Lecture 20:
Objects
I invented the term ObjectOriented, and I can tell you
I did not have C++ in mind.
— Alan Kay
CS200: Computer Science
University of Virginia
Computer Science
David Evans
http://www.cs.virginia.edu/evans
Menu
• A better counter
• PS8 Demo (Spencer)
• Programming with Objects
13 March 2003
CS 200 Spring 2003
2
nextx from Lecture 16
global
(define x 0)
environment
(define (nextx)
+ : #<primitive:+>
(set! x (+ x 1))
nextx:
x
:
24
x)
> (nextx)
1
parameters:
> (set! x 23)
body:
(begin (set! x (+ x 1)) x)
> (next x)
24
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CS 200 Spring 2003
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A Better Counter
• The place that keeps track of the count
would be part of the counter, not part of
the global environment
• Can we do this?
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Recall from Lecture 17:
Application
1. Construct a new frame, enclosed in the
environment of this procedure
2. Make places in that frame with the
names of each parameter
3. Put the values of the parameters in those
places
4. Evaluate the body in the new
environment
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A Better Counter
(define (make-counter)
((lambda (count)
(lambda ()
(set! count (+ 1 count))
count))
0))
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(define (make-counter)
((lambda (count)
(lambda ()
(set! count (+ 1 count))
count))
0))
> (define mycount
(make-counter))
> (mycount)
1
> (mycount)
2
> (mycount)
3
13 March 2003
global
environment
count :
+ : #<primitive:+>
make-counter:
mycount:
3
1
0
2
parameters:
body: ((lambda …
parameters:
body: (lambda ()
(set! count …)
CS 200 Spring 2003
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An Even Better Counter
(define (make-ocounter)
((lambda (count)
(lambda (message)
(if (eq? message 'reset) (set! count 0)
(if (eq? message 'next)
(set! count (+ 1 count))
(if (eq? message 'how-many)
count)))))
0))
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CS 200 Spring 2003
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Using Counter
>
>
>
>
>
3
>
>
0
(define bcounter (make-ocounter))
(bcounter 'next)
(bcounter 'next)
(bcounter 'next)
(bcounter 'how-many)
(bcounter 'reset)
(bcounter 'how-many)
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Objects
• When we package state and
procedures together we have
an object
• Programming with objects is
object-oriented programming
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PS8 Demo
Charlottesville Music Database
CS200 Spring 2002
Spencer Stockdale
and Russ O’Reagan
http://www.people.virginia.edu/~sas9t/csfinal/index.php3
13 March 2003
CS 200 Spring 2003
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Why Your PS8 Sites Should be
2.357 Times Better
• Last year’s students got 15 days for PS8,
you get 25 days!
• Their team was 2 students, your teams
can be 1-6 students (preferably 3-4)
– Start thinking about projects and talking to
potential teammates now, so you can request
your team by March 31
> (* (/ 25 15) (sqrt (/ 4 2)))
2.3570226039551585
13 March 2003
CS 200 Spring 2003
Expectations scale as
sqrt of number of people
involved, because of
management overhead.
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Why Your PS8 Sites Should be
3.877 Times Better
• According to Tyson, you are 4.1%
smarter and better prepared than last
year’s students!
• According to Moore, you have 58%
more computing power than last
year’s students!
> (* (/ 25 15) (sqrt (/ 4 2)) 1.041 1.58)
3.876783638533366
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Why Your PS8 Sites Should be
8 Million Times Better!
• This year’s guest speaker’s web site
gets 213M unique visitors per month;
last year’s guest speaker’s web site
gets maybe 100.
> (* (/ 25 15) (sqrt (/ 4 2)) 1.041 1.58 (/ 213000000 100))
8257549.150076069
Note: my expectations are very reasonable. I will settle for
PS8 projects that are only 1M times better than Spencer’s.
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Object-Oriented
Programming
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Who was the first
object-oriented
programmer?
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By the word operation, we mean any process which
alters the mutual relation of two or more things, be this
relation of what kind it may. This is the most general
definition, and would include all subjects in the universe.
Again, it might act upon other things besides number,
were objects found whose mutual fundamental relations
could be expressed by those of the abstract science of
operations, and which should be also susceptible of
adaptations to the action of the operating notation and
mechanism of the engine... Supposing, for instance,
that the fundamental relations of pitched sounds in the
science of harmony and of musical composition were
susceptible of such expression and adaptations, the
engine might compose elaborate and scientific pieces of
music of any degree of complexity or extent.
Ada, Countess of Lovelace, around 1830
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Simula
• Considered the first “object-oriented”
programming language
• Language designed for simulation by
Kristen Nygaard and Ole-Johan Dahl
(Norway, 1962)
• Had special syntax for defining classes
that packages state and procedures
together
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Counter in Simula
class counter; integer count;
begin
procedure reset(); count := 0; end;
procedure next();
count := count + 1; end;
integer procedure how-many();
how-many := count; end;
end
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XEROX Palo Alto Research Center (PARC)
1970s:
• Bitmapped display
• Graphical User Interface
– Steve Jobs paid $1M to visit and PARC, and
returned to make Apple Lisa/Mac)
•
•
•
•
Ethernet
First personal computer (Alto)
PostScript Printers
Object-Oriented Programming
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CS 200 Spring 2003
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Dynabook, 1972
(Just a model)
13 March 2003
The best way to predict
the future is to invent it.
Alan Kay
CS 200 Spring 2003
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Dynabook
• Tablet computer
• Intented as tool for learning
• Kay wanted children to be able to program it
also
• Hallway argument, Kay claims you could
define “the most powerful language in the
world in a page of code”
• Proof: Smalltalk
– Scheme is as powerful, but takes two pages
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BYTE
Magazine,
August
1981
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Smalltalk
• Everything is an object
• Objects communicate by sending and
receiving messages
• Objects have their own state (which may
contain other objects)
• How do you do 3 + 4?
send the object 3 the message “+ 4”
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Counter in Smalltalk
class name counter
instance variable names count
new count <- 0
next count <- count + 1
how-many ^ count
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Counter in Scheme
(define (make-ocounter)
((lambda (count)
(lambda (message)
(if (eq? message 'reset) (set! count 0)
(if (eq? message 'next)
(set! count (+ 1 count))
(if (eq? message 'how-many)
count)))))
0))
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Counter in Scheme using let
(define (make-ocounter)
(let ((count 0))
(lambda (message)
(if (eq? message 'reset) (set! count 0)
(if (eq? message 'next)
(set! count (+ 1 count))
(if (eq? message 'how-many)
count))))))
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Defining ask
(ask Object Method)
> (ask bcounter 'how-many)
0
> (ask bcounter 'next)
> (ask bcounter 'how-many)
1
(define (ask object message)
(object message))
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Charge
• PS6: out today
– Programming with Objects
• Monday: Subtyping
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