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Chapter 2
Using Classes
Classes are the building block for object-oriented programming (OOP). In fact, programs
created using OOP consist primarily of a network of interacting class instances (i.e., objects).
The Objective-C language provides full support for OOP, including language features that enable
design-time specification of classes and the runtime creation of class instances, along with multiple
mechanisms to support object interaction.
This chapter focuses on Objective-C’s key elements and unique features for developing classes.
It covers key areas such as Objective-C class structure, class design and implementation, and
some additional language features that support class development and OOP. The chapter also includes
plenty of examples (along with a program that you will develop) to reinforce your understanding of the
concepts. Sounds like fun, right? OK, so let’s get started!
Developing Your First Class
Object-oriented programming is a style of computer programming that emphasizes the creation
and use of software objects to write programs. A software object provides a representation of the
characteristics or attributes of the thing/concept being modeled (its state) along with a definition
of the things it can do (its methods). With Objective-C, you create the specification or blueprint for
an object with a class interface and a corresponding implementation. In the interface, you specify
the structure of the class (i.e., its properties and methods). In the implementation, you specify the
variables that store the classes’ internal state, and implement its logic by defining its properties and
methods. The Objective-C language also provides several additional features for developing classes,
specifically protocols and categories, which you’ll learn about later in this chapter. For now, let’s start
with the basics by adding a class to the Elements project.
Adding a Class to the Project
From Xcode, select the Elements folder in the navigator area and then create a new file by selecting
New ➤ File . . . from the Xcode File menu. The New File template will be displayed. Next, select
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