Book Image

Learning Object-Oriented Programming

By : Gaston C. Hillar
Book Image

Learning Object-Oriented Programming

By: Gaston C. Hillar

Overview of this book

Table of Contents (16 chapters)
Learning Object-Oriented Programming
About the Author
About the Reviewers

Recognizing attributes/fields

We already know the information required for each of the shapes. Now, it is time to design the classes to include the necessary attributes that provide the required data to each instance. In other words, we have to make sure that each class has the necessary variables that encapsulate all the data required by the objects to perform all the tasks.

Let's start with the Square class. It is necessary to know the length of side for each instance of this class, that is, for each square object. Thus, we need an encapsulated variable that allows each instance of this class to specify the value of the length of side.


The variables defined in a class to encapsulate data for each instance of the class are known as attributes or fields. Each instance has its own independent value for the attributes or fields defined in the class.

The Square class defines a floating point attribute named LengthOfSide whose initial value is equal to 0 for any new instance of the class. After you create an instance of the Square class, it is possible to change the value of the LengthOfSide attribute.

For example, imagine that you create two instances of the Square class. One of the instances is named square1, and the other is square2. The instance names allow you to access the encapsulated data for each object, and therefore, you can use them to change the values of the exposed attributes.

Imagine that our object-oriented programming language uses a dot (.) to allow us to access the attributes of the instances. So, square1.LengthOfSide provides access to the length of side for the Square instance named square1, and square2.LengthOfSide does the same for the Square instance named square2.

You can assign the value 10 to square1.LengthOfSide and 20 to square2.LengthOfSide. This way, each Square instance is going to have a different value for the LengthOfSide attribute.

Now, let's move to the Rectangle class. We can define two floating-point attributes for this class: Width and Height. Their initial values are also going to be 0. Then, you can create two instances of the Rectangle class: rectangle1 and rectangle2.

You can assign the value 10 to rectangle1.Width and 20 to rectangle1.Height. This way, rectangle1 represents a 10 x 20 rectangle. You can assign the value 30 to rectangle2.Width and 50 to rectangle2.Height to make the second Rectangle instance, which represents a 30 x 50 rectangle.

The following table summarizes the floating-point attributes defined for each class:

Class name

Attributes list










The following image shows a UML (Unified Modeling Language) diagram with the four classes and their attributes: