Chapter 1

© 2000 McGraw-Hill Modified by C.W.Pang with author's permission

Intro to OOP with Java--Wu Chapter 1 - 1

Chapter 1

Introduction to Object-oriented Programming

and

Software Development



Chapter 1 Objectives

After you have read and studied this chapter, you should be able to

Name the basic components of object-oriented programming.

Differentiate classes and objects.

Differentiate class and instance methods.

Differentiate class and instance data values.

Draw object diagrams using icons for classes, objects, and other components of object-oriented programming.

Describe the significance of inheritance in object-oriented programs.



Classes and Objects

Object-oriented programs use objects.

An object is a thing, both tangible and intangible. Account, Vehicle, Employee, etc.

To create an object inside the computer program, we must provide a definition for objects—how they behave and what kinds of information they maintain —called a class.

An object is called an instance of a class.



Graphical Representation of an Object

Account

SV129

The class name is placed inside the object icon.

The class name is placed inside the object icon.

The object’s name appears on top of the icon.

The object’s name appears on top of the icon.

An icon for an object is the rounded rectangle.

An icon for an object is the rounded rectangle.

The class name may be omitted when it is clear from the context which class the object belongs to.

The class name may be omitted when it is clear from the context which class the object belongs to.

customer1



Graphical Representation of a Class

AccountThe class name appears on top of the icon.

The class name appears on top of the icon.

An icon for a class is the rectangle.

An icon for a class is the rectangle.



Instance-of Relationship

Employee

Employee

Bill

Employee

Steve

Employee

AndyThe class name can be omitted since it is clear which class these objects belong to .

The class name can be omitted since it is clear which class these objects belong to .

The dotted line shows the instance-of relationship.

The dotted line shows the instance-of relationship.

Before you can create instances of a class, the class must be defined.

Before you can create instances of a class, the class must be defined.



Messages and Methods

To instruct a class or an object to perform a task, we send a message to it.

You can send a message only to the classes and objects that understand the message you sent to them.

A class or an object must possess a matching method to be able to handle the received message.

A method defined for a class is called a class method, and a method defined for an object is called an instance method.

A value we pass to an object when sending a message is called an argument of the message.



Sending a Message

Message deposit with the argument 250.00 is sent to chk-008.

Message deposit with the argument 250.00 is sent to chk-008.

Account

chk-008

depositdeposit 250.00

Message name is usually omitted in the diagram.

Message name is usually omitted in the diagram.

deposit250.00



Sending a Message and Getting an Answer

This message has no argument.

This message has no argument.

Account

chk-008

getMonthlyFee

monthly fee

The method returns the value monthly fee back to the message sender.

The method returns the value monthly fee back to the message sender.



Calling a Class Method

Account

getAverageBalance

average balance

The average balance of all accounts is returned.

The average balance of all accounts is returned.



Summary of Class and Object Icons

<result>

<Class Name> Squared corners are used for a class and class methods.

Squared corners are used for a class and class methods.

An instance method icon is drawn in a dotted line.

An instance method icon is drawn in a dotted line.

<Class Name>

<Object Name>

<result>

Rounded corners are used for an object and instance methods.

Rounded corners are used for an object and instance methods.



Class and Instance Data Values

An object is comprised of data values and methods.

An instance data value is used to maintain information specific to individual instances. For example, each Account object maintains its balance.

A class data value is used to maintain information shared by all instances or aggregate information about the instances.

For example, minimum balance is the information shared by all Account objects, whereas the average balance of all Account objects is an aggregate information.



Sample Instance Data Value

Account

SV129

Account

SV506

Account

SV008

current balance current balance current balance

All three Account objects possess the same instance data value current balance.

All three Account objects possess the same instance data value current balance.

The actual dollar amounts are, of course, different.

The actual dollar amounts are, of course, different.

908.55 1304.98 354.00



Sample Class Data Value

Account

SV129

Account

SV506

Account

SV008

current balance current balance current balance908.55 1304.98 354.00

Account

minimum balance

100.00

There is one copy of minimum balance for the whole class and shared by all instances.

There is one copy of minimum balance for the whole class and shared by all instances.



Variable and Constant Data Values

There are two types of data values:

Account

Account

SV129

minimum balance

100.00current balance

908.55

account prefix

6427opening balance

100.00A constant whose value must remain fixed over time.

A constant whose value must remain fixed over time.

A variable whose value can change over time.

A variable whose value can change over time.



Inheritance

In object-oriented programming, we use a mechanism called inheritance to design two or more entities that are different but share many common features.

First we define a class that contains the common features of the entities. Then we define classes as an extension of the common class inheriting everything from the common class.

We call the common class the superclass and all classes that inherit from it subclasses. We also call the superclass an ancestor and the subclass a descendant.



Account and Its Subclasses

Account

minimum balance

100.00

Savings Checking

minimum balance

250.00

This class becomes the superclass of two subclasses…

This class becomes the superclass of two subclasses…

when (sub)classes inherit from it.

when (sub)classes inherit from it.

Inherited components are not shown in the subclasses unless…

Inherited components are not shown in the subclasses unless…

the subclass(es) overrides them.

the subclass(es) overrides them.



Sample Inheritance HierarchyAccount

Savings Checking

SuperSaver Regular StudentInterestBearing

ATMChecking



Having Fun with Java

/*Program FunTime

The program will allow you to draw a picture by dragging a mouse (move the mouse while holding the left mouse button down; hold the button on Mac). To erase the picture and start over, click the right mouse button (command-click on Mac).

*/

import javabook.*;

class FunTime{

public static void main(String[ ] args){

SketchPad doodleBoard;

doodleBoard = new SketchPad();

doodleBoard.setVisible( true );}

}

Declare a nameDeclare a name

Create an objectCreate an object

Make it visibleMake it visible



Object Diagram for FunTime

FunTime

main

SketchPad

doodleBoard

setVisibletrue



Execution Flow of the FunTime Program



doodleBoard.setVisible( true );


doodleBoard


SketchPadSketchPad

doodleBoard.setVisible( true );

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