table.display();
}

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example of default constructor

class Laptop {
  String? brand;
  int? prize;

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

void main() {
  // Here laptop is object of class Laptop.
  Laptop laptop = Laptop();
}

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Example of parameterized constructor, constructor that has parameters

 class Student {
      String? name;
      int? age;
      int? rollNumber;
      // Constructor
      Student(this.name, this.age, this.rollNumber);
    }
    
    void main(){
        // Here student is object of class Student. 
        Student student = Student("John", 20, 1);
        print("Name: ${student.name}");
        print("Age: ${student.age}");
        print("Roll Number: ${student.rollNumber}");
    }
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constructor with named parameters

   class Student {
      String? name;
      int? age;
      int? rollNumber;
    
      // Constructor
      Student({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(name: "John", age: 20, rollNumber: 1);
        print("Name: ${student.name}");
        print("Age: ${student.age}");
        print("Roll Number: ${student.rollNumber}");
    }

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parameterized constructor with default values

   class Student {
      String? name;
      int? age;
    
      // Constructor
      Student({String? name = "John", int? age = 0}) {
        this.name = name;
        this.age = age;
      }
    }
    
    void main(){
        // Here student is object of class Student. 
        Student student = Student();
        print("Name: ${student.name}");
        print("Age: ${student.age}");
    }

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example of constructor with named parameters

class Chair {
String? name;
String? color;

// Constructor
Chair({this.name, this.color});

// Method
void display() {
  print("Name: ${this.name}");
  print("Color: ${this.color}");
}
}

void main(){
Chair chair = Chair(name: "Chair1", color: "Red");
chair.display();
}

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program to create a named constructor which means constructor has a name

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}");
}

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Another example of named constructor

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

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

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

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

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program to create two named constructors

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}");
}

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program to create a constant constructor

class Point {
  final int x;
  final int y;

  const Point(this.x, this.y);
}

void main() {
  // p1 and p2 has the same hash code.
  Point p1 = const Point(1, 2);
  print("The p1 hash code is: ${p1.hashCode}");

  Point p2 = const Point(1, 2);
  print("The p2 hash code is: ${p2.hashCode}");
  // without using const
  // this has different hash code.
  Point p3 = Point(2, 2);
  print("The p3 hash code is: ${p3.hashCode}");

  Point p4 = Point(2, 2);
  print("The p4 hash code is: ${p4.hashCode}");
}

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Another example of constant constructor

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

  // Constant Constructor
  const Student({this.name, this.age, this.rollNumber});
}

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

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constant constructor with named parameters

class Car {
  final String? name;
  final String? model;
  final int? prize;

  // Constant Constructor
  const Car({this.name, this.model, this.prize});
}

void main() {
  // Here car is object of class Car.
  const Car car = Car(name: "BMW", model: "X5", prize: 50000);
  print("Name: ${car.name}");
  print("Model: ${car.model}");
  print("Prize: ${car.prize}");
}

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