Computer Programming with GNU Smalltalk - Free

Objects, Messages and Classes: Part II 55
Chapter 5: Objects, Messages and Classes: Part II
Encapsulation
Inheritance
Ever wondered how the electronic circuits inside your cell phone works? Ever needed to know that?
Most probably not. Because, we don't need to know all the manufacturing details about our cell phone
to use it. Just reading the manual is enough to use all of its features. So, we cannot know all details, all
scientific laws, all properties of an object we use and we don't need to. Object oriented programming
introduces encapsulation concept to apply this real life fact in our applications.
Smalltalk does not allow you to directly modify internal state of an object. Objects can only be
communicated by sending messages to them. So, Smalltalk encapsulates the unnecessary information.
Beside encapsulation allows us to think in a real world manner, it also provides some kind of
information hiding which helps our program to be solid and reusable. This means, if you hide an
object's implementation from the programmer and provide him/her just a protocol to communicate with
that object then the programmer won't be affected by implementation changes made in the future as
long as the protocol is preserved.
For example, you created a Calculator class which is able to calculate the roots of a second degree
equation. You just send the equation's information to it and it returns the roots... Maybe it uses an
inefficient algorithm which causes it to calculate the roots in hours. But, well, it is better than nothing
and you distributed the code to friends (we shall call them clients). Maybe, after distribution, you found
a revolutionary approach which reduces the calculation time into seconds. The algorithm is changed but
the protocol does not need to because the clients just need to provide the equality to the calculator and
want it to solve the equation. How calculator solves the equation is not their business. So you can
benefit the advantages of encapsulation and distribute your new code to clients. Clients just need to
replace the Calculator class and does not need to change their own program.
Human beings like categorizing things. Because categorizing brings order and order brings easymanipulation.
Similar elements come together to form groups (or categories) and some special
properties of them are the thing which distinguishes them from other group members. We may group
some groups together, according to their similarities to form some other groups and also group them to
form other groups. This process can go on until we reach one main group which is an ancestor of all
others. This is actually a simulation of life itself.
The elements we mentioned above are objects and the groups are classes. What is the most general
expression we use for elements? The answer is object. We can call everything as an object and then
derive a special kind of object from it to which we give names like car, house, animal etc. But
remember, we call objects which we use to create instances as classes. So, when we say car, house,