void main() {
  // ? Operator
  Coke myCoke = Coke(volume: 600);
  // this is unsafe as anyone can change volume before printing.
  // for example, myCoke = null
  // then it gonna throw an error.

  print(myCoke.volume); // 600

  // so let's use ? as null check operator to ensure this won't happen
  // means if it is null, if null, stop and return null
  // otherwise continue ".volumne"

  myCoke = null;
  print(myCoke?.volume); // null

  // ?? Operator
  // To set default value
  print(myCoke?.volume ?? '550'); // 550

  myCoke = Coke(volume: 500);
  myCoke.volume = 700;
  print(myCoke?.volume ?? '550'); // 700;
}

class Coke {
  // 300 ml
  int volume = 330;
  Coke({ this.volume });
}