Working with optional fields in Swift vs Dart

coffee.dart

class Coffee {
  String? _temperature;

  void heat() {
    _temperature = 'hot';
  }

  void chill() {
    _temperature = 'iced';
  }

  void checkTemp() {
    var temperature = _temperature;
    if (temperature != null) {
      print('Ready to serve ' + temperature + '!');
    }
  }

  String serve() => _temperature! + ' coffee';
}

Coffee.swift

struct Coffee {
    var temperature: String?
    
    mutating func heat() {
        temperature = "hot"
    }
    
    mutating func chill() {
        temperature = "iced"
    }
    
    func checkTemp() {
        guard let unwrappedTemperature = temperature else {
            print("Temperature is unknown :(")
            return
        }
        print("Ready to serve " + unwrappedTemperature + "!")
    }
    
    func serve() -> String {
        temperature! + " coffee";
    }
}

let myCoffee = Coffee()

myCoffee.checkTemp()

https://dart.dev/null-safety/understanding-null-safety#working-with-nullable-fields