Inheritance
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 that are shown below.
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 Field 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 Field Object, we simply say that a Password Field Object "is a" Text Field with the added feature that it echoes asteriskes instead of what the user types in.
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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 cannot say 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.
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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.
NEXT
Selena Sol contributes to the JavaBoutique's Introduction to Java. Selena curently works for Barclays Capital in London, one of the leading global investment banks in Europe and has worked as a software developer for the National Center for Human Genome research, Microline Software, Neuron Data, and Electric Eye in Singapore. Selena is perhaps best-known for creating the Public Domain Web Script Archive (Extropia) and writing several books on Web Programming (Perl, CGI, Java).
Email: selena@extropia.com
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