Transcript Inheritance - University of Virginia

Lecture 23:
Inheritance
CS200: Computer Science
University of Virginia
Computer Science
David Evans
http://www.cs.virginia.edu/evans
Menu
• Objects Review
• Inheritance
• PS6
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CS 200 Spring 2003
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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 22:
(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…
•
•
•
•
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)
Same as in
(lambda (self other)
make-number
(+ (ask self 'value) (ask other 'value)))
((eq? message ‘get-numerator)
Note: our add
(lambda (self) numerator))
method evaluates
to a number, not
((eq? message ‘get-denominator)
a fraction object
(lambda (self) denominator))
(which would be
)))))
better).
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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>)
|
|
|
|
|
|
|
|
|
|
(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
1
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make-number
make-fraction
> (trace ask)
> (trace eq?)
> (ask half 'add half)
|(ask #<procedure> add #<procedure>)
|
|
|
|
|
|
|
|
|
|
(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
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 definition of one
class to make a new kind of class
make-fraction uses make-number
“fraction inherits from number”
• Often confused with subtyping which is
saying one kind of object can be used
where another kind of object is expected
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Subtyping
• Subtyping is very important in statically
typed languages (like C, C++, C#, Java,
Pascal) where you have to explicitly
declare a type for all variables:
method Number add (Number n) { … }
Because of subtyping, either a Number or a Fraction
(subtype of Number) could be passed as the argument
• CS200 won’t cover subtyping (although we
will talk more about types later)
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PS6
Make an adventure game
programming with objects
Many objects in our game have
similar properties and behaviors,
so we use inheritance.
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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
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Alyssa P. Hacker
place
Cabal
Hall
Recursa
(make-place name)
evaluates to an object
that is an instance of
the class place.
police-officer
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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
Behavior
RotationPathInterpolator
Leaf
Object
SceneGraphObject
Not at all uncommon to have
class hierarchies like this!
Java
Class
Hierarchy Diagram
173D
March
2004
CS 200 Spring 2003
http://java.sun.com/products/java-media/3D/collateral/j3dclass.html
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Charge
• PS6
– Programming with Objects
– Due Date change:
• Previously Monday, now due Friday 26
March
• Or, turn in Monday without doing
questions 7 and 8
• Friday: Is there anything that cannot be
computed?
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