Principles of Object-Oriented ProgrammingPrinciples of Object-Oriented Programming

Why OOP is popular?Why OOP is popular? Language and thoughtsLanguage and thoughts A new way of viewing the worldA new way of viewing the world Elements of OOPElements of OOP Coping with complexityCoping with complexity

Difficult QuestionsDifficult Questions

• What is Object-Oriented Programming?

• Why is it so popular?

– OOP is a revolutionary idea, totally unlike anything that has come before in programming languages

– OOP is an evolutionary step, following naturally on the heels of earlier programming abstractions

Why is OOP Popular?Why is OOP Popular?

A few possible reasons why OOP is popular:

• Hope that it will quickly and easily lead to increased productivity and increased reliability (solve the software crises)

• Similarity to techniques of thinking about problems in other domains

• Hope for easy transition from existing languages (e.g., C or Pascal)

A New ParadigmA New Paradigm

We start by considering the definition of the term ``paradigm'':

Par a digm n. 1. A list of all the inflectional forms of a word taken as illustrative example of the conjugation or declension to which it belongs. An example or model. [Late Latein paradigma, from Greek paradeigma, modern paradeiknunai, to compare, exhibit.]

OO programming is a new paradigm• New way of thinking about what it means to compute, and about what we ca

n structure information inside a computer

Let’s first see the relationship between languages and thoughts

Language and ThoughtsLanguage and Thoughts

In linguistics there is a hypothesis that the languages we speak directly influence the way in which we view the world (called

Sapir-Whorf Hypothesis)

Controversial example of languages influencing thoughts – Eskimo (or Innuit) languages have many words to describe snow – Arabic languages and camels

Should not over-generalize the conclusion– With time and training, we can also distinguish snow; however, our language does not

force me into doing so, so it is not natural to me– Different languages can lead one (but does not require one) to view the world in a diff

erent fashion This is also applicable to computer programming languages

– Using OO language can naturally lead one to view the world in OO fashion, but does not force one to do

An Example from Computer LanguagesAn Example from Computer Languages

The programming language chosen to solve a problem will decide fundamentally the way in which an algorithm is developed– A student working in DNA research had the task of finding repeated sequences of M

values in a long sequence of values:

ACTCGGATCTTGCATTTCGGCAATTGGACCCTGACTTGGCCA ...

Wrote the simplest (and therefore, most efficient?) Fortran program:

DO 10 I = 1, N-M DO 10 J = I+1, N-M FOUND = .TRUE. DO 20 K = 1, M 20 IF X[I+K-1] .NE. X[J+K-1] THEN FOUND = .FALSE. IF FOUND THEN ... 10 CONTINUE

Took a depressingly long time.

A Better SolutionA Better Solution

A friend writing in APL found a much better solution by rearranging the data and sorting.

A C T C G G positions 1 to M C T C G G A positions 2 to M+1 T C G G A T positions 3 to M+2 C G G A T T positions 4 to M+3 G G A T T C positions 5 to M+4 G A T T C T positions 6 to M+5 . . . T G G A C C G G A C C C . . .

Ran surprisingly fast

What lead to the discovery?What lead to the discovery?

Why did the APL programmer find the better solution?

• Fortran programmer was blinded by a culture that valued loops, simple programs

• Sorting is a built-in operation in APL, so good APL programmers try to find solutions based on sorting

• APL programmers were “naturally” led to better solutions

Programming language directs the programmer’s mind to view the problem in certain way

Church's ConjectureChurch's Conjecture

Language affects the way of thinking is believable, but Sapir-Whorf hypothesis is controversial– It claims that it is possible for one working in one language to imagine thoughts t

hat cannot be translated or understood in different languages In computer science, we have a directly opposite assertion:

Church's Conjecture: Any computation for which there exists an effective procedure can be realized by a Turing machine.

Any programming language can simulate Turing machine– All languages are equivalent, so anything can be done in any language– But it may simply be easier or more efficient to use one language or another

Power of OO programming languages should be understood in a similar way; they make it easy to maintain large S/W projects

Imperative Programming ParadigmImperative Programming Paradigm

Imperative programming is the ``traditional'' model of computation.

• State • Variables • Assignment • Loops

A processing unit is separate from memory, and ``acts'' upon memory– Computer is the data manager– wandering through memory, pulling values in memory slots– transforming them in some manner, and pushing results back in some other

slots– Although this is exactly what happens inside a computer, it is not the way peo

ple do for solving problems

Visualization of Imperative ProgrammingVisualization of Imperative Programming

Sometimes called the ``pigeon-hole'' model of computation.

