1. Understanding Objects and Classes
Barb Ericson
Jan 2005
Georgia Institute of Technology

2. 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
Georgia Institute of Technology

3. 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?)
This contrasts with procedural programming in which we focus on the task that needs to be done. You may break that task up into smaller parts.
In object-oriented programming we need to know who (what class) as well as what (procedures).
Object-oriented programming is concerned with responsibility. Who (what object) is responsible for doing the job, not just what jobs need to be done.
Georgia Institute of Technology

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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
Point out objects in the room (like a book, pen, computer, person, etc). Talk about what type of object it is, what data it has, what it can do.
The picture is a cashier in a music store. The data is his name and password. The operations are how to “ring up” and “take payment”, and “make change”.
Georgia Institute of Technology

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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
Objects are instances of a class. This slides shows two book objects of the class book.
Georgia Institute of Technology

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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
Georgia Institute of Technology

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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
Object data is also called instance variables, object variables, and object fields.
Georgia Institute of Technology

8. 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?
There are probably computers in your room. What do we mean by a “computer”? What data does it have (manufacturer, size screen, size memory, etc). What can it do?
Point out that nobody had to tell the students that these where computers. How did they know? They fit their idea of what a computer looks like.
Georgia Institute of Technology

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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
We can’t put the “real” objects into the computer so we need to make software objects that represent the “real” objects.
Georgia Institute of Technology

10. 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
Georgia Institute of Technology

11. 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
When you create an object it has a reference to the object that represent the class that created it. So, objects always know what type (class) they are.
Georgia Institute of Technology

12. 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
Georgia Institute of Technology

13. 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?
Ask people to walk through each of these scenarios. Then go back and see what objects they talk about or use. Bring a deck of cards with you and have two or more people play blackjack.
Georgia Institute of Technology

14. 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
Georgia Institute of Technology

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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?
What type of thing is each item? For example popcorn is a type of food (or snack). For each snack we would need to know at least the name and price.
Georgia Institute of Technology

16. 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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17. Role Playing Exercises
Page at has links to exercises
Powerpoint slides you can hand out developed by Joseph Bergin of Pace University
FirstDayRolePlay-Bergin.ppt
Georgia Institute of Technology

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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
Georgia Institute of Technology