OBJECT-ORIENTED PROGRAMMING - PowerPoint Presentation

Slide1

OBJECT-ORIENTED PROGRAMMINGSMALLTALK

Lecturers:Hesam SetarehSalar Moarref

1

Fall 2009 – Programming languagesSlide2

Topics

History and MotivationStructural OrganizationClasses and SubclassesMessage SendingImplementation

Object-Oriented extensionsEvaluation and Epilog

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History and Motivation

Alan Kay at the University of Utah in the late 1960s, suggested that it is possible to put the power of a room-sized, million dollar computer into a package. (Personal Computer)Personal computer needs a personal programming language.

Older languages were designed for the scientific and commercial applications that occupy large computers.

Kay designed a simulation and graphic-oriented programming language for

nonspecialists

.

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History and Motivation

Kay is a member of FLEX designer team. FLEX took the ideas of classes and objects from Simula.Kay wanted to provide a rich interactive environment for nonspecialists.

Xerox came in: In the 1971 Xerox produced a personal computer, called

Dynabook

.

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The born of Smalltalk

Smalltalk first version, Smalltalk-72, was designed and implement for Dynabook by 1972.Smalltalk-74, Smalltalk-76, Smalltalk-78 , and Smalltalk-80 are other versions of it.

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Topics

History and MotivationStructural OrganizationClasses and SubclassesMessage SendingImplementation

Object-Oriented extensionsEvaluation and Epilog

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In Smalltalk everything is object, including variables, numbers, strings, etc. For example : x*2

Any object has two major parts:7

Properties (Variables)

Behaviors (Methods)

Structural OrganizationSlide8

Scribe goto:500@500.

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ExampleSlide9

Scribe goto:500@500.

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Scribe go:300.

ExampleSlide10

Scribe goto:500@500.

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Scribe go:300.

Scribe turn:90.

ExampleSlide11

Scribe goto:500@500.

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Scribe go:300.

Scribe turn:90.

Scribe go:300.

ExampleSlide12

Scribe goto:500@500.

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Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

ExampleSlide13

Scribe goto:500@500.

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Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

Scribe go:300.

ExampleSlide14

Scribe goto:500@500.

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Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

ExampleSlide15

Scribe goto:500@500.

15

Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

Scribe go:300.

ExampleSlide16

Scribe goto:500@500.

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Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90

ExampleSlide17

Scribe goto:500@500.

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Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90.

Scribe go:300.

Scribe turn:90

4

timesRepeat

: [Scribe go:300. Scribe turn:90]

ExampleSlide18

Class

A class is a plan, which objects are made from.Getting a new object from a class is called instantiation.

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anotherScribe

← pen newAt:200@200Slide19

shape||

scribe penup;goto:loc;turnTo:tilt;pendn. 4 timesRepeat: [scribe go:size;turn:90]show || scribe color ink. self shape

erase || scribe color background. self shape

grow: amount ||

self erase.

size

←

size+amount

.

self show

Class Definition

19

class name

box

Instance variable name

Loc tilt size scribe

Instance message and methodsSlide20

newAt:initialLocation|newBox

| newBox ← self new. newBox setLoc:initialLocation

tilt:0 size:100 scribe:pen new.

newBox

show.

↑

newBox

shape||

scribe

penup;goto:loc;turnTo:tilt;pendn

.

4

timesRepeat

: [scribe go:size;turn:90]

Class Definition

20

class name

box

Instance variable name

Loc tilt size scribe

Instance message and methods

class message and methodsSlide21

Topics

History and MotivationStructural OrganizationClasses and SubclassesMessage SendingImplementation

Object-Oriented extensionsEvaluation and Epilog

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Classes and Subclasses

Smalltalk objects model Real-World!Some of objects have similar properties and behaviors.We can group classes of these objects and make superclass which has common variables and methods.

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dispayObject

box

pen

windowSlide23

Classes and Subclasses

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class name

box

Instance variable name

displayObject

Instance message and methods

superclass

Loc tilt size scribeSlide24

Classes and Subclasses

A Superclass can be subclass of another class.24

dispayObject

box

pen

window

number

…

Object

…Slide25

print ||

↑ realPt print + “+” imagePt print + “i”

Classes and Subclasses

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class name

complex

Instance variable name

realPt

imagePt

Instance message and methods

Subclasses can change the inherent method, this process is called OverridingSlide26

An Important Issue

In Smalltalk a class can’t have several superclass.But, in the Real-World, an object can inherent from more than one object.

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dispayObject

box

pen

bumper

number

inventoryItem

Object

roof

doorSlide27

An Important Issue

27dispayObject

brake

engine

bumper

number

inventoryItem

Object

roof

doorSlide28

A simple solution

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28

dispayObject

box

pen

bumper

number

inventoryItem

Object

roof

door

…

!Slide29

African

S.American

N.American

Another aspect of that issue

The way of classification causes class grouping.

