UML Weekend Crash Course™ - To Parent Directory

Session 3—How to Approach the UML 29
Victor : Person
name : Victor
phone : 555-555-5555
call(integer): boolean
Figure 3-5 Victor on the left (hard at work); abstraction of Victor on the right
(an object icon)
Here’s the crux of the matter: An abstraction benefits you only in so far as it describes
the information you need in order to solve a problem. So when you create an abstraction,
you must first make certain that you know why you need it.
My own working definition for creating an abstraction is: representing something in the
real world in a useful manner to solve a specific problem. Usefulness is measured by how well
it helps you solve the problem you are trying to solve.
The representation is an object. The rules that define the representation make up a class.
For comparison, think of the word apple in a dictionary and an apple in your hand. In software,
the definition in the dictionary is a class, and the apple in your hand is an object. To
make objects from classes, you must use the class definition like a template or a mold.
Although each object may vary somewhat, all objects of the same class must conform to
the class definition. That’s why some people say that an object is an instance of a class.
An object is created, manufactured, or instantiated (made real) from the class definition.
What an object knows
To function properly, every object has to know two kinds of information and two types of
behavior.
Information
First, you can say that an object knows about itself. In other words, there is information
that describes the object. For example, the pencils on my desk have a length, hardness,
brand name, eraser, and so on. A book has pages, a cover, a title, an author, and so on. This
is some of the information that would eventually be captured and manipulated in files or
databases.
Second, you can say that an object knows its own current condition. This condition is
formally called the state of the object. Simply put, the state of an object is a description of
the properties of the object during a particular period of time. Consequently, when any of
the properties of the object change, the state of the object is said to change.
An object can describe itself.
An object knows its current condition (or state).