Wednesday, October 11, 2006

Your feedback on goals of software design

Please take a couple of minutes to give your feedback on this section. Would you like to see any additions, subtractions, or changes? How did you like the audio and video descriptions used in this section? I know they are not optimal, and I hope to improve them, but it will really help me if I get feedback on exactly what should be improved to make the course more effective for you.



Note: This post was originally posted on my blog at http://www.adaptivelearningonline.net

You will find below comments from the original post


COMMENT:
AUTHOR: Rashmi
DATE: 10/18/2006 05:50:25 PM
Sir could you please explain the difference between inheritance and composition through an example code



COMMENT:
AUTHOR: Parag
DATE: 10/20/2006 01:16:47 PM
Thanks for the suggestion. I will add an example to the blog post. Till then, here is a quick code sample.

//this class uses inheritance. Guitar inherits
//from MusicalInstrument
public class Guitar extends MusicalInstrument {
//...
}

//this is an example of composition. The class
//Car is composed of several instances of the
//class Wheel
public class Car {
Wheel wheels[];
//...
}

When a class has other class(es) as instance or class variables then a composition relationship is established between them.
Composition also allows code reuse. In the above example the class Car can invode methods of Wheel thus acheiving functionality without impemented it in it's own class.

Notice the relationship between the classes in both the examples.
A Guitar IS-A MusicalInstrument.
We typically use inheritance when there is an IS-A relationship between two classes.
A Car HAS Wheel(s)
We typically use composition between classes that have a HAS-A or HAS relationship.

now before I wrap up, a couple of questions. Why is it incorrect to use inheritance when there is a HAS-A relationship between two classes. More specifically why should we avoid...
public class Car extends Wheel {}

Here is a very good link that explains the concept in detail:
http://www.artima.com/designtechniques/compoinh.html

--
Parag

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