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.
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.
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
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.
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.