How to create Singleton class and Setter , Getter in dart language

 Singleton class And Setter, Getter

// This is the main () function 

void main() {

  // This constructor calls the factory constructor,

  // which turns around and returns the static instance

  // which was initialized with the `_internal` named constructor

  //This will be true if the two instances have the same hashcode

  // (hint: they do)

  print(Student().hashCode == Student().hashCode);

  //*****************************************

  // Create an object of Student class

  Student st =  Student();

  // setting values to the object using a setter

  st.firstName = "Surya Prakash";

  st.lastName = "Yadav";

  st.age = 28;

  // Display the values of the object

  print("Full Name: ${st.fullName}");

  print("Age: ${st.age}");

}

// Create Student Class

//****************************************************************************

class Student {

  

  //Create singleton constructor

  Student._primaryConstructor();

   static final Student _instance=Student._primaryConstructor();

// Create Factory Method

  factory Student(){

    return _instance;

  }

  

  // Private Properties

  String? _firstName;

  String? _lastName;

  int? _age;

  // Getter to get full name

  String get fullName => this._firstName! + " " + this._lastName!;

  // Getter to read private property _age

  int get age => this._age!;

  // Setter to update private property _firstName

  set firstName(String firstName) => this._firstName = firstName;

  // Setter to update private property _lastName

  set lastName(String lastName) => this._lastName = lastName;

  // Setter to update private property _age

  set age(int age) {

    if (age < 0) {

      throw  Exception("Age can't be less than 0");

    }

    this._age = age;

  }

}