Unhandled Exception: NoSuchMethodError: The method '[]' was called on null (json parse)

Question

我正在尝试解析打印 'confirmed'

的 json

{
  "Delhi": {
    "districtData": {
      "East Delhi": {
        "confirmed": 1,
        "lastupdatedtime": ""
      },
      "South West Delhi": {
        "confirmed": 3,
        "lastupdatedtime": ""
      },
      "West Delhi": {
        "confirmed": 2,
        "lastupdatedtime": ""
      },
      "North Delhi": {
        "confirmed": 3,
        "lastupdatedtime": ""
      },
      "New Delhi": {
        "confirmed": 3,
        "lastupdatedtime": ""
      },
      "South Delhi": {
        "confirmed": 2,
        "lastupdatedtime": ""
      },
      "North East Delhi": {
        "confirmed": 1,
        "lastupdatedtime": ""
      },
      "North West Delhi": {
        "confirmed": 3,
        "lastupdatedtime": ""
      },
      "Unknown": {
        "confirmed": 53,
        "lastupdatedtime": ""
      },
      "Delhi": {
        "confirmed": 1,
        "lastupdatedtime": ""
      }
    }
  }
}

https://api.covid19india.org/state_district_wise.json 是实际的 json 数据并且只反序列化了德里数据，因为我想测试它是对还是错。就像

class IndiaState {
  Delhi delhi;

  IndiaState({
    this.delhi,
  });

  factory IndiaState.fromJson(Map<String, dynamic> json) => IndiaState(
    delhi: Delhi.fromJson(json["Delhi"]),
  );
}

class Delhi {
  DelhiDistrictData districtData;

  Delhi({
    this.districtData,
  });

  factory Delhi.fromJson(Map<String, dynamic> json) => Delhi(
    districtData: DelhiDistrictData.fromJson(json["districtData"]),
  );
}

class DelhiDistrictData {
  DelhiValue eastDelhi;
  DelhiValue southWestDelhi;
  DelhiValue westDelhi;
  DelhiValue delhi;
  DelhiValue southDelhi;
  DelhiValue northEastDelhi;
  DelhiValue northDelhi;
  DelhiValue northWestDelhi;
  DelhiValue unknown;
  DelhiValue newDelhi;

  DelhiDistrictData({
    this.eastDelhi,
    this.southWestDelhi,
    this.westDelhi,
    this.delhi,
    this.southDelhi,
    this.northEastDelhi,
    this.northDelhi,
    this.northWestDelhi,
    this.unknown,
    this.newDelhi,
  });

  factory DelhiDistrictData.fromJson(Map<String, dynamic> json) => DelhiDistrictData(
    eastDelhi: DelhiValue.fromJson(json["East Delhi"]),
    southWestDelhi: DelhiValue.fromJson(json["South West Delhi"]),
    westDelhi: DelhiValue.fromJson(json["West Delhi"]),
    delhi: DelhiValue.fromJson(json["Delhi"]),
    southDelhi: DelhiValue.fromJson(json["South Delhi"]),
    northEastDelhi: DelhiValue.fromJson(json["North East Delhi"]),
    northDelhi: DelhiValue.fromJson(json["North Delhi"]),
    northWestDelhi: DelhiValue.fromJson(json["North West Delhi"]),
    newDelhi: DelhiValue.fromJson(json["New Delhi"]),
    unknown: DelhiValue.fromJson(json["Unknown"]),
  );
}

class DelhiValue {
  int confirmed;
  String lastupdatedtime;


  DelhiValue({
    this.confirmed,
    this.lastupdatedtime,
  });

  factory DelhiValue.fromJson(Map<String, dynamic> json) => DelhiValue(
    confirmed: json['confirmed'],
    lastupdatedtime: json["lastupdatedtime"],
  );
}

现在我正在尝试将确认的结果打印在德里的任何一个地区

  @override
  void initState() {
    // TODO: implement initState
    setState(() {
      isLoading = true;
    });

    givenFunction();

    setState(() {
      isLoading = false;
    });
  }

  Future givenFunction() async {
    final httpRequest = await http.get(districtAPI);
    final json = jsonDecode(httpRequest.body);
    IndiaState firstObject = new IndiaState.fromJson(json);
    print(firstObject.delhi.districtData.eastDelhi.confirmed.toString());
  }

现在我尝试打印 eastDelhi 确认数据时出现错误

E/flutter ( 5895): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method '[]' was called on null.
E/flutter ( 5895): Receiver: null
E/flutter ( 5895): Tried calling: []("confirmed")
E/flutter ( 5895): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
E/flutter ( 5895): #1      new DelhiValue.fromJson (package:coraona2903/models/district_state.dart:77:20)
E/flutter ( 5895): #2      new DelhiDistrictData.fromJson (package:coraona2903/models/district_state.dart:54:23)
E/flutter ( 5895): #3      new Delhi.fromJson (package:coraona2903/models/district_state.dart:21:37)
E/flutter ( 5895): #4      new IndiaState.fromJson (package:coraona2903/models/district_state.dart:9:18)
E/flutter ( 5895): #5      Covid19ScreenState.givenFunction (package:coraona2903/screens/covid_19_screen.dart:37:34)
E/flutter ( 5895): <asynchronous suspension>

