CSCI 383
Object-Oriented Programming & Design
Lecture 3
Martin van Bommel
Imperative Programming
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So, what are the paradigms of programming?
Imperative programming is the “traditional” model of
computation
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State
Variables
Assignment
Loops
A processing unit is separate from memory,
and “acts” upon memory
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Visualization of IP
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Recursive Design
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Why Not Build Program out of Computers?
Alan Kay wondered why conventional computers were
constructed out of pieces that were useless by
themselves
Why not build a whole out of pieces that were similar
at all levels of detail? (Think of fractals)
Idea: A program can be build out of little computing
agents
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Recursive Design (cont’d)
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The structure of the part mirrors the structure of the larger
unit
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Kay's Description of OOP
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Object-oriented programming is based on the principle of
recursive design
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Everything is an object
Objects perform computation by making requests of each other through
the passing of messages
Every object has its own memory, which consists of other objects
Every object is an instance of a class.
A class groups similar objects
The class is the repository for behavior associated with an object
Classes are organized into singly-rooted tree structure, called an
inheritance hierarchy
We can illustrate these principles by considering how to go
about solving a problem in real life
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Illustration of OOP Concepts
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To illustrate the concepts of OOP in an easily
understood framework, consider Chris sending flowers
to Robin.
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Chris can't deliver them directly.
So Chris uses the services of the local Florist
Chris tells the Florist (named Fred) the address for Robin, how
much to spend, and the type of flowers to send
Fred contacts a florist in Robin’s city, who arranges the flowers, then
contacts a driver, who delivers the flowers
If we start to think about it, there may even be other people
involved in this transaction. There is the flower grower, perhaps
somebody in charge of arrangements, and so on
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Agents and Communities
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Our first observation is that results are achieved
through the interaction of agents, which we will call
objects
Furthermore, any nontrivial activity requires the
interaction of an entire community of objects working
together
Each object has a part to play, a service they provide
to the other members of the community
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Elements of OOP - Objects
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So we have Kay's first principle
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Actions in OOP are performed by agents, called
instances or objects
There are many agents working together in the
scenario
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Everything is an object
Chris, Robin, the florist, the florist in Robin’s city, the driver,
the flower arranger, and the grower.
Each agent has a part to play, and the result is
produced when all work together in the solution of a
problem
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Elements of OOP - Messages
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And principle number 2:
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Objects perform computation by making requests of each
other through the passing of messages
Actions in OOP are produced in response to requests
for actions, called messages.
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An instance may accept a message, and in return will
perform an action and return a value
To send the flowers, Chris gives a message to Fred. Fred in
turn gives a message to the florist in Robin’s city, who gives
another message to the driver, and so on
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Information Hiding
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Notice that the user of a service being provided by an
object need only know the name of the messages that
the object will accept
I need not have any idea how the actions performed in
response to my request will be carried out
Having accepted a message, an object is responsible
for carrying it out
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Elements of OOP - Receivers
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Messages differ from traditional function calls in two
very important respects:
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In a message, there is a designated receiver that accepts the
message
The interpretation of the message may be different,
depending upon the receiver
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OOP: The General Principle
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An object oriented program is structured as a
community of interacting agents called objects.
Action is initiated by the transmission of a message to
an agent (an object).
The message encodes the request for action and is
accompanied by additional information (arguments)
needed to carry out the request.
The receiver is the agent to which the message is sent.
If the receiver accepts the message it accepts the
responsibility to carry out the indicated action.
In response to a message the receiver will perform
some method to satisfy the request.
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OOP: A Way to View the World
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Chris wants to send some flowers to his friend Robin
who lives in a distant city.
Because of the distance, Chris cannot pick up the
flowers and bring them to Robin in person.
Chris goes down to Fred, a local florist, and tells him
the number and type of flowers he wants together with
the address they need to be delivered to.
Chris can rest assured that the flowers will be
delivered.
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Agents, Messages, Methods
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Chris finds an appropriate agent (Fred)
And passes to him a message containing a request
It is the responsibility of Fred to satisfy the request
There is some method (a set of operations) used by
Fred to do this
Chris does not need to know the particular method he
will use, this information is hidden from inspection
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Behavior and Interpretation
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Although different objects may accept the same
message, the actions (behavior) the object will
perform will likely be different
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If Fred asked his wife Beth to send some flowers to Robin for
her birthday, she might use a different agent than the florist
Fred
determination of what behavior to perform may be
made at run-time (late binding)
The fact that the same name can mean two entirely
different operations is one form of polymorphism, a
topic we will discuss later
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OOP Element - Recursive Design
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Every object has its own memory, which consists of
other objects
Each object is like a miniature computer itself - a
specialized processor performing a specific task
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Non-interference
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It is important that objects be allowed to perform their
task however they see fit, without unnecessary
interactions or interference with other objects
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“Instead of a bit-grinding processor ... plundering data
structures, we have a universe of well-behaved objects that
courteously ask each other to carry out their various desires”
-- Dan Ingalls
“Ask not what you can do to your data structures, but ask
what your data structures can do for you”
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Elements of OOP - Classes
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Every object is an instance of a class. A class groups
similar objects
The class is the repository for behavior associated
with an object
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The behavior I expect from Fred is determined from a
general idea I have of Florists
We say Fred is an instance of the class Florist
Behavior is associated with classes, not with individual
instances.
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All objects that are instances of a class use the same method
in response to similar messages
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Hierarchies of Categories
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But there is more that I know about Fred than just
that he is a Florist.
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I know he is a ShopKeeper, and a Human, and a Mammal,
and a Material Objects, and so on
At each level of abstraction I have certain information
recorded.
That information is applicable to all lower (more
specialized) levels
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Elements of OOP - Inheritance
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The principle that knowledge of a more general
category is also applicable to a more specific
category is called inheritance
Classes can be organised into a hierarchical
inheritance structure.
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A child class (subclass) will inherit attributes from a parent
class (super class) higher up in the hierarchy.
An abstract parent class is a class for which there are no
direct instances, it is only introduced to group subclasses
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Hierarchies of Categories
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Class Hierarchies
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Elements of OOP - Overriding
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Subclasses can alter or override information inherited
from parent classes:
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All mammals give birth to live young
A platypus is an egg-laying mammal
The search for a method to use in response to a
message starts with the receiver’s class and continues
up the parent chain. When methods with the same
name are available higher in the class hierarchy the
method that executes is said to override the inherited
behavior
Inheritance combined with overriding are where most
of the power of OO originates
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Problem Solving
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Because the OOP view is similar to the way in which
people go about solving problems in real life (finding
another agent to do the real work!), intuition, ideas,
and understanding from everyday experience can be
brought to bear on computing
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However, infinite recursion does not solve the problem
Someone has to do the work!
On the other hand, common sense was seldom useful
when computers were viewed in the process-state
model, since few people solve their everyday problems
using pigeon-holes
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From Newsweek
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“Unlike the usual programming method – writing
software one line at a time – NeXT's ‘object-oriented’
system offers larger building blocks that developers
can quickly assemble the way a kid builds faces on
Mr. Potato Head.”
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