Polymorphism | 37instantiated using the subclasses, and each launched the same method,myMusic( ).
However, with each usage, even though all shared the same datatype (or supertype),
Polymorphism, each instance’s use of the method generates a unique outcome.
Looking at the program and your MP3 library, you may be thinking, “That’s not
nearly enough music categories.” What aboutR&B,Country,Alternative, Hip-Hop,
andCowboymusic? (Cowboy?) We couldn’t agree more. Go ahead and create new
subclasses using polymorphism to add all the categories you want. (This is not one of
those exercises where the answer is at the end of the chapter or the back of the book.
You’re on your own. Programmers don’t get polymorphism right without writing
their own code. See if you’ve got the right stuff.) By the way, notice that you can
make all the changes you want with polymorphism without having to change any of
the other classes. That’s the whole point of polymorphism.
Implementing Polymorphism with Interfaces
The ActionScript 3.0interfacestatement and structure is the other powerful tool for
polymorphism in both object-oriented programming and design patterns. Suppose
you’re building an e-business site. You have no idea how many new products the site
will have, but you know the site will have many different products that change regu-
larly. You’re going to need something that will handle a wide variety of products,
and a way of displaying them.
You know that you’ll need the following operations, but you’re not sure about the
details:
- Description
- Price
- Product display
One way to set up an e-business site would be to create a class that has methods for
all three operations. However, suppose you find out that the client wants different
kinds of displays for the different products. She wants dynamically loaded video for
video products (e.g. TV sets), sound for sound products (e.g. MP3 players) and
graphic images for all other products. The operations for both description and price
methods are pretty standard, but the display method is going to have to be very flexi-
ble. That’s where polymorphism comes in. We could use an abstract class, but to
provide ever more flexibility just in case other unanticipated requirements crop up,
we’ll use an interface.
Example 1-32 through Example 1-36 make up the e-business application. The inter-
face,IBiz, contains the primary operations with unique signatures but not any
details. This will allow us to create the unique details we need as long as we keep all
the signatures the same.