Not truly final fields

fields.dart

class A {
  A(this.text);

  final String? text; // Final field
}

class B implements A {
  bool _first = true;

  @override
  // Not final anymore
  String? get text {
    if (_first) {
      _first = false;
      // On the first call return null
      return 'Some string';
    } else {
      // On the next calls return null
      return null;
    }
  }
}

void main() {
  final A example = B();
  print(example.text); // Prints 'Some string'
  print(example.text); // Prints null
}