Question

I’m trying to learn when static functions should be used, and have had a difficult time finding an answer my questions. I am creating a class User, which is related to a class Group. If I have a user id and I want to get a user object from that, is it better to do something like

$existingUser = User::get($userId); 

where the class is defined like this

class User()
{ 
    public static function get($id){
        $user = new User();
        return $user->findById($id);
    }
    public function findById($id) {
        //find and populate user object
    }
}

or

$existingUser=new User();
$existingUser->findById($userId);

where the class is defined like this

class User()
{ 
    public function findById($id) {
        //find and populate user object
    }
}

---

What about if I were to write a function which returns an array of Group objects based on a user id?

class User()
{ 
    //stuff
    $groupArray = Group::getAllByUserId($this->getId())
    //stuff
}

or

class User()
{ 
    //stuff
    $group = new Group();
    $groupArray = $group->findAllByUserId($this->getId());
    //stuff
}

The second method creates an empty group object which is never used. Does it matter?
Am I misunderstanding the concept of static? I know it is useful for not having to instantiate a class, so if the function instantiates one anyway, does that kind of defeat the purpose? If so, what would be an example of when a static function would be used?

Anything else I should be considering in this over simplified example?

Answer 1

I’m trying to learn when static functions should be used

Oh, it’s so simple: never.

To understand it, read:
http://www.objectmentor.com/resources/articles/ocp.pdf
http://misko.hevery.com/code-reviewers-guide/flaw-brittle-global-state-singletons/