David Evans CS200: Computer Science University of Virginia Computer Science Lecture 22: Objects I invented the term Object- Oriented, and I can tell you I did not have C++ in mind. — Alan Kay

15 March 2004CS 200 Spring Menu A better counter PS5 Comments –Databases Programming with Objects

15 March 2004CS 200 Spring nextx from Lecture 17 (define x 0) (define (nextx) (set! x (+ x 1)) x) > (nextx) 1 > (set! x 23) > (next x) 24 global environment + : # nextx: x : 24 parameters: body: (begin (set! x (+ x 1)) x)

15 March 2004CS 200 Spring 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?

15 March 2004CS 200 Spring 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 Recall from Lecture 17:

15 March 2004CS 200 Spring A Better Counter (define (make-counter) ((lambda (count) (lambda () (set! count (+ 1 count)) count)) 0))

15 March 2004CS 200 Spring > (define mycount (make-counter)) > (mycount) global environment + : # make-counter: parameters: body: ((lambda … count : 0 (define (make-counter) ((lambda (count) (lambda () (set! count (+ 1 count)) count)) 0)) mycount: parameters: body: (lambda () (set! count …) 1 2 3

15 March 2004CS 200 Spring 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))

15 March 2004CS 200 Spring Using Counter > (define bcounter (make-ocounter)) > (bcounter 'next) > (bcounter 'how-many) 3 > (bcounter 'reset) > (bcounter 'how-many) 0

15 March 2004CS 200 Spring Objects When we package state and procedures together we have an object Programming with objects is object-oriented programming

15 March 2004CS 200 Spring 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.

15 March 2004CS 200 Spring 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

15 March 2004CS 200 Spring How big are big databases? Microsoft TerraServer –Claimed biggest in 1998 –Aerial photos of entire US (1 meter resolution)

15 March 2004CS 200 Spring You are here Rotunda Amphitheater

15 March 2004CS 200 Spring AFC? Picture from 2 Apr 1994 You are here

15 March 2004CS 200 Spring Big Databases Microsoft TerraServer –3.3 Terabytes (claimed biggest in 1998) –1 Terabyte = 2 40 Bytes ~ 1 Trillion Bytes Wal-Mart –285 Terabytes (2003) Stanford Linear Accelerator (BaBar) –500 Terabytes (30 KB per particle collision)

15 March 2004CS 200 Spring 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? s ~ 23 days How do expensive databases perform so much faster?

15 March 2004CS 200 Spring Object-Oriented Programming

15 March 2004CS 200 Spring 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

15 March 2004CS 200 Spring 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

15 March 2004CS 200 Spring 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

15 March 2004CS 200 Spring Dynabook, 1972 The best way to predict the future is to invent it. Alan Kay (Just a model)

15 March 2004CS 200 Spring 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

15 March 2004CS 200 Spring BYTE Magazine, August 1981

15 March 2004CS 200 Spring 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”

15 March 2004CS 200 Spring Counter in Smalltalk class name counter instance variable names count new count <- 0 next count <- count + 1 how-many ^ count

15 March 2004CS 200 Spring 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))

15 March 2004CS 200 Spring 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))))))

15 March 2004CS 200 Spring Defining ask > (ask bcounter 'how-many) 0 > (ask bcounter 'next) > (ask bcounter 'how-many) 1 (ask Object Method) (define (ask object message) (object message))

15 March 2004CS 200 Spring Who was the first object-oriented programmer?

15 March 2004CS 200 Spring 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

15 March 2004CS 200 Spring Charge PS6: –Programming with Objects Monday: Subtyping