Flutter: problem in fetching data: type 'Null' is not a subtype of type 'String' error

Issue

I am trying to fetch google book search api data.
https://www.googleapis.com/books/v1/volumes?q=%EA%B2%BD%EC%A0%9C

I followed this one:
https://docs.flutter.dev/cookbook/networking/fetch-data

My class:

class Book {
  final String id;
  final String title;
  final List<String> authors;

  const Book({
    required this.id,
    required this.title,
    required this.authors,
  });

  factory Book.fromJson(Map json) {
    return Book(
      id: json['id'],
      title: json['title'],
      authors: json['author'],
    );
  }
}

request data:

  late Future<List<Book>> futureBooks;
  Future<List<Book>> fetchBooks() async {
    Uri url = Uri.parse(
        'https://www.googleapis.com/books/v1/volumes?q=경제 경영'); //&maxResults=1
    final response = await http.get(url);
    if (response.statusCode == 200) {
      var json = jsonDecode(response.body);
      List<dynamic> items = json['items'];
      List<Book> books = (items.map((item) {
        return Book.fromJson(item);
      })).toList();
      return books;
    } else {
      throw Exception('Failed to load Books');
    }
  }

  @override
  void initState() {
    super.initState();
    futureBooks = fetchBooks();
  }

I think I have same issue with this.
How to solve the "Type Null is not a subtype of type …" error?

So I appended [?] for fields.

class Book {
  final String? id;
  final String? title;
  final List<String>? authors;

It still give me null.

my code:
https://github.com/kangsudal/millie/blob/7f1f912c5a0eba0fe09de67c1c729be73b660da1/lib/screens/0_today/tab_widdget/tab_now.dart#L62

how to get data?

Solution

because title and author is not inside item object, it inside volumeInfo, so you much change fromJson method of your Book class to

factory Book.fromJson(Map json) {
    return Book(
      id: json['id'],
      title: json['volumeInfo']['title'],
      authors: json['volumeInfo']['author'],
    );
  }

Answered By – manhtuan21

Answer Checked By – Mildred Charles (FlutterFixes Admin)