Inheritance

Everything we have done so far has been what is known as 'composition'. This name should suggest composing an object from smaller components, not a homework assignment in high school. You likely already use composition in your programs, but lets define exactly what it is to keep it from blurring later. It is often referred to as a 'has-a' relation as mentioned earlier. Object A has-an object B. A house has-a kitchen, a dog has-a tail, a rebel has-a cause. You might be hard pressed to think of an object that doesn't have-a something or other in it - everything is made of something after all. Or is it? What about ideas? For example, how does an appliance relate to a fridge? How does a monkey wrench relate to wrenches, or tools? How does James Dean relate to Ronald Reagan? to Elvis? to Che Guverra? You can see that once you start organizing ideas and concepts, has-a relations are not going to cut it alone. The tool needed for the job is inheritance - 'is-a' relationships.

Before going further, let's get used to the two ideas. Has-a relations are when one object contains another. A car has-an engine. An engine has-some pistons. Pistons have, uhh, this little black ring thing that goes around them, I think (sorry, that is the limit of my car knowledge!). Is-a relations are when one thing is a type of another thing. So a car is-a vehicle, a vehicle is-a machine, a machine is-a man-made tool with whirring parts that do stuff... (sorry again!). You really should stop now and try to think of a few inheritance systems. Obviously you didn't stop, so here's some detail.

Inheritance is not complicated, but it is hard to visualize. There is nothing in the 'real' world seems to have exactly the same behavior. This is because inheritance is more of a mental categorization system. It's easy to spot a mammal when you talk to one, but there is no such animal as 'mammal', only types of mammals. It doesn't mean the word mammal is useless of course, just don't expect the zoo to have a cage that says 'mammal'. Before we talked about objects as boxes, or containers of some sort. This works well for composition, where you put objects inside other objects, but it breaks down somewhat when dealing with inheritance. We will need to find a slightly different model to 'visualize' inheritance.

We are ahead of ourselves. Let's just make sure we are clear on the difference between has-a and is-a relations between objects (at least the easy ones). Go through the list below and decide if the relation is inheritance or composition. Try to figure out what is common in all inheritance examples, and what is common to all composition examples:

1. Animal, mammal, dog, poodle
2. Computer, motherboard, CPU, circuitry
3. Entertainer, singer, pop-star, Elvis
4. House, kitchen, sink, tap
5. Bookstore, computer section, flash section, actionscript section
6. Book, computer book, flash book, actionscript book
7. Book, chapter, paragraph, sentence, word
8. Continent, country, region, city/town, street, house
9. Asia, China, Fujian, Xiamen, ZengCuoAn, 15
10. http://www.macromedia.com/support/flash/
11. Application, document, scrollbar, thumb
12. Window, dialog, commonDialog, printDialog
13. Food, dessert, pie, banana-cream
14. 01 (204) 435-8301

Let's go over these in (not too painstaking) detail.

1. Animal, mammal, dog, poodle - Inheritance. A poodle is-a dog, a dog is-a mammal, a mammal is-an animal. What properties would each level have? Try attaching the following properties: hair, eyes, obnoxious, domesticated.
2. Computer, motherboard, CPU, circuitry - Composition. A computer has a motherboard, a motherboard has a CPU... If a computer has an on/off switch, does a CPU have an on/off switch? (and don't say 'yes, 5 billion'!). Apply that kind of question to animals and dogs.
3. Entertainer, singer, pop-star, Elvis - Inheritance. Elvis was-a pop-star, a pop-star is-a singer, a singer is-an entertainer. Are all pop-stars singers? Where would you put actors in that list? Can you see how Elvis is an instance, the rest are classes?
4. House, kitchen, sink, tap - Composition. A kitchen has-a sink. A sink is clearly not a house (unless you are a cockroach in a college dorm). You could also, theoretically, cut a kitchen out of a house, put it on the moon, and it would still be a kitchen. It is an object inside a house.
5. Bookstore, computer section, flash section, actionscript section - Composition. Has-a, has-a, has-a... Each one is physically contained in the other. You could imagine exchanging components, by putting the actionscript section in the kitchen that is now on the moon. A great benefit of using well designed objects is that you can reuse them in different places.
6. Book, computer book, flash book, actionscript book - Inheritance. These are all types and subtypes. An actionscript book is-a computer book. Are there any instances in this sequence? What would be an example of an instance if you added one to the end?
7. Book, chapter, paragraph, sentence, word - Composition. Perhaps a tricky one, in that components do not always have to be physical. Of course you can touch a paragraph in any given book, but it still is composition if we are talking about books in general. Books have chapters, sentence are composed of words. The same is true for previous examples of composition, but perhaps it is more obvious here. Kitchens have sinks, and my kitchen has a sink. In both cases the sink is a component in the kitchen. Don't lose focus!
8. Continent, country, region, city/town, street, house - Composition. This time it must be abstract, and yet it is composition. Clearly a street is not a type of country. Clearly concepts can be components of other concepts. The container 'continent' has countries in it. These countries in turn contain regions...
9. Asia, China, Fujian, Xiamen, ZengCuoAn, 15 - Composition. This is the same hierarchy as the previous, just 'instanciated' - Each concept is now filled in with an object, but these objects are certainly components of each other. The continent called Asia has-a country called China. The country called China has-a city called Xiamen.
10. http://www.macromedia.com/support/flash/ - Composition. The domain macromedia.com has-a section called support. Support has a section called Flash. This final section is useful when all else fails, because it has a copy of the manual. Is the folder structure on your computer composition or inheritance? Are all tree structures composition? (no, look up..)
11. Application, document, scrollbar, thumb - Composition. Yes, windows programs are object oriented... documents have scrollbars, scrollbars have thumbs (thumbs are the little things you drag on a scrollbar).
12. Window, dialog, commonDialog, printDialog - Inheritance. Yes, windows programs use inheritance too. The print dialog box is a type of window, so are all dialogs.
13. Food, dessert, pie, banana-cream - Inheritance. Pie is food. Dessert is food. Notice this is a tree structure - you could have many types of dessert, and many types of pies. All of these structures have been trees.
14. 01 (204) 435-8301 - Composition. This is actually the same as saying North America>Manitoba>Miami>my old phone number. The phone system, like the postal system, uses a very structured naming system to find you.

Inheritance, and OO programming in general, becomes most useful when you get enough similar classes that they start to somewhat naturally organize themselves (always good to be hopeful!). This is worth mentioning, because many Flash programs are quite simple and not all that big, and have short life spans (nor is the programming even the central focus of many files). It would be overkill to use multiple levels of inheritance to link buttons to pages. Writing small programs is a bit like being the post-master in a small village. You need nothing more than the name of a person to get your mail delivered. If there is a change - someone new moves into town, someone changes houses, even a street name change - you can just keep track of it in your head. You might even laugh at how seriously big city postmasters take obscure rules, or how they debate for hours tiny changes in the postal codes. This is all fair enough; you both get the mail delivered. As long as you recognize that the village system would not work in the city, there is nothing wrong with being 'quick and dirty' when it meets all the requirements - it's probably advisable in fact. You don't want to subdivide mammals>dogs down to atoms, after all. One of the main skill sets in OO programming is finding the right balance between complexity and completeness (the same dilemma a writer faces when writing a short chapter about object-oriented programming in actionscript in fact!).

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