Flutter，为什么在fromJson方法中使用“static”？

class User {
  final String name;
  final int age;

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

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

Future<User> fetchUser() async {
  final response = await http.get(Uri.parse('https://example.com/user.json'));
  if (response.statusCode == 200) {
    final Map<String, dynamic> jsonResponse = json.decode(response.body);
    return User.fromJson(jsonResponse);
  } else {
    throw Exception('Failed to load user');
  }
}

factory User.fromJson(Map<String, dynamic> json) {
  if (json['name'] == null || json['age'] == null) {
    throw FormatException('Invalid JSON format');
  }
  return User(
    name: json['name'],
    age: json['age'],
  );
}

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