Transcript Inheritance - University of Virginia

Lecture 21:
Inheritance
CS200: Computer Science
University of Virginia
Computer Science
David Evans
http://www.cs.virginia.edu/evans
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Objects Review
Inheritance
PS5: Databases
PS6
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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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make-number
(define make-number
(lambda (n)
(lambda (message)
(cond
((eq? message 'value)
(lambda (self) n))
Why don’t we just use n?
((eq? message 'add)
(Well see why later today.)
(lambda (self other)
(+ (ask self 'value)
(ask other 'value))))))))
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ask
Lecture 20:
(define (ask object message)
(object message))
(define (ask object message . args)
(apply (object message) object args))
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(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))))))))
> (define san
(make-number 3))
> (ask san 'value)
3
> (ask san 'add
(make-number 4))
7
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global
environment
+ : #<primitive:+>
make-number:
san:
n:
3
parameters:
body: ((lambda …
parameters: message
body: (cond ((eq? …
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There are many kinds of numbers…
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Whole Numbers (0, 1, 2, …)
Integers (-23, 73, 0, …)
Fractions (1/2, 7/8, …)
Floating Point (2.3, 0.0004, 3.14159)
• But they can’t all do the same things
– We can get the denominator of a fraction, but
not of an integer
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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))
)))))
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Same as in
make-number
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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.)
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Inheritance
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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))))))
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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
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> (trace ask)
> (trace eq?)
> (ask half 'add half)
|(ask #<procedure> add #<procedure>)
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(eq?
#f
(eq?
#f
(eq?
#f
(eq?
#f
(eq?
#t
add value)
add get-denominator)
add get-numerator)
add value)
add add)
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| (ask #<procedure> value)
| |(eq? value value)
| |#t
| 1/2
| (ask #<procedure> value)
| |(eq? value value)
| |#t
| 1/2
|1
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make-number
make-fraction
> (trace ask)
> (trace eq?)
> (ask half 'add half)
|(ask #<procedure> add #<procedure>)
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(eq?
#f
(eq?
#f
(eq?
#f
(eq?
#f
(eq?
#t
add value)
add get-denominator)
add get-numerator)
add value)
add add)
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| (ask #<procedure> value)
| |(eq? value value)
| |#t
| 1/2
| (ask #<procedure> value)
| |(eq? value value)
| |#t
| 1/2
|1
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Inheritance
Inheritance is using the definition of one
class to make another class
make-fraction uses make-number to
inherit the behaviors of number
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• English
A Fraction is a kind of Number.
Number
• C++
Fraction is a derived class whose base
class is Number
• Java
Fraction
Fraction extends Number.
• Eiffel
Fraction inherits from Number.
Note: people
sometimes draw
this different ways
• Beta
Fraction is a subpattern of Number.
• Smalltalk (72)
Didn’t have inheritance!
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CS 200:
Number
Fraction inherits from Number.
Fraction is a subclass of Number.
Fraction
The superclass of Fraction is
Number.
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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)
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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 199423
Big Databases
• Microsoft TerraServer
– 3.3 Terabytes (claimed biggest in 1998)
– 1 Terabyte = 240 Bytes ~ 1 Trillion Bytes
• Wal-Mart
– 285 Terabytes
• Stanford Linear Accelerator (BaBar)
– 500 Terabytes (30 KB per particle collision)
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PS6
Make an adventure game
programming with objects
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object
PS6 Classes
physical-object
mobile-object
thing
student inherits from person
which inherits from mobile-object
which inherits from physical-object
which inherits from object.
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person
student
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place
make-class is the
procedure for
constructing
objects in the
class class
police-officer
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object
PS6 Objects
physical-object
mobile-object
thing
person
student
Alyssa P. Hacker
place
Cabal
Hall
Recursa
(make-place name)
evaluates to an object
that is an instance of
the class place.
police-officer
Are there class hierarchies
like this in the real world or
just in fictional worlds like
Charlottansville?
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Microsoft Foundation Classes
CButton inherits from CWnd inherits from CObject
“A button is a kind of window is a kind of object”
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Node
PathInterpolator
Interpolator
Selector
RotationPathInterpolator
Leaf
SceneGraphObject
Not at all uncommon to have
class hierarchies like this!
Java
Class
Hierarchy Diagram
173D
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http://java.sun.com/products/java-media/3D/collateral/j3dclass.html
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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?
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