It is possible for some classes to have several ways of classification.

Orthogonal classification:

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Pets

Beasts of burden

Source of food

PestsSlide30

Multiple Inheritance raises difficult problem

Multiple inheritance allows a class to be an immediate subclass of more than one class.It solve one problem but, brings many new problems.

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C

B

ASlide31

Topics

History and MotivationStructural OrganizationClasses and SubclassesMessage SendingImplementation

Object-Oriented extensionsEvaluation and Epilog

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Dynamic versus Static, Strong versus weak

Smalltalk uses dynamic type checking like LISP.Pascal and Ada use static type checking.Dynamic type checking does not imply weak type.32Slide33

Forms of Message Template (Argument Passing)

Message sending syntaxes:Object_name methodnameObject_name methodname : argument

Object_name methodname : 1

st

arg

2

nd

arg_name:2nd

arg_val

We have a problem with arithmetic expression: (x+2)*y (x plus : 2) times : y

It became possible for one-parameter message to use an operation.

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run ||

Keyboard read eval printThe Smalltalk main loop is written in Smalltalk

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class name

userTask

Instance message and methods

Smalltalk is in a loop: read a command, execute the command, print the result and loop

true

whileTrue

: [Display put: user run]Slide35

run ||

sched map: [: Task | Task run]Concurrency is easy to implement

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class name

scheduler

Instance message and methods

Sched

is the name of a set that contains all of the objects that are scheduled to be run concurrently.

S map: B, apply block B to every element of S. Slide36

Topics

History and MotivationStructural OrganizationClasses and SubclassesMessage SendingImplementation

Object-Oriented extensionsEvaluation and Epilog

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Implementation: Classes and objectsMost of Smalltalk system is written in

smalltalkCompiler, decompiler, debugger, editors,… 97% most of implementation data structures are smalltalk objectsSmalltalk-80

VM:not portable6~12KB assembly codeOne man-year to produce a fully

debuged

version

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ImplementationSmalltalk Virtual machine:

Storage managerEncapsulate the representation of objects and organization of memory Fetch the class of an objectFetch and store the fields of objectsCreate new objectsInterpreterPrimitive subroutines

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ImplementationSmalltalk Virtual machine:

Storage managerInterpreter Manager for methodsPrimitive subroutinesCollection of methodsFor performance reasons are implemented in machine code basic I/O functions, integer arithmetic, basic screen graphics functions

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Implementation There are three central ideas in Smalltalk

ObjectsClassesMessage sending40Slide41

Object representationRepresentation of an object must contain just that information that varies from object to object

Instance variables The information stored with the class includes the class methods and instance methods41Slide42

Representation of objects

Len6c.d.

locTilt

Size

scribe

Len

6

c.d.

Loc

Tilt

Size

Scribe

Len

4

c.d.

X

Y

Box

500@600

200

500

point

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Class representation everything in Smalltalk is an object

Even classes! Classes are instances of the class named classInstances variables of a class object contain pointers to objects representing the information that is the same for all instances of the classWhat is this information?43Slide44

Representation of class object

Len8c.d.

nameSuper class

Inst.

Vars

Class

msgs

.

Inst.

Msgs

.

Inst. Size

4

class

“box”

DisplayObject

“loc tilt size scribe”

Message

dict

Message

dict

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Representation of class object(continued)

inst. Size  number of instance variables Needed by storage manager when it instantiates an objectMessage dictionariesA method identified by the keywords

Scribe go:100  go: identifies the methodSpinner newAt

: 500@200 rate:1 

newAt:rate

: is method

Message template

For each message template acceptable to a class or its instances, one of the message dictionaries must contain an entry

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Message dictionary How should method be represented?

Too much slow if the interpreter had to decode the source form every time the method was executedCompile the code into some form of pseudo code that can be rapidly interpretedThe source form is needed for editing and displaying class definitionMessage dictionaries contain two entries for each message template

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Example of message dictionary

. . .

msgmethod

source

.

.

.

Message dictionary

“grow:”

“grow: amount || self release.

Size<-

size+amount

. Self show”

Push ‘

slef

’

Send ‘erase’,0

Pop

Push ‘size’

Push ‘size’

Push ‘amount’

Send ‘+’,1

Send ‘<-’,1

pop

method

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Implementation of message sendingThere is a strong resemblance between message passing in Smalltalk and procedure calls in other languages

Activation records: primary vehicle for procedure implementationThe same is the case in SmalltalkActivation records hold all of the information relevant to one activation of a method 48Slide49

Structure of an activation recordEnvironment part

The context to be used for execution of the methodInstruction partThe instruction to be executed when this method is resumedSender partThe activation record of the method that sent the message invoking this method49Slide50

Instruction part Must designate a particular instruction in a particular method

Methods are themselves objects  instances of class methodTwo coordinate system is used for identifying instructions:An object pointer defines the method-objectA relative offset identifies the particular instruction within the method object

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Environment partLocal environment

Parameters to methodTemporary variablesHidden temporary variables such as the intermediate results of expressionsNonlocal environment: all other visible variablesInstance variablesClass variables

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Parts of an activation record

localsparameters

Temporary vars

intermediates

Non locals

Static link

Instruction part

method

instruction

Sender part

Dynamic link

Length

c.d.

