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 ::   Intro to Perl/CGI and HTML Forms
 ::   Intro to Windows Perl
 ::   Intro to Perl 5
 ::   Intro to Perl
 ::   Intro to Perl Taint mode
 ::   Sherlock Holmes and the Case of the Broken CGI Script
 ::   Writing COM Components in Perl

Java tutorials
 ::   Intro to Java
 ::   Cross Browser Java

Misc technical tutorials
 ::   Intro to The Web Application Development Environment
 ::   Introduction to XML
 ::   Intro to Web Design
 ::   Intro to Web Security
 ::   Databases for Web Developers
 ::   UNIX for Web Developers
 ::   Intro to Adobe Photoshop
 ::   Web Programming 101
 ::   Introduction to Microsoft DNA

Misc non-technical tutorials
 ::   Misc Technopreneurship Docs
 ::   What is a Webmaster?
 ::   What is the open source business model?
 ::   Technical writing
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Offsite tutorials
 ::   ISAPI Perl Primer
 ::   Serving up web server basics
 ::   Introduction to Java (Parts 1 and 2) in Slovak


Introduction to Web Programming
  • But the benefits of objects do not stop there.

  • Objects also have the special ability to inherit functionality from other objects.

  • Let's look at the standard cat metaphor that most books use:

  • A cat is certainly a cat. Since it is a cat, we know that it has properties including whiskers, retractable claws and sharp teeth. We also know that it has methods including the fact that it purrs when it is happy, it hunts mice, and it rips up the sofa when it is sharpening its claws.

  • But a cat is more than a cat. It is also a mammal. As a mammal, it inherits certain attributes shared by all mammals. For example, it has mammary glands, it is warm-blooded, it gestates its young internally, it breathes, it maintains homeostasis, etc....

  • Okay, now let's consider a more realistic object that you might use in your code and see how the same logic applies. Consider the HTML FORM Text Field and Password Field objects from Day One that are shown below.

    Text Field

  • As we noted earlier, the password object is exactly the same as a Text Field object except for the fact that it does not echo what the user types in.

  • So the password field is like the cat and the text filed is like the mammal. The password field inherits all of the functionality of the text field but it adds another feature that makes it distinct.

  • In the case of the password object, we simply say that a password object "is a" text field with the added feature that it echoes stars instead of what the user types in.

When thinking about inheritance and objects, it is useful to distinguish between an "is a" relationship and a "has a" relationship because objects can have both types of relationship with each other.

For example, while you can say a cat object is a mammal object, and has a brain object, you cannotsay a cat object has a mammal object and is a brain object.

In our Password Field example above we can say that a Password Field "is a" Text Field and "has an" area to type in.

  • What is the benefit of inheritance?

  • Well, with inheritance, if you find that someone has already developed an object that is "mostly" what you need, you can easily add new features without breaking the old ones, or even having to understand how the old ones work.

  • In other words, inheritance allows you to easily reuse code.

  • Thus, we do not need to rewrite the entire functionality of a Text Field in order to create a password object, you just inherit.

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