Lecturers: Hesam Setareh Salar Moarref 1 Fall 2009 Programming
languages
Slide 2
Topics History and Motivation Structural Organization Classes
and Subclasses Message Sending Implementation Object-Oriented
extensions Evaluation and Epilog 2
Slide 3
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. 4
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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. 5
Slide 6
Topics History and Motivation Structural Organization Classes
and Subclasses Message Sending Implementation Object-Oriented
extensions Evaluation and Epilog 6
Slide 7
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) Properties
(Variables) Behaviors (Methods) Structural Organization
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Scribe goto:500@500. 8 Example
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Scribe goto:500@500. 9 Scribe go:300. Example
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Scribe goto:500@500. 10 Scribe go:300. Scribe turn:90.
Example
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Scribe goto:500@500. 11 Scribe go:300. Scribe turn:90. Scribe
go:300. Example
Class A class is a plan, which objects are made from. Getting a
new object from a class is called instantiation. 18 anotherScribe
pen newAt:200@200
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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 namebox
Instance variable nameLoc tilt size scribe Instance message and
methods
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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
namebox Instance variable nameLoc tilt size scribe Instance message
and methods class message and methods
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Topics History and Motivation Structural Organization Classes
and Subclasses Message Sending Implementation Object-Oriented
extensions Evaluation and Epilog 21
Slide 22
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. 22 dispayObject boxpenwindow
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Classes and Subclasses 23 class namebox Instance variable name
displayObject Instance message and methods superclass Loc tilt size
scribe
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Classes and Subclasses A Superclass can be subclass of another
class. 24 dispayObject boxpenwindow number Object
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print || realPt print + + imagePt print + i Classes and
Subclasses 25 class namecomplex Instance variable namerealPt
imagePt Instance message and methods Subclasses can change the
inherent method, this process is called Overriding
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An Important Issue In Smalltalk a class cant have several
superclass. But, in the Real-World, an object can inherent from
more than one object. 26 dispayObject box penbumper
numberinventoryItem Object roof door
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An Important Issue 27 dispayObject brakeenginebumper
numberinventoryItem Object roof door
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A simple solution 28 dispayObject box penbumper number
inventoryItem Object roof door ! !
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AfricanS.AmericanN.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: 29 Pets Beasts of burden Source of food Pests
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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. 30 C
B A
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Topics History and Motivation Structural Organization Classes
and Subclasses Message Sending Implementation Object-Oriented
extensions Evaluation and Epilog 31
Slide 32
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. 32
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Forms of Message Template (Argument Passing) Message sending
syntaxes: Object_namemethodname Object_namemethodname : argument
Object_namemethodname : 1 st arg 2 nd arg_name:2 nd 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 print The Smalltalk main loop is
written in Smalltalk 34 class nameuserTask 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]
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run || sched map: [: Task | Task run] Concurrency is easy to
implement 35 class namescheduler 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.
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Topics History and Motivation Structural Organization Classes
and Subclasses Message Sending Implementation Object-Oriented
extensions Evaluation and Epilog 36
Slide 37
Implementation: Classes and objects Most of Smalltalk system is
written in smalltalk Compiler, decompiler, debugger, editors, 97%
most of implementation data structures are smalltalk objects
Smalltalk-80 VM:not portable 6~12KB assembly code One man-year to
produce a fully debuged version 37
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Implementation Smalltalk Virtual machine: Storage manager
Encapsulate the representation of objects and organization of
memory Fetch the class of an object Fetch and store the fields of
objects Create new objects Interpreter Primitive subroutines
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Implementation Smalltalk Virtual machine: Storage manager
Interpreter Manager for methods Primitive subroutines Collection of
methods For performance reasons are implemented in machine code
basic I/O functions, integer arithmetic, basic screen graphics
functions 39
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Implementation There are three central ideas in Smalltalk
Objects Classes Message sending 40
Slide 41
Object representation Representation 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 methods 41
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Representation of objects Len6 c.d. loc Tilt Size scribe Len6
c.d. Loc Tilt Size Scribe Len4 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 class Instances
variables of a class object contain pointers to objects
representing the information that is the same for all instances of
the class What is this information? 43
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Representation of class object Len8 c.d. name Super class Inst.
Vars Class msgs. Inst. Msgs. Inst. Size4 class box DisplayObject
loc tilt size scribe Message dict 44
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Representation of class object(continued) inst. Size number of
instance variables Needed by storage manager when it instantiates
an object Message dictionaries A method identified by the keywords
Scribe go:100 go: identifies the method Spinner 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 45
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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 executed Compile the code into some form of pseudo
code that can be rapidly interpreted The source form is needed for
editing and displaying class definition Message dictionaries
contain two entries for each message template 46
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Example of message dictionary. msgmethodsource. Message
dictionary grow: grow: amount || self release. Size
