Object Oriented Programming (or OOP) may be classified by three main principles.
1) Encapsulation
2) Inheritance
3) Polymorphism
1) Encapsulation
encapsulation is the hiding of data implementation by restricting access to accessors and mutators.
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.
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.
All these seem like intimidating words but are really fairly simple
concepts to comprehend. If you wish to learn how to program with java,
you will need to know these concepts. So let's have a look at our first
main principle of OOP, encapsulation. Encapsulation just means that we
wish to limit the access that other pieces of code have to our
particular object. So, for example, if you have a Person object, and
this Person object has a first and last name as attributes. In the
event that another piece of code tries to change your Person object's
first name to be say "Frank3", you could have a look at what the first
name is trying to be set to, and strip out any digits so we're just left
with "Frank". Without having encapsulation, we wouldn't be able to
stop "silly programmers" from changing the values of our variables to
something that wouldn't make sense, or even worse, break the application
.
2) Inheritance
The second principle of OOP, and a very important concept if you wish to
learn how to program with Java, is Inheritance.
This principle refers
to a super class (or parent class) and a sub-class (or child class) and
the idea that a child class gains all the attributes of its parent. You
can think of it in terms of a real world situation, just like a real
parent and child.
A child has a tendency to inherit certain
characteristics from his or her parents, like say, eye colour or hair
colour. Let's think about another example in terms of programming, say
we have super class "Vehicle" and sub-classes "Car" and "Motorcycle". A
"Vehicle" has wheels, and therefore through inheritence so would a
"Car" and a "Motorcycle", but a "Car" has doors, and a "Motorcycle" does
not. So it wouldn't be accurate to say that a "Vehicle" has doors, as
that statement would be incorrect. So you can see how we could
determine all the aspects that are common between a "Car" and a
"Motorcycle" and define them inside of the "Vehicle" super class.
3) Polymorphism
The 3rd principle of OOP is Polymorphism. This principle sounds the
most intimidating, but I can explain it in simple terms. Polymorphism
means that an object (i.e. Animal) can take on several forms while your
program is running.
Let's imagine you have created an Animal class and
defined the method "Speak". You then asked three of your friends to
make types of animals and have them implement the "Speak" method. You
won't know what type of animals your friends create, nor how their
Animals will speak, until you actually hear those animals speak.
This
is very much like how Java handles this problem. It's known as dynamic
method binding, which simply means, Java won't know how the specific
Animal speaks until runtime. So perhaps your friends have created a
Dog, Cat and Snake. These are three kinds of Animals, and they each
speak differently. When Java asks the Dog to speak, it says "woof".
Whenever Java asks the Cat to speak, it says "meow". Whenever Java
asks the snake to speak, it hisses. There's the advantage of
polymorphism, all we did was define an Animal interface with a Speak
method, and we can create a whole bunch of types of animals that speak
in their own distinctive way.
