article will briefly explain the 4 major principles that make a
language object-oriented: Encapsulation, Data Abstraction,
Polymorphism and Inheritence. All examples will be in VB.Net,
because in my opinion its easier for a new OOP programmer to read and
understand at first. Certainly don’t think I’m saying you should use
one .Net based language over another, as they all are based on the
CLR/CLS and all end up as the same assembly language when compiled. Its
your preference that determines what language you use. Of course,
there are other OOP languages out there, such as Ruby, a pure OOP
language, and hybrid languages such as Python, C++ and Java to mention
a few.
Encapsulation
What is encapsulation? Well, in a nutshell, encapsulation is the
hiding of data implementation by restricting access to accessors and
mutators. First, lets define accessors and mutators:
Accessor
An accessor is a method that is used to ask an object about itself. In
OOP, these are usually in the form of properties, which have, under
normal conditions, a get method, which is an accessor method.
However, accessor methods are not restricted to properties and can be
any public method that gives information about the state of the object.
Mutator Mutators are public methods that are used to modify
the state of an object, while hiding the implementation of exactly how
the data gets modified. Mutators are commonly another portion of the
property discussed above, except this time its the set method that lets the caller modify the member data behind the scenes.
Ok, now lets look at a different example that contains an accessor and a mutator:
So, the use of mutators and accessors provides many advantages. By
hiding the implementation of our Person class, we can make changes to
the Person class without the worry that we are going to break other
code that is using and calling the Person class for information. If we
wanted, we could change the fullName from a String to an array of
single characters (FYI, this is what a string object actually is behind
the scenes) but they callers would never have to know because we would
still return them a single FullName string, but behind the scenes we
are dealing with a character array instead of a string object. Its
transparent to the rest of the program. This type of data protection
and implementation protection is called Encapsulation. Think of accessors and mutators as the pieces that surround the data that forms the class.
Abstraction
Data abstraction is the simplest of principles to understand. Data
abstraction and encapuslation are closely tied together, because a
simple definition of data abstraction is the development of classes,
objects, types in terms of their interfaces and functionality, instead
of their implementation details. Abstraction denotes a model, a view,
or some other focused representation for an actual item. Its the
development of a software object to represent an object we can find in
the real world. Encapsulation hides the details of that implementation.
Abstraction
is used to manage complexity. Software developers use abstraction to
decompose complex systems into smaller components. As development
progresss, programmers know the functionality they can expect from as
yet undeveloped subsystems. Thus, programmers are not burdened by
considering the waysin which the implementation of later subsystesm
will affect the design of earlier development.
The best
definition of abstraction I’ve ever read is: “An abstraction denotes
the essential characteristics of an object that distinguish it from all
other kinds of object and thus provide crisply defined conceptual
boundaries, relative to the perspective of the viewer.” — G. Booch,
Object-Oriented Design With Applications, Benjamin/Cummings, Menlo
Park, California, 1991.
Lets look at this code for a person
object. What are some things that a person can do? Those things must be
represented here in our software model of a person. Things such as how
tall the person is, and the age of the person; we need to be able to
see those. We need the ability for the person to do things, such as
run. We need to be able to ask the person if they can read.
So, there we have started to create a software model of a person
object; we have created an abstract type of what a person object is to
us outside of the software world. The abstract person is defined by the
operations that can be performed on it, and the information we can get
from it and give to it. What does the abstracted person object look
like to the software world that doesn’t have access to its inner
workings? It looks like this:
You can’t really see what the code is that makes the person run. This is encapsulation that we discuseed.
So,
in short, data abstraction is nothing more than the implementation of
an object that contains the same essential properties and actions we
can find in the original object we are representing.
Inheritance
Now lets discuss inheritance. Objects can relate to eachother
with either a “has a”, “uses a” or an “is a” relationship. “Is a”
is the inheritance way of object relationship. The example of
this that has always stuck with me over the years is a library (I think
I may have read it in something Grady Booch wrote). So, take a
library, for example. A library lends more than just books, it
also lends magazines, audiocassettes and microfilm. On some
level, all of these items can be treated the same: All four types
represent assets of the library that can be loaned out to people.
However, even though the 4 types can be viewed as the same, they are
not identical. A book has an ISBN and a magazine does not.
And audiocassette has a play length and microfilm cannot be checked out
overnight.
Each of these library’s assets should be represented by its own
class definition. Without inheritance though, each class must
independently implement the characteristics that are common to all
loanable assets. All assets are either checked out or available
for checkout. All assets have a title, a date of acquisition and
a replacement cost. Rather than duplicate functionality,
inheritance allows you to inherit functionality from another class,
called a superclass or base class.
