Dart-理解空安全中的的操作符

class Person {
  Children children;
  work() {
    print("人在工作");
  }

  toString() {
    return "这是一个人";
  }
}

class Children {
  eat() {
    print("孩子吃饭啦");
  }
}

void main() {
  Person? p;
}

Person? p;
p.work();

void main() {
  Person? p;
  /// 跳过运行
  p.work();
  print("程序结束");
}

class HttpResponse {
  final int code;
  final String? error;

  HttpResponse.ok()
      : code = 200,
        error = null;
  HttpResponse.notFound()
      : code = 404,
        error = 'Not found';

  @override
  String toString() {
    if (code == 200) return 'OK';
    return 'ERROR $code ${error.toUpperCase()}';
  }
}

String toString() {
  if (code == 200) return 'OK';
  return 'ERROR $code ${error!.toUpperCase()}';
}