Flutter: TypeError when I try to access elements of JSON data

Question

I am trying to display individual elements of a JSON data on a dynamic listview. However, I keep getting "type 'int' is not a subtype of type 'String'" error and I have no idea why.

The code works if I include just the left() function in the widget located under the Row in the buildFlightsColumn function. But once I include the middle() and right() functions, I get the error.

Widget buildListView() {
    print(data);
    return ListView.builder(
      itemCount: data == null ? 0 : data.length,
      itemBuilder:  (context, index) {
        return buildFlightsColumn(data[index]); 
      }
    );
  }


  Widget buildFlightsColumn(dynamic item) => Container( 
    height: 150.0, 
    decoration: BoxDecoration(
    ),
    child: new Row( 
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: <Widget>[ 
        left(item['PlaceId']),
        middle(item['IataCode']),
        right()
      ],
    ),
  );

  Container left(dynamic item) {
    return new Container (
      child: Text(   
        item,
        textAlign: TextAlign.left,
        style: TextStyle(
          fontSize: 25.0,
          color: Colors.red,
        )
      ),
    );
  }

  Container middle(dynamic item) {
    return new Container( 
      child: Text(   
        item,
        textAlign: TextAlign.left,
        style: TextStyle(
          fontSize: 25.0,
          color: Colors.red,
        )
      ),
    );
  }

  Container right() {
    return new Container(
      child: RaisedButton( 
        onPressed: () { 
        },
        child: Text('Book Flights'),
      )
    );
  }

The data passed into the buildFlightsColumn function is JSON data returned by the API request:

[{PlaceId: 65368, IataCode: LAX, Name: Los Angeles International, Type: Station, SkyscannerCode: LAX, CityName: Los Angeles, CityId: LAXA, CountryName: United States}, {PlaceId: 81727, IataCode: SFO, Name: San Francisco International, Type: Station, SkyscannerCode: SFO, CityName: San Francisco, CityId: SFOA, CountryName: United States}]

Please read Under what circumstances may I add “urgent” or other similar phrases to my question, in order to obtain faster answers? - the summary is that this is not an ideal way to address volunteers, and is probably counterproductive to obtaining answers. Please refrain from adding this to your questions. — halfer
– halfer, Commented Mar 28, 2020 at 21:39

Ahmed Khattab · Accepted Answer · 2020-03-28 15:59:09Z

1

Text widgets cannot display int s, they can only interpet strings , so your error is coming from this code

  Widget buildFlightsColumn(dynamic item) => Container( 
    height: 150.0, 
    decoration: BoxDecoration(
    ),
    child: new Row( 
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: <Widget>[ 
        left(item['PlaceId']) // item['placeId'] is int,
        middle(item['IataCode']),
        right()
      ],
    ),
  );

  Container left(dynamic item) {
    return new Container (
      child: Text(   
        item, // here item is int, which is not allowed <-----------------------
        textAlign: TextAlign.left,
        style: TextStyle(
          fontSize: 25.0,
          color: Colors.red,
        )
      ),
    );
  }

you can change it number to string using the .toString() method, or string interpolation


  Container left(dynamic item) {
    return new Container (
      child: Text(   
        item.toString(), // here item is String <-----------------------
        textAlign: TextAlign.left,
        style: TextStyle(
          fontSize: 25.0,
          color: Colors.red,
        )
      ),
    );
  }

answered Mar 28, 2020 at 15:59

Ahmed Khattab

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1 Comment

Ahmed Khattab Over a year ago

please accept the answer if it solves your question, thanks

Morez · Accepted Answer · 2020-03-28 16:03:13Z

0

I think the problem is that PlaceId is of type int but you try to use it as a String.

Change your code like this:

Container left(dynamic item) {
return new Container (
  child: Text(   
    item.toString(), //change the type here
    textAlign: TextAlign.left,
    style: TextStyle(
      fontSize: 25.0,
      color: Colors.red,
    )
  ),
);
}

Or like this:

 Widget buildFlightsColumn(dynamic item) => Container( 
height: 150.0, 
decoration: BoxDecoration(
),
child: new Row( 
  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
  children: <Widget>[ 
    left(item['PlaceId'].toString()) // convert item['placeId'] to String
    middle(item['IataCode']),
    right()
  ],
),

);

answered Mar 28, 2020 at 16:03

Morez

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Comments

Mohith7548 · Accepted Answer · 2020-03-28 17:27:49Z

0

Coming to your question, the Flutter Text widget accepts only the String data type. Try using x.toString() to convert int to String or use "$x" or "${x}" in Text widget.

To be honest, it is actually a bad practice to use JSON objects as it is in the flutter code. You should consider doing serialization & deserialization as it increases robustness. In simple terms, deserialization means converting your JSON into class object and serialization is the reverse. Here is an example from flutter docs.

class User {
  final String name;
  final String email;

  User(this.name, this.email);

  User.fromJson(Map<String, dynamic> json)
      : name = json['name'],
        email = json['email'];

  Map<String, dynamic> toJson() =>
    {
      'name': name,
      'email': email,
    };
}

It is strongly recommended to use this technique to have more control over data and their data-types.

Map userMap = jsonDecode(jsonString);
var user = User.fromJson(userMap); // convert json to obj

print('Howdy, ${user.name}!');
print('We sent the verification link to ${user.email}.');

String json = jsonEncode(user); // convert obj to json

More on this topic: https://flutter.dev/docs/development/data-and-backend/json

answered Mar 28, 2020 at 17:27

Mohith7548

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1 Comment

Taiosquare Over a year ago

I will carry out your recommendations.

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