Answer 1

只需检查修改后的代码：

以下是您提供的本地json：

{
    "Delhi": {
      "districtData": {
        "East Delhi": {
          "confirmed": 1,
          "lastupdatedtime": ""
        },
        "South West Delhi": {
          "confirmed": 3,
          "lastupdatedtime": ""
        },
        "West Delhi": {
          "confirmed": 2,
          "lastupdatedtime": ""
        },
        "North Delhi": {
          "confirmed": 3,
          "lastupdatedtime": ""
        },
        "New Delhi": {
          "confirmed": 3,
          "lastupdatedtime": ""
        },
        "South Delhi": {
          "confirmed": 2,
          "lastupdatedtime": ""
        },
        "North East Delhi": {
          "confirmed": 1,
          "lastupdatedtime": ""
        },
        "North West Delhi": {
          "confirmed": 3,
          "lastupdatedtime": ""
        },
        "Unknown": {
          "confirmed": 53,
          "lastupdatedtime": ""
        },
        "Delhi": {
          "confirmed": 1,
          "lastupdatedtime": ""
        }
      }
    }
  }

后来我为此创建了一个模型 class :

// To parse this JSON data, do
//
//     final indiaState = indiaStateFromJson(jsonString);

import 'dart:convert';

IndiaState indiaStateFromJson(String str) => IndiaState.fromJson(json.decode(str));

String indiaStateToJson(IndiaState data) => json.encode(data.toJson());

class IndiaState {
    Delhi delhi;

    IndiaState({
        this.delhi,
    });

    factory IndiaState.fromJson(Map<String, dynamic> json) => IndiaState(
        delhi: Delhi.fromJson(json["Delhi"]),
    );

    Map<String, dynamic> toJson() => {
        "Delhi": delhi.toJson(),
    };
}

class Delhi {
    Map<String, DistrictDatum> districtData;

    Delhi({
        this.districtData,
    });

    factory Delhi.fromJson(Map<String, dynamic> json) => Delhi(
        districtData: Map.from(json["districtData"]).map((k, v) => MapEntry<String, DistrictDatum>(k, DistrictDatum.fromJson(v))),
    );

    Map<String, dynamic> toJson() => {
        "districtData": Map.from(districtData).map((k, v) => MapEntry<String, dynamic>(k, v.toJson())),
    };
}

class DistrictDatum {
    int confirmed;
    String lastupdatedtime;

    DistrictDatum({
        this.confirmed,
        this.lastupdatedtime,
    });

    factory DistrictDatum.fromJson(Map<String, dynamic> json) => DistrictDatum(
        confirmed: json["confirmed"],
        lastupdatedtime: json["lastupdatedtime"],
    );

    Map<String, dynamic> toJson() => {
        "confirmed": confirmed,
        "lastupdatedtime": lastupdatedtime,
    };
}

下面是我在列表视图中显示您的数据的主要文件，请查看：

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:sample_project_for_api/model.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  bool isLoading = false;
  List<Cities> citilesList = List();

  // here i have taken the  json locally which you posted on stack 
  Future<String> loadFromAssets() async {
    return await rootBundle.loadString('json/parse.json');
  }

  @override
  void initState() {
    super.initState();
    givenFunction();
  }

  Future givenFunction() async {
    setState(() {
      isLoading = true;
    });

    //final httpRequest = await http.get(districtAPI);
    //final json = jsonDecode(httpRequest.body);
    // you can make the http call above just uncomment is and comment the below line
    String jsonString = await loadFromAssets();
    // Here you can just replace down your httpRequest.body with jsonString
    final indiaState = indiaStateFromJson(jsonString);
    indiaState.delhi.districtData.forEach((key, value) {
      // This column is show you the values are getting fetched and printed below
      print('This is the key : ' + key);
      print('Confirmed :' + value.confirmed.toString());
      citilesList.add(Cities(key, value.confirmed));
    });
    setState(() {
      isLoading = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: isLoading
            ? CircularProgressIndicator()
            : ListView.builder(
                itemCount: citilesList.length,
                itemBuilder: (BuildContext context, int index) {
                  return Container(
                    child: Card(
                      margin:
                          EdgeInsets.symmetric(vertical: 10, horizontal: 10),
                      child: Padding(
                        padding: const EdgeInsets.all(15.0),
                        child: Row(
                          children: <Widget>[
                            Text(citilesList[index].state + ": "),
                            Text(citilesList[index].confirmed.toString())
                          ],
                        ),
                      ),
                    ),
                  );
                },
              ),
      ),
    );
  }
}

void main() {
  runApp(HomePage());
}

class Cities {
  final String state;
  final int confirmed;

  Cities(this.state, this.confirmed);
}

刚刚为数据获取项添加了示例图像：让我知道它是否有效

Unhandled Exception: NoSuchMethodError: The method '[]' was called on null (json parse)

Unhandled Exception: NoSuchMethodError: The method '[]' was called on null (json parse)

json

jsonp

dart

flutter