IntroJB06.pdf

Part II: Object-Oriented Programming

In Part I, “Fundamentals of Programming,” you learned how to

write simple Java applications using primitive data types,

control statements, methods, and arrays, all of which are

features commonly available in procedural programming

languages. Java, however, is a class-centric object-oriented

programming language that uses abstraction, encapsulation,

inheritance, and polymorphism to provide great flexibility,

modularity, and reusability for developing software. In this

part of the book you will learn how to define, extend, and

work with classes and their objects.

Chapter 6 Programming with Objects and Classes

Chapter 7 Strings

Chapter 8 Class Inheritance and Interfaces

Chapter 9 Object-Oriented Software Development

218

Objects and Classes

Objectives

•

Understand objects and classes and the relationship

between them.

•

Learn how to define a class and how to create an

object of the class.

•

Comprehend the roles of constructors.

•

Know object references and how to pass objects to

methods.

•

Understand instance variables and methods.

•

Understand static variables, constants, and methods.

•

Use objects as array elements.

•

Use UML graphical notations to describe classes and

objects.

•

Understand the scope of variables in the context of a

class.

• Become familiar with the organization of the Java API.

219

Introduction

Programming in procedural languages like C, Pascal, BASIC,

Ada, and COBOL involves choosing data structures, designing

algorithms, and translating algorithms into code. An object-

oriented language like Java combines the power of procedural

languages with an added dimension that provides such

benefits as abstraction, encapsulation, reusability, and

inheritance.

In procedural programming, data and operations on the data

are separate, and this methodology requires sending data to

procedures and functions. Object-oriented programming places

data and the operations that pertain to them within a single

entity called object ; this approach solves many of the

problems inherent in procedural programming. The object-

oriented programming approach organizes programs in a way

that mirrors the real world, in which all objects are

associated with both attributes and activities. Programming

in Java involves thinking in terms of objects; a Java

program can be viewed as a collection of cooperating

objects.

This chapter introduces the fundamentals of object-oriented

programming: declaring classes, creating objects,

manipulating objects, and making objects work together.

Objects and Classes

Object-oriented programming (OOP) involves programming using

objects. An object represents an entity in the real world

that can be distinctly identified. For example, a student, a

desk, a circle, and even a mortgage loan can all be viewed

as objects. An object has a unique identity, a state and

behaviors. The state of an object consists of a set of

fields, or fields with their current values. The behavior of

an object is defined by a set of methods. Figure 6.1 shows a

diagram of an object with its data fields and methods.

***Insert Figure 6.1 (Same as Figure 5.1 in the 3 rd Edition,

p139)

Figure 6.1

An object contains both state and behavior. The state

defines the object, and the behavior defines what the object

does.

A Circle object, for example, has a data field radius, which

is the property that characterizes a circle. One behavior of

a circle is that its area can be computed. A Circle object

is shown in Figure 6.2.

220

***Insert Figure 6.2 (Same as Figure 5.2 in the 3 rd Edition,

p139)

Figure 6.2

A Circle object contains the radius data field and the

findArea method.

Classes are constructs that define objects of same type. In

a Java class, data are used to describe properties, and

methods are used to define behaviors. A class for an object

contains a collection of method and data definitions. Here

is an example of the class for a circle:

class Circle

{

/**The radius of this circle*/

double radius = 1.0;

/**Return the area of this circle*/

double findArea()

{

return radius*radius*3.14159;

}

This class is different from all of the other classes you

have seen thus far. The Circle class does not have a main

method. Therefore, you cannot run this class; it is merely a

definition used to declare and create Circle objects. For

convenience, the class that contains the main method will be

referred to as the main class in this book.

A class is a blueprint that defines what an object’s data

and methods will be. An object is an instance of a class.

You can create many instances of a class (see Figure 6.3).

Creating an instance is referred to as instantiation . The

terms object and instance are often interchangeable. The

relationship between classes and objects is analogous to the

relationship between apple pie recipes and apple pies. You

can make as many apple pies as you want from a single

recipe.

***Insert Figure 6.3

221

Circle

UML Graphical notation for classes

radius: double

UML Graphical notation for fields

UML Graphical notation for methods

findArea(): double

new Circle()

circle1: Circle

circlen: Circle

UML Graphical notation

for objects

radius = 2

...

radius = 5

Figure 6.3

A class can have many different objects.

NOTE: Figure 6.3 uses the graphical notations

adopted in the Unified Modeling Language (UML)

to illustrate classes and objects. UML has

become the standard for object-oriented

modeling. For more information on UML, see

www.rational.com/uml . For a summary of the

graphical notations used in this book, see

Appendix G, “UML Graphical Notations.”

Creating Objects and Object Reference Variables

Objects are created using the new operator as follows:

new ClassName();

For example, new Circle() creates an object of the Circle

class. Newly created objects are allocated in the memory.

Objects are accessed via object reference variables , which

contain references to the objects. Such variables are

declared using the following syntax:

ClassName objectReference;

The types of reference variables are known as reference

types . The following statement declares the variable

myCircle to be of the Circle type:

Circle myCircle;

The variable myCircle can reference a Circle object. The

following statement creates an object and assigns its

reference to myCircle.

myCircle = new Circle();

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