Skip to main content

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

Comments

Popular posts from this blog

Commenting your code

Comments are an integral part of any program, even though they do not contribute to the logic. Appropriate comments add to the maintainability of a software. I have heard developers complain about not remembering the logic of some code they wrote a few months back. Can you imagine how difficult it can be to understand programs written by others, when we sometimes find it hard to understand our own code. It is a nightmare to maintain programs that are not appropriately commented. Java classes should contain comments at various levels. There are two types of comments; implementation comments and documentation comments. Implementation comments usually explain design desicisions, or a particularly intricate peice of code. If you find the need to make a lot of implementation comments, then it may signal overly complex code. Documentation comments usually describe the API of a program, they are meant for developers who are going to use your classes. All classes, methods and variables ...

Inheritance vs. composition depending on how much is same and how much differs

I am reading the excellent Django book right now. In the 4th chapter on Django templates , there is an example of includes and inheritance in Django templates. Without going into details about Django templates, the include is very similar to composition where we can include the text of another template for evaluation. Inheritance in Django templates works in a way similar to object inheritance. Django templates can specify certain blocks which can be redefined in subtemplates. The subtemplates use the rest of the parent template as is. Now we have all learned that inheritance is used when we have a is-a relationship between classes, and composition is used when we have a contains-a relationship. This is absolutely right, but while reading about Django templates, I just realized another pattern in these relationships. This is really simple and perhaps many of you may have already have had this insight... We use inheritance when we want to allow reuse of the bulk of one object in other ...

Planning a User Guide - Part 3/5 - Co-ordinate the Team

Photo by  Helloquence  on  Unsplash This is the third post in a series of five posts on how to plan a user guide. In the first post , I wrote about how to conduct an audience analysis and the second post discussed how to define the overall scope of the manual. Once the overall scope of the user guide is defined, the next step is to coordinate the team that will work on creating the manual. A typical team will consist of the following roles. Many of these roles will be fulfilled by freelancers since they are one-off or intermittent work engagements. At the end of the article, I have provided a list of websites where you can find good freelancers. Creative Artist You'll need to work with a creative artist to design the cover page and any other images for the user guide. Most small to mid-sized companies don't have a dedicated creative artist on their rolls. But that's not a problem. There are several freelancing websites where you can work with great creative ...