Parsing double values from JSON in Flutter and Dart

Question

I have problems while I try to get double values from JSON in Flutter.

class MapPoint {
  String title;
  double lat;
  double lng;

  MapPoint({this.title, this.lat, this.lng});

  factory MapPoint.fromJson(Map<String, dynamic> json) {
    return MapPoint(
        title: json["title"] as String,
        lat: json["lat"] as double,
        lng: json["lng"] as double
    );
  }
}

For some reason I got error

Dart Error: Unhandled exception: type 'double' is not a subtype of type 'String'

I tried some to user double.parse(json["lng"]) instead but got same error.
Same time this way of getting data from JSON works fine with other types.

This is JSON example

{ 
   point: {
     title: "Point title",
     lat: 42.123456,
     lng: 32.26567
  }
}

What JSON causes this error? If your values are wrapped in quotes, they won't be interpreted as double. — Günter Zöchbauer
– Günter Zöchbauer, Commented Jan 20, 2019 at 14:07
JSON is fine because I can get all other values (sting and int) from it — moonvader
– moonvader, Commented Jan 20, 2019 at 14:09
Either the JSON doesn't fit your code or your code doesn't fit your JSON. Without providing the JSON that causes the error you can as well delete the question because it just doesn't provide the required information to diagnose the issue. — Günter Zöchbauer
– Günter Zöchbauer, Commented Jan 20, 2019 at 14:11
I added example of JSON. One thing that can be unusual is that double values are not presented as strings. — moonvader
– moonvader, Commented Jan 20, 2019 at 14:17
That is not valid JSON. In JSON keys must be quoted and there is a , missing after "Point title". Double values are never represented as String in JSON. — Günter Zöchbauer
– Günter Zöchbauer, Commented Jan 20, 2019 at 14:19

Gilson Cavalcanti · Accepted Answer · 2019-09-02 13:46:33Z

30

The Dart JSON parser converts the property inside json and apparently is clever enough to spit a double type.

someVariable as double expects a String at the left side.

Whats probably happening is that you're trying to convert a double into a double.

I would try something like this:

lat: json["lat"].toDouble(),

This would cover the case of the data in your JSON comes like "5". In this case the dart json converter would cast the type to int and it would break your code if you are always expecting a double.

edited Sep 2, 2019 at 13:46

answered Sep 2, 2019 at 12:29

Gilson Cavalcanti

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2 Comments

Hedron Dantas Over a year ago

lat: double.parse(json["lat"])

Sisir Over a year ago

If you are not sure whethe rthe type is int or double; better to declare it as num

Matheus Toniolli · Accepted Answer · 2020-06-15 20:26:15Z

8

I was having the same problem, the way I think it was this:

class MapPoint {
  String title;
  double lat;
  double lng;

  MapPoint({this.title, this.lat, this.lng});

  factory MapPoint.fromJson(Map<String, dynamic> json) {
    return MapPoint(
        title: json["title"] as String,
        lat: json["lat"] is int ? (json['lat'] as int).toDouble() : json['lat'],
        lng: json["lng"] is int ? (json['lng'] as int).toDouble() : json['lng']
    );
  }
}

answered Jun 15, 2020 at 20:26

Matheus Toniolli

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1 Comment

Fahmida Over a year ago

Thanks a lot. I have tried several ways, but this works :D

Alex Correia · Accepted Answer · 2022-09-16 22:23:00Z

6

Rewrite your fromJson method as follows:

factory MapPoint.fromJson(Map<String, dynamic> json) {
    return MapPoint(
        title: json["title"] as String,
        lat : double.parse(json["lat"].toString()),
        lng : double.parse(json["lng"].toString()),
    );
  }

If your lat field is nullable, for example:

double? lat

In your fromJson method, instead of:

lat : double.parse(json["lat"].toString()),

Use:

lat : double.tryParse(json["lat"].toString()),

edited Sep 16, 2022 at 22:23

answered Aug 6, 2022 at 0:38

Alex Correia

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1 Comment

Alex Correia Over a year ago

Hello @moonvader, please do consider on marking the answer as accepted, if it has helped to solve your problem (or to answer your question).

Günter Zöchbauer · Accepted Answer · 2019-01-20 14:21:24Z

2

I can not reproduce

void main() {
  final json = {
    "point": {"title": "Point title", "lat": 42.123456, "lng": 32.26567}
  };
  final p = MapPoint.fromJson(json);
  print(p);
}

class MapPoint {
  String title;
  double lat;
  double lng;

  MapPoint({this.title, this.lat, this.lng});

  factory MapPoint.fromJson(Map<String, dynamic> json) {
    return MapPoint(
        title: json["title"] as String,
        lat: json["lat"] as double,
        lng: json["lng"] as double);
  }
}

answered Jan 20, 2019 at 14:21

Günter Zöchbauer

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Comments

Uditha Chathura Banadara · Accepted Answer · 2021-05-22 22:27:30Z

2

i can fix this problem this way

class MapPoint {
  String title;
  double lat;
  double lng;

  MapPoint({this.title, this.lat, this.lng});

  factory MapPoint.fromJson(Map<String, dynamic> json) {
    return MapPoint(
        title: json["title"] as String,
        lat: json["lat"] *1.0,
        lng: json["lng"] *1.0
    );
  }
}

answered May 22, 2021 at 22:27

Uditha Chathura Banadara

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