Lecture 22:
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
• PS5 Comments
– Databases
• Programming with Objects
15 March 2004
CS 200 Spring 2004
2
nextx from Lecture 17
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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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
15 March 2004
global
environment
count :
+ : #<primitive:+>
make-counter:
mycount:
3
1
0
2
parameters:
body: ((lambda …
parameters:
body: (lambda ()
(set! count …)
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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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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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PS5
How are commercial databases different
from what you implemented for PS5?
UVa’s Integrated Systems Project to
convert all University information
systems to use an Oracle database was
originally budgeted for $58.2 Million
(starting in 1999). Actual cost is likely
to be $100 Million.
http://www.virginia.edu/isp/
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Real Databases
• Atomic Transactions: a transaction may involve many
modifications to database tables, but the changes should only
happen if the whole transaction happens (e.g., don’t charge the
credit card unless the order is sent to the shipping dept)
• Security: limit read/write access to tables,
entries and fields
• Storage: need to efficiently store data on disk,
provide backup mechanisms
• Scale: to support really big data tables,
real databases do lots of clever things
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How big are big databases?
• Microsoft TerraServer
– Claimed biggest in 1998
– Aerial photos of entire US (1 meter resolution)
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Rotunda
You are here
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Amphitheater
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You are here
AFC?
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Picture from 2 Apr 199415
Big Databases
• Microsoft TerraServer
– 3.3 Terabytes (claimed biggest in 1998)
– 1 Terabyte = 240 Bytes ~ 1 Trillion Bytes
• Wal-Mart
– 285 Terabytes (2003)
• Stanford Linear Accelerator (BaBar)
– 500 Terabytes (30 KB per particle collision)
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How much work?
• table-select is (n) where n is the number
of entries in the table
– Would your table-select work for Wal-Mart?
– If 1M entry table takes 1s, how long would it
take Wal-Mart to select from 285TB ~ 2
Trillion Entries? 2 000 000s ~ 23 days
How do expensive databases perform so
much faster?
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Object-Oriented
Programming
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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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Dynabook, 1972
(Just a model)
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The best way to predict
the future is to invent it.
Alan Kay
CS 200 Spring 2004
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Dynabook 1972
• Tablet computer
• Intended 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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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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Charge
• PS6:
– Programming with Objects
• Monday: Subtyping
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