Let us look at loanable assets base class. This will be used
as the base for assets classes such as book and audiocassette:
This LibraryAsset is a superclass, or base class, that maintains
only the data and methods that are common to all loanable assets.
Book, magazine, audiocassette and microfilm will all be subclasses or derived classes
or the LibraryAsset class, and so they inherit these
characteristics. The inheritance relationship is called the “is
a” relationship. A book “is a” LibraryAsset, as are the other 3
assets.
Let’s look at book and audiocassette classes that inherit from out LibraryAsset class:
Now, lets create an instance of the book class so we can record a new book into the library inventory:
You see, when we create a new book, we have all the properties of
the LibraryAsset class available to us as well, because we inherited
the class. Methods can be inherited as well. Let’s add a
few methods to our LibraryAsset class:
Now, our “myBook” we created above automatically inherited these
methods, and we didn’t even have to touch the Book class in order for
it to happen. The book and audiocassette classes above
automatically inherited the abilities to be checked out and checked
in. In our “myBook” above, now we can check the book out by
calling “myBook.CheckOut()”. Simple! One of the most
powerful features of inheritance is the ability to extend components
without any knowledge of the way in which a class was implemented.
Declaration options, such as Public and Private, dictate which
members of a superclass can be inherited. For more information on
this, see the Declaration Option section of Eric’s post.
Polymorphism
Polymorphism means one name, many forms. Polymorphism
manifests itself by having multiple methods all with the same name, but
slighty different functionality. Many VB6ers are familiar
with interface polymorphism. I’m only going to discuss
polymorphism from the point of view of inheritance because this is the
part that is new to many people. Because of this, it can be
difficult to fully grasp the full potential of polymorphism until you
get some practice with it and see exactly what happens under different
scenarios. We’re only going to talk about polymorphism, like
the other topics, at the basic level.
There are 2 basic types of polymorphism. Overridding, also
called run-time polymorphism, and overloading, which is referred to as
compile-time polymorphism. This difference is, for method
overloading, the compiler determines which method will be
executed, and this decision is made when the code gets compiled.
Which method will be used for method overriding is determined at
runtime based on the dynamic type of an object.
Let’s look at some code:
You see our library asset class. Pay attention to the
overridable function CalculateFineTotal(). In LibraryAsset, we
have defined the default functionality for this method that any derived
classes can use. Any class derived from LibraryAsset can use this
default behavior and calculate fines based on the default
implementation of $1.25 per day late. This is true for our
Magazine class. We didn’t override the function so when late fees
are calculated for late magazine returns, it will use the default
implementation.
Now look at the book class. We have overridden the
CalculateFineTotal to use a different value when determining late
fees. The overrides keywork in VB tells the caller that any
method call will use the virtual method found in Book, not the default
implementation found in LibraryAsset. We have implemented runtime
polymorphism – method overriding.
Lets move on to AudioCassette. Here we have the same method
overriding we found in the book class. Fines are calculated based
on $0.25 per day. Notice we’ve added something extra. We’ve
added the Overloads keywork to our function and to a new function with
the same name, except the new function now accepts a parameter.
Now the caller can call either method, and depending on whether or not
a parameter is passed, that determines with method will be
executed. Notice we do not include the overrides keywork in the
2nd function with a parameter. This is because not method exists
in LibraryAsset with that same signature (accepting a parameter of type
double). You can only override methods with the same signature in
a base class.
Now lets look at some code that creates all these library items and
checks them in and cacluates our fines based on returning them 3 days
late:
The output will look like the following:
Magazine: 3.75
Book: 2.25
AudioCassette1: 0.75
AudioCassette2: 9
You can see how all of our output was different, based on the method
that was executed. We created a new Magazine, which is a type of
LibraryAsset. That is why the instantiation says “myMagazine As
LibraryAsset”. However, since we actually want a magazine, we
create a “New Magazine”. Same thing with book. For Book,
its a little bit more tricky. Since we created a Book of the type
LibraryAsset, this is where the polymorphism comes into play.
Book overrides the CalculateFineTotal of LibraryAsset.
Audiocassette is a little bit different. It actually extends the
implementation of LibraryAsset by including an overloaded function for
CalculateFineTotal(). If we weren’t going to use the function
that took a parameter, we would create it the same way we created the
Book and Magazine classes. But in order to use the overloaded
function, we have to create a new AudioCassette of the type
AudioCassette, because LibraryAsset doesn’t support the overloaded
function.
Only the Magazine used the default method found in the base
class. Book and AudioCassette used their own implementations of
the method. Also, at compile time, the decision was made which
method would be used when we calculate amountDue for the AudioCassette
class. The first call used the 1st method in AudioCassette
without parameters. The 2nd call used the 2nd method with a
parameter
