Question

I’m trying to design some class hierarchy and I got “stuck” at this part.

Lets say that I have following classes

abstract class Video 
{
    const TYPE_MOVIE = 1;
    const TYPE_SHOW  = 2;

    abstract public function getTitle();
    abstract public function getType();
}

class Movie extends Video 
{
    // ...

    public function getType() 
    {
        return self::TYPE_MOVIE;
    }
}

class Show extends Video 
{
    // ...

    public function getType() 
    {
        return self::TYPE_SHOW;
    }
}

In the diffrent part of the system I have (Parser) class that encapsulates creation of
movie and show objects and returns obj. to the client.

Question: What is the best way to get a type of a obj. returned from parser/factory class, so that client can do something like

$video = $parser->getVideo('Dumb and Dumber');

echo $video->getTitle();

// Way 1
if($video->getType == 'show') {
    echo $video->getNbOfSeasons();
}

// Way 2
if($video instanceof Show) {
    echo $video->getNbOfSeasons();
}

// Current way
if($video->getType == Video::TYPE_SHOW) {
    echo $video->getNbOfSeasons();
}

Is there a better way than my solution (read as: does my solution suck?)?

Answer 1

Is there a better way than my solution (read as: does my solution suck?)?

Your solution does not suck, per se. However, whenever someone is trying to determine the subtype to perform some actions, I tend to wonder; why? This answer might be a little theoretical and perhaps even a little pedantic, but here goes.

You shouldn’t care. The relationship between a parent and a child class is that the child class overwrites the behaviour of the parent. A parent class should always be substitutable by it’s children, regardless which one. If you find yourself asking: how do I determine the subtype, you’re usually doing one of two things “wrong”:

1. You’re attempting to perform an action based upon subtype. Normally, one would opt for moving that action to the class itself, instead of “outside” of the class. This makes for more manageable code as well.

2. You’re attempting to fix a problem you’ve introduced yourself by using inheritance, where inheritance isn’t warranted. If there is a parent, and there are children, each of which are to be used differently, each of which have different methods, just stop using inheritance. They’re not the same type. A film is not the same a tv-serie, not even close. Sure, you can see both on your television, but the resemblance stops there.

If you’re running into issue number 2, you’re probably using inheritance not because it makes sense, but simply to reduce code duplication. That, in and on itself, is a good thing, but the way you’re attempting to do so might not be optimal. If you can, you could use composition instead, although I have my doubts where the duplicated behaviour would be, apart from some arbitrary getters and setters.

That said, if your code works, and you’re happy with it: go for it. This answer is correct in how to approach OO, but I don’t know anything about the rest of your application, so the answer is generic.