Instance

vars

instance

Length

c.d.

Superclass

Class variable

Class

Length

c.d.

Superclass

Super class variables

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Message sending create an activation record for the receiver (

callee)Identify the method being invoked by extracting the template form from the message and then looking it up in the message dictionary for the receiving object’s class or super classTransmit the parameters to the receiver’s activation recordsSuspend the sender(caller) by saving its state in its activation record.Establish a path(dynamic link) from the receiver back to the sender and establish the receiver’s activation record as the active one

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Returning from a method transmit the returned object(if any) from the receiver back to sender

Resume execution of the sender by restoring its state from its activation record54Slide55

Returning from a method transmit the returned object(if any) from the receiver back to sender

Resume execution of the sender by restoring its state from its activation record deallocation of the activation record???The storage manager handles allocation and deallocation of all objects, this include AR objectsWe don’t explicitly deallocate

activation records Smalltalk doesn’t allocate its activation records on a stackconcurrency

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Topics

History and MotivationStructural OrganizationClasses and SubclassesMessage SendingImplementation

Object-Oriented extensionsEvaluation and Epilog

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Object oriented extensions Many languages have been extended for object oriented programming

Fifth generation languages such as LISPThird and fourth generation languages such as C and Ada83Simula-67 is essentially an object-oriented extension to algol-60Programming by extensionSuggested by object oriented languagesAbility to extend and refine software systems without needing to recompile and retest the parts already implemented

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Tagged types support programming by extension-Ada

Package Display_Objects is type display_object is tagged [private] record loc: display_location;

end record;Procedure Go_to ( The_object

:

display_object

;

new_loc

: display_location

);

Procedure Show (

The_object:display_object

);

Procedure Erase (

The_object:display_object

);

End

display_objects

;

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Tagged types support programming by extension-Ada

With display_objects;Package Boxes is type Box is new display_objects.display_object with [private] record tilt, size :float;

The_scribe: scribe; end record;

procedure shape (

The_Box

: Box);

.

.

.

End Boxes;

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Package

All_Display_Objects type Display_Object is tagged private;

… operations for Display_Object

…

type Box is new

Display_Object

with private;

… operations for Box …

type Window is new

Display_Object

with private;

… operations for Window …

… etc. …

end

All_Display_Objects

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Types and Their Refinements Can be Defined is a Single PackageSlide61

Abstract declarations in

display_object:Procedure shape (The_object: display_Object) is abstract;In Box:Procedure shape (The_box:Box

); In package body for display_objects:procedure show(

The_object

:

display_object

) is

Begin

Set_color

(

The_object

);

Shape(

The_object

);

End show;

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Ada’s support for object oriented programming is complex

The complexity comes from grafting the object oriented features onto a type system that was designed for different trade-offs(e.g. efficiency on small embedded computers)62Slide63

C++ extends C for object-oriented programmingC++ is largely the work of

Bjarne StroustrupC++ supports:Class declarationMultiple inheritanceVirtual  abstract in Ada

Template  generic in Ada

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Java is a more advanced object-oriented language derived from CDesigned for networked and distributed programming environments

Greater emphasis on security, robustness, machine independence, portabilityJava emits some C++ features: operator overloading, pointers,…Add others such as automatic garbage collection64Slide65

Topics

History and MotivationStructural OrganizationClasses aMessage Sendingnd Subclasses

ImplementationObject-Oriented extensions

Evaluation and Epilog

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Evalution and epilogSmalltalk is small, flexible, and extensible

Small, simple, regular languageSmall number of independent concepts  easy to learnHas demonstrated its flexibility by being Used in a number of applications: simulations, graphics, artificial intelligenceDesign of Smalltalk has been dictated by several powerful principles: simplicity, regularity, abstraction, security and information hiding

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Evalution and epilogSmalltalk is an example of a programming environment

Has introduced important ideas other than language itself, like widowsIs also an excellent example of an advanced programming environmentIntegration of graphics, windows and an advanced language67Slide68

Evalution and epilogSmalltalk introduces a new programming paradigm

Although most of its ideas have appeared in other languages, smalltalk’s integration of these ideas is novelThe essence of the Smalltalk view is that programming is simulation programs model, sometimes in a very abstract form, some aspects of the real world

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Evalution and epilog

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The End

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