Abstract Factory Method and Polymorphism

Question

Being a PHP programmer for the last couple of years, I'm just starting to get into advanced programming styles and using polymorphic patterns. I was watching a video on polymorphism the other day, and the guy giving the lecture said that if at all possible, you should get rid of if statements in your code, and that a switch is almost always a sign that polymorphism is needed. At this point I was quite inspired and immediately went off to try out these new concepts, so I decided to make a small caching module using a factory method. Of course the very first thing I have to do is create a switch to decide what file encoding to choose. DANG!

class Main {
    public static function methodA($parameter='')
    {
        switch ($parameter)
        {
            case 'a':
                $object = new \name\space\object1();
                break;

            case 'b':
                $object = new \name\space\object2();
                break;

            case 'c':
                $object = new \name\space\object3();
                break;

            default:
                $object = new \name\space\object1();
        }

        return (sekretInterface $object);
    }
}

At this point I'm not really sure what to do. As far as I can tell, I either have to use a different pattern and have separate methods for each object instance, or accept that a switch is necessary to "switch" between them. What do you guys think?

Answer 1

switch and if are generally required for primitives (which $parameter is), although PHP allows you to create class names and call methods from variables, so you can get away with a little magic.

For example you could do this:

public static function methodA($parameter = '') {
   $class = '\name\space\object' . $parameter;
   return new $class;
}

This of course requires that $parameter and the existing class names are all named appropriately, and it does not account for the default case.

The other alternative requires that $parameter be an object. You can remove the switch, but it doesn't cut down on verbosity (although it is polymorphic). One possible way:

public static function methodA(EncodingType $parameter = null) {
   return new $parameter->getObject();
}

...which would return the required object string.

I should note that the principle of avoiding conditionals in contemplation of using polymorphism is great, especially in theory -- but if you try to eliminate all conditionals in your code you can half kill yourself and end up writing unnecessarily complicated code. It may also be a signal to rethink your design.

I will say that one common spot where you find conditionals is in factory methods, which is what your example seems to be.