David Evans CS200: Computer Science University of Virginia Computer Science Lecture 21: Inheritance

17 March 2003CS 200 Spring Menu Objects Review Inheritance PS5: Databases PS6

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

17 March 2003CS 200 Spring make-number (define make-number (lambda (n) (lambda (message) (cond ((eq? message 'value) (lambda (self) n)) ((eq? message 'add) (lambda (self other) (+ (ask self 'value) (ask other 'value)))))))) Why don’t we just use n ? (Well see why later today.)

17 March 2003CS 200 Spring ask (define (ask object message. args) (apply (object message) object args)) (define (ask object message) (object message)) Lecture 20:

17 March 2003CS 200 Spring global environment + : # make-number: parameters: body: ((lambda … n : 3 (define make-number (lambda (n) (lambda (message) (cond ((eq? message 'value) (lambda (self) n)) ((eq? message 'add) (lambda (self other) (+ (ask self 'value) (ask other 'value)))))))) san: parameters: message body: (cond ((eq? … > (define san (make-number 3)) > (ask san 'value) 3 > (ask san 'add (make-number 4)) 7

17 March 2003CS 200 Spring There are many kinds of numbers… Whole Numbers (0, 1, 2, …) Integers (-23, 73, 0, …) Fractions (1/2, 7/8, …) Floating Point (2.3, , ) But they can’t all do the same things –We can get the denominator of a fraction, but not of an integer

17 March 2003CS 200 Spring make-fraction (define make-fraction (lambda (numerator denominator) (lambda (message) (cond ((eq? message 'value) (lambda (self) (/ numerator denominator)) ((eq? message 'add) (lambda (self other) (+ (ask self 'value) (ask other 'value))) ((eq? message ‘get-numerator) (lambda (self) numerator)) ((eq? message ‘get-denominator) (lambda (self) denominator)) ))))) Same as in make-number

17 March 2003CS 200 Spring Why is redefining add a bad thing? Cut-and-paste is easy but… There could be lots of number methods (subtract, multiply, print, etc.) Making the code bigger makes it harder to understand If we fix a problem in the number add method, we have to remember to fix the copy in make-fraction also (and real, complex, float, etc.)

17 March 2003CS 200 Spring Inheritance

17 March 2003CS 200 Spring make-fraction (define (make-fraction numer denom) (let ((super (make-number #f))) (lambda (message) (cond ((eq? message 'value) (lambda (self) (/ numer denom))) ((eq? message 'get-denominator) (lambda (self) denom)) ((eq? message 'get-numerator) (lambda (self) numer)) (else (super message))))))

17 March 2003CS 200 Spring Using Fractions > (define half (make-fraction 1 2)) > (ask half 'value) 1/2 > (ask half 'get-denominator) 2 > (ask half 'add (make-number 1)) 3/2 > (ask half 'add half) 1

17 March 2003CS 200 Spring > (trace ask) > (trace eq?) > (ask half 'add half) |(ask # add # ) | (eq? add value) | #f | (eq? add get-denominator) | #f | (eq? add get-numerator) | #f | (eq? add value) | #f | (eq? add add) | #t | (ask # value) | |(eq? value value) | |#t | 1/2 | (ask # value) | |(eq? value value) | |#t | 1/2 |1 1

17 March 2003CS 200 Spring > (trace ask) > (trace eq?) > (ask half 'add half) |(ask # add # ) | (eq? add value) | #f | (eq? add get-denominator) | #f | (eq? add get-numerator) | #f | (eq? add value) | #f | (eq? add add) | #t | (ask # value) | |(eq? value value) | |#t | 1/2 | (ask # value) | |(eq? value value) | |#t | 1/2 |1 1 make-number make-fraction

17 March 2003CS 200 Spring Inheritance Inheritance is using the definition of one class to make another class make-fraction uses make-number to inherit the behaviors of number

17 March 2003CS 200 Spring Number Fraction Note: people sometimes draw this different ways English A Fraction is a kind of Number. C++ Fraction is a derived class whose base class is Number Java Fraction extends Number. Eiffel Fraction inherits from Number. Beta Fraction is a subpattern of Number. Smalltalk (72) Didn’t have inheritance!

17 March 2003CS 200 Spring CS 200: Fraction inherits from Number. Fraction is a subclass of Number. The superclass of Fraction is Number. Number Fraction

17 March 2003CS 200 Spring Inheritance and Subtyping Inheritance: reusing the implementation of one object to make a new kind of object Often confused with subtyping which is saying one kind of object can be used where another kind of object is expected –CS200 won’t cover subtyping (take CS201J) –We will cover types (Class 29)

17 March 2003CS 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.

17 March 2003CS 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

17 March 2003CS 200 Spring How big are big databases? Microsoft TerraServer –Claimed biggest in 1998 –Aerial photos of entire US (1 meter resolution)

17 March 2003CS 200 Spring You are here Rotunda Amphitheater

17 March 2003CS 200 Spring AFC? Picture from 2 Apr 1994 You are here

17 March 2003CS 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 Stanford Linear Accelerator (BaBar) –500 Terabytes (30 KB per particle collision)

17 March 2003CS 200 Spring PS6 Make an adventure game programming with objects

17 March 2003CS 200 Spring PS6 Classes object physical-object place mobile-object thing person student police-officer make-class is the procedure for constructing objects in the class class student inherits from person which inherits from mobile-object which inherits from physical-object which inherits from object.

PS6 Objects object physical-object place mobile-object thing person student police-officer Cabal Hall Recursa Alyssa P. Hacker (make-place name) evaluates to an object that is an instance of the class place.

17 March 2003CS 200 Spring Are there class hierarchies like this in the real world or just in fictional worlds like Charlottansville?

17 March 2003CS 200 Spring Microsoft Foundation Classes CButton inherits from CWnd inherits from CObject “A button is a kind of window is a kind of object”

17 March 2003CS 200 Spring Java 3D Class Hierarchy Diagram RotationPathInterpolator PathInterpolator Interpolator Selector Node Leaf SceneGraphObject Not at all uncommon to have class hierarchies like this!

17 March 2003CS 200 Spring Charge PS5 –Return now, comments will be available Wednesday PS6 –Programming with Objects Wednesday and Friday: –How will solving the (generalized) Cracker Barrel Peg Board puzzle help cure cancer?