Flutter add property for object - flutter

Question

My code works but I want to update this with objects. But I don't see how to add a property value to an object. It must be declared in the constructor ??
For exemple I have a user object with some property.
But in my code I need to classify users by those with the closest salary to a certain value.
For that I want to add an index key which I then use to organize them in order.
This key is just used to classify my users to have the order I want
This index key is not base defined in my object.
My class user :
class Users{
List<User> user;
Users({this.user});
Users.fromJson(Map<String, dynamic> json) {
if (json['user'] != null) {
user= new List<User>();
json['user'].forEach((v) {
user.add(new User.fromJson(v));
});
}
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
if (this.user!= null) {
data['user'] = this.user.map((v) => v.toJson()).toList();
}
return data;
}
#override
String toString() {
return '{${this.user}}';
}
}
class User{
String name;
int gender;
num salary;
User(
{this.name,
this.gender,
this.salary,
});
User.fromJson(Map<String, dynamic> json) {
name= json['name'];
gender= json['gender'];
salary= json['salary'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['name'] = this.name;
data['gender'] = this.gender;
data['salary'] = this.salary;
return data;
}
#override
String toString() {
return '{ '
'${this.name},'
'${this.gender},'
'${this.salary},'
'}';
}
void add(String key, dynamic value) {
// ... implementation
}
}
Here an exemple of the function the classify my user and adding the index key in a functions.dart file
List<User> func_orderedItemByClosestSalary(List<User> filteredUsers)
{
switch (filteredUsers.length)
{
case 1:
// One result
print('un seul resultat ...');
filteredUsers[0].index = 0; // Not work for adding the index key
break;

Answer 1

The reason why filteredUsers[0].index = 0; doesn't work is because index isn't defined on Users object. Add index on Users to be able to update its value.
class User{
int index;
String name;
int gender;
num salary;
...
}