How to parse a list inside JSON in flutter

In your code, you used map to map some objects and then assigned them to children of a widget, also use .toList() after the map.

Something that I couldn't get from you code is that you have a Wellcome model that contains a list of posts and comments and a profile, your sample JSON is equal to one Welcome but in your code when you fetched the data you tried to map it as a list of Wellcome also in your widget.

The other thing is in the future builder, it's better to define what type it is going to get:

...
FutureBuilder<Welcome>(
    future: http.getPost(),
    builder: (context, AsyncSnapshot<Welcome> snapshot) {

...

Also future takes some times to complete so in cases it doesn't have data(not completed yet) you should return another widget, here I simply used:

return Center(child: CircularProgressIndicator());

I changed your code a little bit, here is the result:

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:http/http.dart' as http;

void main() {
  runApp(MaterialApp(
    home: News(),
  ));
}

class Welcome {
  List<Post> posts;
  List<Comment> comments;
  Profile profile;

  Welcome({
    this.posts,
    this.comments,
    this.profile,
  });

  factory Welcome.fromJson(Map<String, dynamic> json) => Welcome(
        posts: List<Post>.from(json["posts"].map((x) => Post.fromJson(x))),
        comments: List<Comment>.from(
            json["comments"].map((x) => Comment.fromJson(x))),
        profile: Profile.fromJson(json["profile"]),
      );

  Map<String, dynamic> toJson() => {
        "posts": List<dynamic>.from(posts.map((x) => x.toJson())),
        "comments": List<dynamic>.from(comments.map((x) => x.toJson())),
        "profile": profile.toJson(),
      };
}

class Comment {
  int id;
  String body;
  int postId;

  Comment({
    this.id,
    this.body,
    this.postId,
  });

  factory Comment.fromJson(Map<String, dynamic> json) => Comment(
        id: json["id"],
        body: json["body"],
        postId: json["postId"],
      );

  Map<String, dynamic> toJson() => {
        "id": id,
        "body": body,
        "postId": postId,
      };
}

class Post {
  int id;
  String title;

  Post({
    this.id,
    this.title,
  });

  factory Post.fromJson(Map<String, dynamic> json) => Post(
        id: json["id"],
        title: json["title"],
      );

  Map<String, dynamic> toJson() => {
        "id": id,
        "title": title,
      };
}

class Profile {
  String name;

  Profile({
    this.name,
  });

  factory Profile.fromJson(Map<String, dynamic> json) => Profile(
        name: json["name"],
      );

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

class HttpService {
  final String url = "https://my-json-server.typicode.com/typicode/demo/db";

  Future<Welcome> getPost() async {
    var response = await http.get(url);

    if (response.statusCode == 200) {
      var body = json.decode(response.body);
      return Welcome.fromJson(body);
    }
  }
}

class News extends StatefulWidget {
  @override
  _NewsState createState() => _NewsState();
}

class _NewsState extends State<News> {
  final HttpService http = HttpService();

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: DefaultTabController(
        length: 4,
        child: Scaffold(
          backgroundColor: Colors.white,
          appBar: AppBar(
            backgroundColor: Colors.white,
            title: Text(
              'Trending Topics',
              style: TextStyle(color: Colors.black, fontSize: 25),
            ),
            automaticallyImplyLeading: false,
            elevation: 0,
            actions: <Widget>[
              Padding(
                padding: const EdgeInsets.symmetric(horizontal: 10),
                child: Center(
                  child: Text(
                    'see all',
                    style: TextStyle(fontSize: 20, color: Colors.grey[600]),
                  ),
                ),
              ),
            ],
            bottom: TabBar(
              labelColor: Colors.black,
              unselectedLabelColor: Colors.black,
              tabs: <Widget>[
                Tab(
                  text: 'Tech',
                ),
                Tab(
                  text: 'Art',
                ),
                Tab(
                  text: 'Sports',
                ),
                Tab(
                  text: 'Nation',
                ),
              ],
            ),
          ),
          body: TabBarView(
            children: <Widget>[
              Container(
                child: FutureBuilder<Welcome>(
                  future: http.getPost(),
                  builder: (context, AsyncSnapshot<Welcome> snapshot) {
                    if (snapshot.hasData) {
                      Welcome welc = snapshot.data;
                      return ListView(
                          children: welc.posts
                              .map((post) => ListTile(
                                    title: Text(post.title),
                                  ))
                              .toList());
                    }
                    return Center(child: CircularProgressIndicator());
                  },
                ),
              ),
              Container(),
              Container(),
              Container(),
            ],
          ),
        ),
      ),
    );
  }
}