Recursive DesignRecursive Design

The structure of the part mirrors the structure of the larger unit.

Alan Kay's Description of OO ProgrammingAlan Kay's Description of OO Programming

OO programming is based on the principle of recursive design.

1. Everything is an object 2. Objects perform computation by making requests of each other

through the passing of messages (bundled with arguments)3. Every object has it's own memory, which consists of other objects. 4. Every object is an instance of a class. A class groups similar

objects. 5. The class is the repository for behaviors associated with an object 6. Classes are organized into singly-rooted tree structure, called an

inheritance hierarchy.

We can illustrate these principles by considering how I go about solving a problem in real life.

Illustration of OOP Concepts Illustration of OOP Concepts - Sending Flowers to a Friend- Sending Flowers to a Friend

To illustrate the concepts of OOP in an easily understood framework, consider the problem of sending flowers to a friend who lives in a different city.

I can't deliver them myself. So I use my local Florist.

I tell my Florist (named Flo) the address for my friend, how much I want to spend, and the type of flowers I wish to send.

Flo contacts a florist in my friends 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 arrangments, and so on.

And so we see, that to solve my problem requires the interaction of an entire community of individuals.

Elements of OOP – ObjectsElements of OOP – Objects

1. Everything is an object.

Actions in OOP are performed by agents, called instances or objects.

There are many agents working together in my scenario. We have myself, my friend, the florist, the florist in my friends 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.

Elements of OOP – MessagesElements of OOP – Messages

2. 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 messges. An instance may accept a message, and in return will perform an action and return a value.

To begin the process of sending the flowers, I give a message to Flo. She in turn gives a message to the florist in my friends city, who gives another message to the driver, and so on.

Information HidingInformation Hiding

Notice that I, as a 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.

Elements of OOP – ReceiversElements of OOP – Receivers

Messages differ from traditional function calls in two very important respects:

• In a message there is a designated receiver that accepts the message (in function calls, there is no receiver

• The interpretation of the message may be different, depending upon the receiver

Different ActionsDifferent Actions

var Flo : Florist; Beth : Wife; Ken : Dentist;

begin Flo.sendFlowersTo(myFriend); { will work } Beth.sendFlowersTo(myFriend); { will also work but in different way} Ken.sendFlowersTo(myFriend); { will probably not work }

end;

Behavior and InterpretationBehavior and Interpretation

Although different objects may accept the same message, the actions (behavior) the object will perform will likely be different.

Usually, the specific receiver for any given message will not be known until run-time, so the determination of what behavior (code) to perform may be made at run-time, a form of late binding.• Different from early binding of name to code in function calls

The fact that the same name can mean two entirely different operations is one form of polymorphism.

Responsibility and Non-interferenceResponsibility and Non-interference

• Behavior is described in terms of responsibilities• My message indicates only the desired outcome• I do not want to interfere with how the florist achieves the

outcome• Responsibility is closely related to non-interference

• Implies greater independence between agents, which is a critical factor in solving complex systems

• OOP is responsibility-driven software design process

• The difference between traditional and OO perspectives can be summarized by the following:

``Ask not what you can do to your data structures, but ask

what your data structures can do for you''

Elements of OOP - Recursive DesignElements of OOP - Recursive Design

3. Every object has it's own memory, which consists of other objects.

Each object is like a miniature computer itself - a specialized processor performing a specific task.

Elements of OOP – ClassesElements of OOP – Classes

4. Every object is an instance of a class. A class groups similar objects.

5. The class is the repository for behavior associated with an object.

The behavior I expect from Flo is determined from a general idea I have of the behavior of Florists.

We say Flo is an instance of the class Florist.

Behavior is associated with classes, not with individual instances. All objects that are instances of a class use the same method in response to similar messages.

Hierarchies of CategoriesHierarchies of Categories

But there is more that I know about Flo then just that she is a Florist. I know she 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.

Class HierarchiesClass Hierarchies

Elements of OOP – InheritanceElements of OOP – Inheritance

6. Classes are organized into a singly-rooted tree structure, called an inheritance hierarchy

Information (data and/or behavior) I associate with one level of abstraction in a class hierarchy is automatically applicable to lower levels of the hierarchy.

Elements of OOP – OverridingElements of OOP – Overriding

Subclasses can alter or override information inherited from parent classes for exceptional cases

• All mammals give birth to live young • Some mammals lays eggs

Summary: OOP View of ComputingSummary: OOP View of Computing

The OOP view of computation is similar to creating a universe of interacting computing objects

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.

