Georgia Institute of Technology

Understanding Objects and Classes

Barb Ericson

ericson@cc.gatech.edu

Jan 2005

Georgia Institute of Technology

Learning Goals

• What does object-oriented mean?

• What are objects?

• What are classes?

• Exercises for conceptually understanding objects and classes– Scenarios– Role-playing

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What is Object-Oriented?

• This means that we focus on the objects not just the procedures– Focus on – who (what objects?)– as well as – what (what do the

objects do?)

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What is an Object?

• A person, place, or thing– That knows something about

itself• Has data (attributes, fields)• A cashier has a id, name, and a

password

– And can do something• Has operations (methods)• A cashier can total the items,

take payment, make change

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What is a Class?

• The type of an object – The way we classify

an object• “The Idiot” by

Dostoevsky is a book• “War and Peace” by

Tolstoy is a book• Mary is a cashier• Tasha is a cashier

• Grouping of objects with the same data and operations

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Class: Example

• Mary is a cashier• Tasha is a cashier• Cashier is a class

– All cashiers have an id, name and password

• Each will have a different id, name, and password

– All cashiers can total an order, take payment, make change

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Object Data

• Each object has its own data– Tasha’s id is 4 and

password is mhall– Mary’s id is 3 and

password is smile4

• All cashier objects have an id, name, and password

• Changing Mary’s data won’t affect Tasha’s data

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Understanding Objects and Classes• What “types” of objects

are in the classroom? • What data is known about

each type and what operations can objects of that type do?

• Are there several objects of the same “type”.– How are they the same and

how different?

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Simulation

• Object-oriented development means creating a simulation of the problem– We need to know the

objects in the problem• So we can create

software objects to represent them

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Classes Define the Objects

• The computer doesn’t know what we mean by a car or cashier

• We define the class Cashier so that the computer will understand what a cashier or bank account “is” and what it can “do”– In the context of the problem we are trying to

solve

• Then the computer can create objects to do the simulation

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Classes Create Objects

• The class can be thought of as a recipe, blueprint, or factory

• Many objects can be created from one class

• Objects keep track of the class that created them– I am an object (instance) of

the Cookie class

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Software Objects are Models

• The objects we create in software are models of the physical object– We can’t stick a

person in our software– We can create a

model of the person with the information we need to know for that person for our task

Cashier

id

name

password

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Using Scenarios to Identify Objects

• What objects are involved in going to the doctor because you are sick?

• What objects are involved in going to the movies?

• What objects are involved in a card game like blackjack?

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Objects have Responsibilities

• An object-oriented design– Determines the tasks to be done– Determines what objects will be responsible

for each task• No one object does everything• Objects work together to accomplish tasks

– The assignment of responsibilities is the key skill in object-oriented design

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Identifying Classes

• What type of thing was each thing in the scenario? How would you classify it?– Patient, Doctor, Nurse, Insurance …– Movie, Ticket Seller, Ticket Taker, Ticket,

Popcorn, Drink …– Card, Deck, Player, Suit …

• How many objects of that types were in each scenario?– What data did each object have associated

with it? What could it do?

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Learning about Objects and Classes

• Walk through scenarios• Do role-playing• Underline nouns in scenarios to identify

the objects and classes• Create CRC cards• Draw UML diagrams• Write methods and classes• Specialize a parent class• Use a debugger to step through execution

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Role Playing Exercises

• Page at http://web.sbu.edu/cs/dlevine/RolePlay/roleplay.html has links to exercises

• Powerpoint slides you can hand out developed by Joseph Bergin of Pace University– FirstDayRolePlay-Bergin.ppt

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Summary• Object-oriented programs are simulations

– Objects simulate the “real world” and interact to accomplish a task

– Classes define the data all objects will have and what things they can do

• Ways to start understanding Objects and Classes – Scenarios– Role-Playing