Flutter – How to store, restore and add to JSON file, SQLite database or text file?

I used Mr. Naqib's intervention and asked the artificial intelligence site to merge what the professor wrote with what I want specifically, so the result was as follows.

import 'dart:convert';
import 'dart:io';

import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';

class UserList {
  List<User> userList;

  UserList({required this.userList});

  factory UserList.fromJson(Map<String, dynamic> json) {
    var userList = json['user'] as List;
    List<User> users = userList.map((userJson) => User.fromJson(userJson)).toList();
    return UserList(userList: users);
  }

  Map<String, dynamic> toJson() {
    return {
      'user': userList.map((user) => user.toJson()).toList(),
    };
  }
}

class User {
  String name;
  int age;

  User({required this.name, required this.age});

  factory User.fromJson(Map<String, dynamic> json) {
    return User(
      name: json['name'],
      age: json['age'],
    );
  }

  Map<String, dynamic> toJson() {
    return {
      'name': name,
      'age': age,
    };
  }
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  List<User> userList = [];

  void _saveData() async {
    // Create a UserList object and add the current list of users
    UserList userListObject = UserList(userList: userList);

    // Convert the UserList object to a JSON object
    Map<String, dynamic> json = userListObject.toJson();

    // Get the directory where the file should be saved
    final directory = await getApplicationDocumentsDirectory();

    // Create the file
    final file = File('${directory.path}/users.json');

    // Write the JSON data to the file
    file.writeAsStringSync(jsonEncode(json));

    // Show a snackbar to indicate that the data has been saved
    ScaffoldMessenger.of(context).showSnackBar(
      SnackBar(
        content: Text('Data saved successfully'),
      ),
    );
  }

  void _showData() async {
    // Get the directory where the file is saved
    final directory = await getApplicationDocumentsDirectory();

    // Open the file
    final file = File('${directory.path}/users.json');

    if (!file.existsSync()) {
      // Show a snackbar to indicate that the file doesn't exist
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(
          content: Text('No data found'),
        ),
      );
      return;
    }

    // Read the JSON data from the file
    String jsonString = await file.readAsString();

    // Convert the JSON data to a UserList object
    Map<String, dynamic> json = jsonDecode(jsonString);
    UserList userListObject = UserList.fromJson(json);

    // Update the list of users
    setState(() {
      userList = userListObject.userList;
    });
  }

  void _addUser() {
    // Create a new User object and add it to the list of users
    User user = User(name: 'New User', age: 0);
    setState(() {
      userList.add(user);
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('User List'),
        ),
        body: Column(
          children: [
            Expanded(
              child: ListView.builder(
                itemCount: userList.length,
                itemBuilder: (context, index) {
                  final user = userList[index];
                  return ListTile(
                    title: Text(user.name),
                    subtitle: Text('Age: ${user.age}'),
                  );
                },
              ),
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceEvenly,
              children: [
                ElevatedButton(
                  onPressed: _saveData,
                  child: Text('Save'),
                ),
                ElevatedButton(
                  onPressed: _showData,
                  child: Text('Show Data'),
                ),
                ElevatedButton(
                  onPressed: _addUser,
                  child: Text('Add User'),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

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