NoSuchMethodError: The method 'isBefore' was called on null

Question

I try to filter the JSON data by date. I'm stuck with comparing the date. The input is a list from the GET request then convert to the actual data type to use( DateTime, String). After that, I will pick a range of dates then compare them to the date of an object in the list. I don't know why I always got errors about Null.

class BudgetDateRangePicker extends StatefulWidget {
  final List<Transaction> transactions;
  BudgetDateRangePicker({Key key, this.transactions}) : super(key: key);
  @override
  State<StatefulWidget> createState() => _BudgetDateRangePickerState();
}

class _BudgetDateRangePickerState extends State<BudgetDateRangePicker> {
  // DateTime _toDate = DateTime.now();
  // DateTime _fromDate = DateTime.now().subtract(Duration(days: 7));
  DateTime _toDate = DateTime.now();
  DateTime _fromDate = DateTime.now().subtract(Duration(days: 7));
  List<Transaction> trans = [];
  Future<void> _selectDate(BuildContext context) async {
    final List<DateTime> picked = await DateRangePicker.showDatePicker(
        context: context,
        initialLastDate: new DateTime.now(),
        initialFirstDate: (new  DateTime.now().subtract(Duration(days: 7))),
        firstDate: new DateTime(2015),
        lastDate: new DateTime(DateTime.now().year+1));
    if (picked != null && picked.length == 2) {
      setState(() {
        _fromDate = picked[0];
        _toDate = picked[1];
        //trans = widget.transactions.where((t)=>(t.date.millisecondsSinceEpoch >= picked[0].millisecondsSinceEpoch  ) && t.date.millisecondsSinceEpoch>=picked[1].millisecondsSinceEpoch).toList();
        for (var item in widget.transactions) {
          // var t = DateFormat('yyyy-MM-dd').parse(item.date);
          var t = item.date;
          if (t.isBefore(_toDate) &&
              t.isAfter(_fromDate)) {
            print(item);
            trans.add(item);
          }
        }
        }
      );
    }
  }

//Main class
body: TabBarView(
              controller: _tabController,
              children: <Widget>[
                new FutureBuilder<List<Transaction>>(
                  future: fetchBudgetTrans(new http.Client()),
                  builder: (context, snapshot) {
                    if (snapshot.hasError) print(snapshot.error);

                    if (snapshot.hasData) {
                      // BudgetDateRangePicker tl = new BudgetDateRangePicker(
                      //     transactions: snapshot.data);
                      BudgetDateRangePicker tl = new BudgetDateRangePicker(
                          transactions: snapshot.data);
                      return tl;
                    } else
                      return new Center(child: new CircularProgressIndicator());
                  },
                ),
//Model
class Transaction {
  final String to;
   DateTime date;
  final String amount;
  final String category;

  Transaction({this.to, this.date, this.amount, this.category});
  factory Transaction.fromJson(Map<String, dynamic> json) {
    return Transaction(
        to: json["to"] as String,
        date: json["date"] as DateTime,
        amount: json["amount"] as String,
        category: json["category"] as String);
  }
}
//GET REQUEST
Future<List<Transaction>> fetchBudgetTrans(http.Client client) async {
  final response =  await client.get(Uri.parse('http://192.168.191.1:8000/budget/trans'));
  // await client.get(Uri.parse('http://127.0.0.1:8000/budget/trans'));
  // Use the compute function to run parse response in a separate isolate
  return compute(parseDataBudget, response.body);
}


// A function that will convert a response body into a List<Stock>
List<Transaction> parseDataBudget(String responseBody) {
  final parsed = json.decode(responseBody).cast<Map<String, dynamic>>();
  return parsed
      .map<Transaction>((json) => new Transaction.fromJson(json))
      .toList();
}

Solution

in the Transaction.fromJson factory you override the type of json['date'] as DateTime, but it isn't, so instead you need to convert the json data to the native DateTime type. Usually something like DateTime.fromMillisecondsSinceEpoch(json['date']?.toInt() ?? 0) works well for me.

Comments

[Mahesh Jamdade]

can you add a sample response of the json in the question that you are trying to parse?

[NoName]

{ "amount": "-105", "date": "2020-01-09", "to": "Unknown" }, { "amount": "-28", "date": "2020-01-30", "to": "Unknown" }, { "amount": "-59", "date": "2020-01-01", "to": "AH" }, { "amount": "-124", "date": "2020-01-03", "to": "JUMBO" },

[NoName]

t.millisecondsSinceEpoch > _fromDate.millisecondsSinceEpoch So is this the way to compare date? (t from JSON, _fromDate is DateTime object)

[NoName]

I still got Unhandled Exception: NoSuchMethodError: The getter 'millisecondsSinceEpoch' was called on null.