Hi,
If you "type" as a superclass you are generally programming to an
interface, so you can just as easily type to an interface.
Using interfaces is generally more flexible - it's easier to work
with "has a" (interfaces) than "is a" (subclass), but sometimes you need
a default implementation for your interface, which is why there are a
number of abstract classes in AS3 that implement the interface - you
need some generic behaviour, it can go in a super-class, but concrete
classes are often difficult to deal with when you want to swap between
interfaces.
E.g. unlike C++ with multiple inheritance, you can only extend a
single class, but you can implement as many interfaces as you want.
People will generally tell you to work with interfaces, but there are
times - and you will find out somewhere along the line, where only a
superclass will do.
For a simple example of this, look at what we do with Sprites and
MovieClips when we extend them.
Hope this helps. If you have not looked at "Head First Design
Patterns", or a similar book about patterns, then they will give you a
more in-depth grounding than I can.
Glen
On 23/09/2010 17:42, Doug Lambert wrote:
Inheritance and abstract classes
The examples I've seen are set up this way:
Super class (abstract class) contains all methods
Subclass overrides methods in super class
Class instances are typed as super class and instantiated as subclass.
I'm wondering why the examples show instances typed as a super class and
instantiated as a subclass. This makes it necessary for all methods to exist in
the super(abstract) class and overridden in the subclass. Is this on purpose so
the abstract class acts as an interface? Wouldn't it be more flexible to type
and instantiate as the subclass so methods can be added to the subclass that
don't exist in the super(abstract) class?
I like the idea of abstract classes. I'm just trying to figure out the best way
to use them.
Thanks.
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