Drupal 8, override User Class for a custom added field I am not a great POO specialist, maybe my ask will seems stupid x) 

When I create new field from the user UI administration, I would like find quickly this field when I work from my IDE in customs modules. 

So, I think I need add a custom method for the User Class, like : 

public function mycustomfield_value($user){
    return $user-&gt;mycustomfield-&gt;value;
}


From my IDE, if I type "$user->" the IDE auto-complete must show me "$user->mycustomfield_value(..." method.

My ask is : how can I add this method ? I must create a new Class implement/extends User Drupal 8 Class ? I don't know how can I do that..
Don't. The field system doesn't work like that, there is currently no useful way to get autocomplete.

You could replace the entity class with your own and add methods for your fields, but it's not worth the trouble IMHO as you might get into trouble of some other module wants to replace the class as well.

You could add helper method or functions somewhere else and pass in the entity.