Flutter 'Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'List<class>'

When you are doing feeds: json['feeds'], You are basically trying to assign a Map<String,dynamic> to a class FeedItem , Moreover you want to assign a complete list as List<Map<String,dynamic>> to List<FeedItem , The problem is that flutter just can't parse the data automatically and assign like this , You need to go deep level by level and assign Map<String,dynamic> to FeedItem and then finally convert it to a List

I will share a piece of code based on my knowledge, how I tackle this type of problem and I believe that will give you better insight about your problem ( if not solve it )

Future<void> getData() async {
http.Response response =
    await http.get('Example address');
if (response.statusCode == HttpStatus.ok) {
//if the status code is 200 or 'OK' parse the data and convert Outer layer as List
//after that , Map each element as a FeedItem and finally convert the result to List
  feeds = (json.decode(response.body) as List)
      .map<FeedItem>(
          (_feed) => FeedItem.fromJSON(_feed))
      .toList();
} else {
  throw response;
}

}

The error is because you are not passing json['feeds'] as a List. Your FeedData class will have to be changed to the following, this is assuming that your FeedItem class also has a fromJson method :

class FeedData {
  String data;
  List<FeedItem> feeds;

  FeedData({this.data, this.feeds});

  factory FeedData.fromJson(Map<String, dynamic> json) {
    return FeedData(
      data: json['data'],
      feeds: List.from(json['feeds']).map((item)=>FeedItem.fromJson(item)).toList(),
    );
  }
}

Please see the working code based on your data. You can also load the following code on DartPad https://dartpad.dev/24694a911ddd0619277f5780573086f1

void main() {

  Map<String, dynamic> json = {
    "data": "default",
    "feeds": [
        {
            "id": 2,
            "photoUrl": "https://cartalkio-image-storage-dev.s3.eu-west-2.amazonaws.com/o85DvOaTXXE.jpg",
            "description": "Magna duis consectetur sit ut commodo non eiusmod.",
            "dateAdded": "0001-01-01T00:00:00",
            "isMain": false,
            "publicId": null,
            "isImage": true,
            "mainImage": "https://randomuser.me/api/portraits/men/76.jpg",
            "userId": 9,
            "likers": null,
            "username": "Larsen",
            "thumbnail": null,
            "likes": 0
        },
        {
            "id": 4,
            "photoUrl": "https://cartalkio-image-storage-dev.s3.eu-west-2.amazonaws.com/lXR833PRh3g.jpg",
            "description": "Magna duis consectetur sit ut commodo non eiusmod.",
            "dateAdded": "0001-01-01T00:00:00",
            "isMain": false,
            "publicId": null,
            "isImage": true,
            "mainImage": "https://randomuser.me/api/portraits/men/4.jpg",
            "userId": 8,
            "likers": null,
            "username": "Lloyd",
            "thumbnail": null,
            "likes": 0
        },
      ]
};
  
  
  FeedData feedData = FeedData.fromJson(json);
  print(feedData.feeds.length);
}


class FeedData {
  String data;
  List<FeedItem> feeds;

  FeedData({this.data, this.feeds});

  factory FeedData.fromJson(Map<String, dynamic> json) {
    return FeedData(
      data: json['data'],
      feeds: List.from(json['feeds']).map((item)=>FeedItem.fromJson(item)).toList(),
    );
  }
}


class FeedItem {
  int id;
  String photoUrl;
  String description;
  String dateAdded;
  bool isMain;
  Null publicId;
  bool isImage;
  String mainImage;
  int userId;
  Null likers;
  String username;
  Null thumbnail;
  int likes;

  FeedItem(
      {this.id,
      this.photoUrl,
      this.description,
      this.dateAdded,
      this.isMain,
      this.publicId,
      this.isImage,
      this.mainImage,
      this.userId,
      this.likers,
      this.username,
      this.thumbnail,
      this.likes});

  FeedItem.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    photoUrl = json['photoUrl'];
    description = json['description'];
    dateAdded = json['dateAdded'];
    isMain = json['isMain'];
    publicId = json['publicId'];
    isImage = json['isImage'];
    mainImage = json['mainImage'];
    userId = json['userId'];
    likers = json['likers'];
    username = json['username'];
    thumbnail = json['thumbnail'];
    likes = json['likes'];
  }
}

You are not converting feedItems to objects

  factory FeedData.fromJson(Map<String, dynamic> json) {
    return FeedData(
      data: json['data'],
      feeds: json['feeds'],  # <- here is the problem
    );

Should be something like this:

feeds: (json['feeds'] as List)
        ?.map((e) =>
            e == null ? null : FeedItem.fromJson(e as Map<String, dynamic>))
        ?.toList(),

Please change this code

print(res.feeds);    -->  print(res.feeds[0]);
print(res.feeds.id); --> print(res.feeds[0].id);