singleton_class_example.dart

// Example of a singleton class
class DatabaseHelper {
  // Singleton databasehelper
  static DatabaseHelper? _databaseHelper;

  // Named constructor to create an instance
  DatabaseHelper._createInstance();

  // returns new instance if and only there are not any existing instances!!!
  factory DatabaseHelper() {
    _databaseHelper ??= DatabaseHelper._createInstance();
    return _databaseHelper!;
  }
}

// Not a singleton class
class Note {
  final int id;
  final String title;

  Note(this.id, this.title);
}

void main() {
  DatabaseHelper databaseHelper1 = DatabaseHelper();
  DatabaseHelper databaseHelper2 = DatabaseHelper();
  DatabaseHelper databaseHelper3 = DatabaseHelper();

  print(databaseHelper1.hashCode);
  print(databaseHelper2.hashCode);
  print(databaseHelper3.hashCode);

  Note note1 = Note(1, "First Note");
  Note note2 = Note(2, "Second Note");

  // as they are not maintaining singleton pattern therefore their hashCode will be different
  print(note1.hashCode);
  print(note2.hashCode);
}