Re: Abstract class or interface?


"ozbear" <ozbear@xxxxxxxxxxx> wrote in message
news:4277546e.81458468@xxxxxxxxxxxxxx
> On Sun, 1 May 2005 19:35:19 -0400, "Brett" <no@xxxxxxxx> wrote:
>
>>I'm still trying to figure out concrete reasons to use one over the other.
>>I understand the abstract class can have implementation in its methods and
>>derived classes can only inherit one abstract class. The interface has
>>implied abstract methods/properties and derived classes can inherit
>>multiple
>>interfaces. The interface properties/methods have no implementation.
>>
>>Besides definitions of the two, what are some conceptual reasons to use
>>one
>>over the other? Perhaps examples of this project uses an abstract classes
>>vs. this one uses an interface...for these reasons.
>>
>>I have one project that uses an interface. I chose the interface over an
>>abstract class because the derived classes all do the same thing, they
>>just
>>go about doing it in slightly different ways. Their results are different
>>but conceptually what they do is the same thing. I don't ever see the
>>need
>>for adding more methods or properties to the interface. If there is every
>>such a need, I can encapsulate this variability into one of the derived
>>classes, since it will be "one" that has such a need rather than all.
>>That
>>make sense?
>>
>>Thanks,
>>Brett
>
> I have a 3-D package that under user control needs to be able to move
> and rotate "things" along all three X, Y, and Z coordinate axis.
>
> The "things" could be sinple points (Vertex class), lines (Line
> class), planar faces (Faces class), or solid things (Solids class).
>
> All these things are defined as supporting the I3D interface whch
> means they must implement methods such as Rotate(x angle, y angle,
> z angle), Translate(some axis), Scale(some percentage), and so forth.
>
> Rotating a Vertex is different than a Line, but the interface doesn't
> know nor care what the differences are. Furthermore, I can place all
> the different things in an ArrayList by their interface and then run
> a simple...
>
> foreach (I3D i3dobj in arraylist)
> i3dobj.Rotate(xangle,yangle,zangle);
>
> to rotate all the objects in the scene.
>
> In this example it would be possible to define an abstract class
> called, say, Ab3D, and have Vertex, Line, Face, etc., derive from
> Ab3D and override the Rotate, Translate, etc. methods in each.
> THis implies some closer coupling between these classes than I
> would like, furthermore...
>
> What if I want to include rotated text or something else where
> the bulk of the work is already defined in an existing class which
> does not derive from Ab3D? If I don't have control of that existing
> class' definition, or if it itself is a derived class I cannot make it
> derive from Ab3D to override those methods.
>
> With interfaces however, I can derive a new class from that already
> existing one, state that it supports the I3D interface, and then
> write the supporting Rotate, Translate, etc., methods for it. Then I
> can include it in the arraylist above and perform the 3D actions on
> it with no changes to manipulation code. I do not have to change
> the interface to make new things 3D manipulatable.
>
> Oz
> --
> A: Because it fouls the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing on usenet and in e-mail?

Thanks. Great example. Say your interface has three methods - X, Y, Z.
You want to add a forth - Time or T. What now? This is completely
unexpected. I know you could allow your implementation to inherit from
another interface, perhaps called ITime. What is best?

Brett


.

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