David Evans cs.virginia/evans

1. Lecture 20: Objects I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. — Alan Kay David Evans http://www.cs.virginia.edu/evans CS200: Computer Science University of Virginia Computer Science

2. Menu • A better counter • PS8 Demo (Spencer) • Programming with Objects CS 200 Spring 2003

3. nextx from Lecture 16 (define x 0) (define (nextx) (set! x (+ x 1)) x) > (nextx) 1 > (set! x 23) > (next x) 24 global environment + : #<primitive:+> nextx: x : 24 parameters: body: (begin (set! x (+ x 1)) x) CS 200 Spring 2003

4. 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? CS 200 Spring 2003

5. Recall from Lecture 17: Application • Construct a new frame, enclosed in the environment of this procedure • Make places in that frame with the names of each parameter • Put the values of the parameters in those places • Evaluate the body in the new environment CS 200 Spring 2003

6. A Better Counter (define (make-counter) ((lambda (count) (lambda () (set! count (+ 1 count)) count)) 0)) CS 200 Spring 2003

7. (define (make-counter) ((lambda (count) (lambda () (set! count (+ 1 count)) count)) 0)) global environment + : #<primitive:+> make-counter: mycount: > (define mycount (make-counter)) > (mycount) 1 > (mycount) 2 > (mycount) 3 0 3 1 2 count : parameters: body: ((lambda … parameters: body: (lambda () (set! count …) CS 200 Spring 2003

8. 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)) CS 200 Spring 2003

9. Using Counter > (define bcounter (make-ocounter)) > (bcounter 'next) > (bcounter 'next) > (bcounter 'next) > (bcounter 'how-many) 3 > (bcounter 'reset) > (bcounter 'how-many) 0 CS 200 Spring 2003

10. Objects • When we package state and procedures together we have an object • Programming with objects is object-oriented programming CS 200 Spring 2003

11. PS8 DemoCharlottesville Music DatabaseCS200 Spring 2002Spencer Stockdale and Russ O’Reagan http://www.people.virginia.edu/~sas9t/csfinal/index.php3 CS 200 Spring 2003

12. 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 Expectations scale as sqrt of number of people involved, because of management overhead. > (* (/ 25 15) (sqrt (/ 4 2))) 2.3570226039551585 CS 200 Spring 2003

13. 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 CS 200 Spring 2003

14. Why Your PS8 Sites Should be8 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. CS 200 Spring 2003

15. Object-Oriented Programming CS 200 Spring 2003

16. Who was the first object-oriented programmer? CS 200 Spring 2003

17. 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 CS 200 Spring 2003

18. 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 CS 200 Spring 2003

19. 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 CS 200 Spring 2003

20. 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 CS 200 Spring 2003

21. Dynabook, 1972 The best way to predict the future is to invent it. Alan Kay (Just a model) CS 200 Spring 2003

22. 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 CS 200 Spring 2003

23. BYTE Magazine, August 1981 CS 200 Spring 2003

24. 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” CS 200 Spring 2003

25. Counter in Smalltalk classname counter instance variable names count new count <- 0 next count <- count + 1 how-many ^ count CS 200 Spring 2003

26. 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)) CS 200 Spring 2003

27. 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)))))) CS 200 Spring 2003

28. Defining ask (ask ObjectMethod) > (ask bcounter 'how-many) 0 > (ask bcounter 'next) > (ask bcounter 'how-many) 1 (define (ask object message) (object message)) CS 200 Spring 2003

29. Charge • PS6: out today • Programming with Objects • Monday: Subtyping CS 200 Spring 2003