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.

Coping with ComplexityCoping with Complexity

Another way to understand OOP is to place it in a historical context.

Non-linear behavior of complexity– As programming projects become large, they tend to require a team of progra

mmers– Interesting phenomenon: a task that takes 2 months with 1 programmer requir

es more than 1 month even with 2 programmers

``software crises'' came out when people realized the major problem in software development were not algorithmic, but were caused by communication difficulties and the management of complexity.

Interconnections - the Bane of ComplexityInterconnections - the Bane of Complexity

Many software systems are complex not because they are large, but because they have many interconnections.

Interconnections make it difficult to understand pieces in isolation, or to carry them from one project to the next.

The inability to cleanly separate out components makes it difficult to divide a task between several programmers.

Complexity can only be managed by means of abstraction, by eliminating information that a programmer do not need to know.

If we examine the history of mechanisms used to solve the problem

of complexity we can better appreciate the role of OOP.

Assembly LanguagesAssembly Languages

Assembly languages and linkers were perhaps the first tools used to abstract features of the bare machine.

• Addresses could be represented symbolically, not as a number.

• Symbolic names for operations.

• Linking of names and locations performed automatically

Procedures and FunctionsProcedures and Functions

Libraries of procedures and functions (such as mathematical or input/output libraries) provided the first hints of information hiding.

They permit the programmer to think about operations in high level terms, concentrating on what is being done, not how it is being performed.

But they are not an entirely effective mechanism of information hiding, and they only partially solved the problem of multiple programmers making use of the same names

An example - A stack

The Problem of StacksThe Problem of Stacks

int datastack[100]; int datatop = 0;

void init() // initialize the stack { datatop = 0; } void push(int val) // push a value on to the stack { if (datatop < 100)

datastack [datatop++] = val; } int top() // get the top of the stack { if (datatop > 0)

return datastack [datatop - 1]; return 0; } int pop() // pop element from the stack { if (datatop > 0)

return datastack [--datatop]; return 0; }

Block Scoping Didn't Solve the ProblemBlock Scoping Didn't Solve the Problem

Problems with previous implementation– we need to use global variables for the array (no access control)– method names cannot be used elsewhere

More control over name visibility using block scoping in Algol and Pascal, but any scope that accesses the four methods can access the data as well

begin var datastack : array [ 1 .. 100 ] of integer; datatop : integer; procedure init; ... procedure push(val : integer); ... function pop : integer; ...

... end;

ModulesModules

Modules basically provide collections of procedures and data with import and export statements [Parnas 1972]– they can distinguish public part that are accessible outside and private part th

at are accessible only within the module– Enforces need-to-know principle

Solves the problem of encapsulation -- but what if your programming task requires two or more stacks? – Only one stack can be manipulated with a module– Need to define a different module for another stack- with what names?

Modules provide information hiding but do not allow instantiation, which is the ability to make multiple copies of the data area– need to develop a new concept

Abstract Data TypesAbstract Data Types

An Abstract Data Type is a programmer-defined data type that can be manipulated in a manner similar to system-provided data types

• Have the ability to instantiate many different copies of the data type.• E.g., define stack as data type and instantiate it as many as you want • Data type can be implemented using provided operations, without knowledge

of internal representation.• Packages in CLU and Ada can define abstract data types

Objects - ADT's with Message PassingObjects - ADT's with Message Passing

Characteristics of Objects

• Encapsulation -- similar to modules • Instantiation -- similar to ADT's • Messages -- dynamic binding of procedure names to behavior

• Activity is initiated by a request to a specific object, not by calling a function (do you call push with a stack and a data value, or do you ask a stack to push a data value onto itself?)

• Added to message passing, overloading of names and reusing software • Interpretation of message -- it depends on objects receiving it

• Push for stack and push for door are different• Inheritance -- Allow different data types to share the same code• Polymorphism -- Allow the shared code to be tailored to fit the specific circu

mstances

SummarySummary

• Object-oriented programming is not simply features added to a programming language. Rather, it is a new way of thinking

• Object-oriented programming views a program as a community of agents, termed objects. Each object is responsible for a specific task.

• An object is an encapsulation of state (data values) and behavior (operations).

• The behavior of objects is dictated by the object class.

• An object will exhibit its behavior by invoking a method (similar to executing a procedure) in response to a message.

• Objects and classes extend the concept of abstract data types by adding the notion of inheritance.