Skipping an Abstract Generation

May 8, 2009 at 5:59 am · Filed under PHP, tips, tutorials

One of the issues of working with an existing OOP codebase is that the original author may have programmed abstract classes with a fixed class depth in mind. For example:

abstract class animal
{
    abstract function speak();
}

class dog extends animal
{
    function speak() {
        print "woof";
    }
}

class cat extends animal
{
    function speak() {
        print "meow";
    }
}

All is well in this quadroped example, but what if you decide you’d like to break cats down into seperate sub-species, but leave dogs as is? Presuming that you already have hundreds of animal extension classes written, it would be tedious to reconfigure the abstract base class to handle this exception. Luckily, abstract requirements can skip a generation:

abstract class animal
{
    abstract function speak();
}

class dog extends animal
{
    function speak() {
        print "woof";
    }
}

abstract class cat extends animal
{
    // some general cat methods
}

class kitten extends cat
{
    function speak() {
        print "meow";
    }
}

class lion extends cat
{
    function speak() {
        print "roar";
    }
}

The intermediate class needs to be declared as abstract to indicate that any requirements of both itself and its parent(s) are to be passed to its child. Because the child classes, such as kitten and lion, contain the speak() method, they have satisfied the abstract requirements of the grandparent class, animal. Meanwhile, the dog class still functions as before.

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