Workshop for Programming And
Systems Management Teachers
Chapter 2
Introduction to Object-Oriented
Programming
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Learning Goals
• Understand at a conceptual level
– What is object-oriented programming?
– What is an object?
– What is a class?
– An object-oriented program is a simulation
– Objects are responsible for their data and
operations
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What does Object-Oriented mean?
• The focus is on objects
– Finding objects
– Describing objects
– Classifying objects (What type of thing?)
– Describing how objects interact to accomplish
a task
– Determining the relationships between types
of objects
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Object-Oriented Example
• What happens when
you go to a restaurant
to get a meal?
• How many objects
are involved (people
and other things)?
• What things do the
objects know about?
• What things
(operations) can they
do?
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Objects and Classes
• The objects in the restaurant are the
people or things doing the work or being
acted upon
• The classes are how we classify the
objects
– What type of thing is the object?
• What job classifications are there in a
restaurant?
• How many people do you need for each
job classification?
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What is an Object?
• Person, place, or thing
– that knows something about itself
• has attributes (data)
– and can do something
• has operations (behaviors)
• Example object
– It is a dog named Spot. He is a small dog.
– He can wag his tail. He can bark.
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Objects are Responsible
• Objects are responsible for
their data
– The data is private
– Methods are used to modify
the data
• An object may refuse to do the
requested action because the
data would be left in an incorrect
state
– Like a negative account balance
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Objects know how to do their jobs
• Other objects don’t worry about how the
object handles the request
– They just assume it knows how
• This makes it easy to change one object
without affecting others
– Meaning the programs can be assembled
from existing objects
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What is a Class?
• The class is the abstract
basis for the object.
– It describes the data that
all objects of that class
have
• All dogs have a name, and
a size
– and the things they can do
• All dogs can wag their tails
and bark
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Class as Blueprint
• The class can be thought of
as the blueprint, cookie
cutter, or factory
– It gives the objects created
from it their shape.
– You can make many objects
from the same blueprint or
cookie cutter.
• A software class creates
the objects of that class
– An object is an instance of a
class.
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Object and Class Example
• These are objects
(instances) of the car
class
– Cars have data
• a manufacturer, model,
year, etc
– Cars have operations
• can go forward, go
backward, turn, stop
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Computation as Simulation
• An object-oriented program is a simulation of
the domain
• Objects send each other messages to
accomplish a task
– A message is a request
– Requests can be refused
Stop Playing
No
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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
Person
identifier
name
address
phoneNumber
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Gopher Bash Exercise
• Go to
http://javaboutique.int
ernet.com/Gopher/
• What objects do you
see?
• What type of things
are they?
• What data do they
have and what can
they do?
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Object-Oriented Principles
• Abstract Data Types - Objects and Classes
– Encapsulation of data and associated operations
– The ability to create your own data types (classes)
• Inheritance
– The ability to factor common things into a parent class
(generalize)
– This ability to do something different (specialize)
• Polymorphism
– A variable can be of different types (classes) at runtime
– Requires dynamic (run-time) binding
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Encapsulation
• Objects encapsulate attributes (data) and
operations (methods)
– The data is modified by the methods
– Other objects can ask an object to do
something to its’ data by sending a message
data
methods
data
message
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methods
Inheritance
• Attributes and operations are inherited
from the parent class.
– Dogs and cats are types of mammal and thus
bear live young
• The mammal class is a generalization
• The dog and cat classes are specializations
Mammal
Cat
Dog
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Advantage of Inheritance
• Handles commonality
– Common attributes and operations are
factored out and put in one place
– Not copied to several locations
• Easier to maintain, extend, and reuse
• Handles differences
– Children can inherit the parts that are
common from the parent
– They can add attributes and operations to
handle how they differ from the parent
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Polymorphism – Many Forms
• Dynamic binding
– The method depends on the run-time type
• Say we are creating an animal symphony
– All animals can make a noise
– The noise that is made depends on which animal
Animal
makeNoise()
Cat
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Dog
Pig
Kinds of Polymorphism
• Unrestricted
– No types are declared
– Any object can be of any type at run-time
– All binding of operations is done at run-time
• Restricted (inheritance-based)
– Types are declared
– Objects can be the declared type or a child of
the declared type at run-time
– May be able to do some compile-time binding
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Advantages of Polymorphism
• Used to create general algorithms that
work on objects of different types
– Collections that hold objects
• Makes it easy to add new types
– Just create the new class and implement the
required operations
Animal
makeNoise()
Cat Dog Pig Bird
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Summary
• Object-oriented programs are simulations
• Object-oriented programs have objects that
interact by sending each other messages
• Objects have data (fields) and operations
(methods)
• The three principles of object-oriented programs
are
– Abstract data types (objects)
– Inheritance
– Polymorphism
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