SE-1010 Dr. Mark L. Hornick 1 Introduction to Object-Oriented Programming (OOP) Part 1

SE-1010Dr. Mark L. Hornick

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Introduction to Object-Oriented Programming (OOP)

Part 1


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Objectives

Name the basic components of object-oriented programming.

Differentiate classes and objects.

Define class methods and attributes.

Draw UML diagrams for classes and objects.


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OOP is about classes and objects

These are two very basic concepts in OOP

…but what are they?

Java is an OOP language


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Alan Kay, who was instrumental in the creation of the first Apple Macintosh, was also the creator of the first OOP language called Smalltalk.

He defined OOP as follows:

http://en.wikipedia.org/wiki/Image:Alan-Kay.jpg


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1. Everything is an object

An object is a thing, both tangible…

Objects often represent physical entities


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1. Everything is an object

… and intangible, like• Time

• Date

• Bank Account

• Grocery List

Or, objects can just represent ideas or concepts


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2. Every object has a type of class

Classes are more difficult to describe:• A Class is an abstraction (a blueprint,

or template) that defines the attributes and behavior of Objects that belong to the Class

• Objects assume the characteristics of a class

We say an Object is an Instance of a Class, kind of how a cake is an instance of a recipe for a cake.

We can also say that a Class Instance is an Object.


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3. An object is comprised of attributes and methods defined by its class

attributes are data that define an object’s properties• Each object has its own variables where it

can store the values of its attributes

methods are behaviors• Methods are executed when an object

receives a message to execute it. • Methods often manipulate attributes

Alan Kay’s OOP definitions, continued


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Exercise 1

List some attributes and behaviors for a BankAccount class Pretend it’s used within the software of an ATM

machine (or within a personal finance app) What does it represent? What can it do? What are its properties?


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Exercise 2

List some attributes and behaviors for a Printer class What does a printer do? What properties does it have?


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Unified Modeling Language (UML)

A notation for objects and classes. Can be applied to any OOP language (not just Java).

cd Data Model

<class name>

- <attribute1>: int- <attribute2>: int

+ <method1>() : void+ <method2>() : void

UML class diagram


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We’re not done with Alan Kay’s definitions yet…


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4. Object-oriented programs use objects

• An object-oriented program is a bunch of objects telling each other what to do by sending and receiving messages to and from one another• A message instructs an object to

execute one of it’s methods

Alan Kay’s OOP definitions, continued


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The Relationship between Messages and Methods

To instruct an object to do something, we “send a message” to it.

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

In Java (and other OO languages like C++, C#, or VB), messages are sent by calling a method defined within the object’s class we also say “execute a method” or “invoke a method” and sometimes we use “function” instead of “method”

We don’t usually send messages to classes, although there are exceptions we’ll learn about later


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UML again

A UML Sequence Diagram illustrating messages being sent to an instance of a class.

hp1:Printerpurge

The message purge

The object’s name hp1 The object’s

class Printer

print(“report.txt”)


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Passing values in messages

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

hp1:Printer

The message print with theargument “report.txt”

The object’s name hp1

The object’s class Printer

print(“report.txt”)


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Two-way communication

Many times, a method will return a message back to the sender.

hp11:Plotter

getPaperLevel()

The method

The object’s name The object’s

classNo argument

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The return value


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Many arguments can be sent in an originating message, but only one return value can be sent back

hp11:Plotter

setFont(“Arial”, “italic”)

The method

The object’s name The object’s

classTwo arguments

“ok”

The return value


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Object-oriented programming

• The object-oriented (OO) approach provides tools for the programmer to represent elements in the problem, or domain space

• Elements in the problem space, and their representation in the solution space, are referred to as “objects”

• OO allows a programmer to define a class (the type of an object) to fit the problem, rather than being forced into existing data types representing units of storage in a machine

• Object-orientation allows you to describe the problem in terms of the problem, rather than in terms of the solution

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SE-1010 Dr. Mark L. Hornick 1 Introduction to Object-Oriented Programming (OOP) Part 1