1. David Evans http://www.cs.virginia.edu/~evans
Lecture 21:
Inheritance
CS200: Computer Science
University of Virginia
Computer Science
David Evans

2. Menu
Objects Review
Inheritance
PS6
8 March 2002
CS 200 Spring 2002

3. When we package state and procedures together we have an object
Objects
When we package state and procedures together we have an object
Programming with objects is object-oriented programming
8 March 2002
CS 200 Spring 2002

4. 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.)
8 March 2002
CS 200 Spring 2002

5. ask (define (ask object message) (object message))
Lecture 20:
(define (ask object message)
(object message))
(define (ask object message . args)
(apply (object message) object args))
8 March 2002
CS 200 Spring 2002

6. 3 > (define san (make-number 3)) > (ask san 'value) 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))))))))
global
environment
+ : #<primitive:+>
make-number:
san:
> (define san
(make-number 3))
> (ask san 'value) 3
> (ask san 'add
(make-number 4)) 7 3
n :
parameters: body: ((lambda …
parameters: message body: (cond ((eq? …
8 March 2002
CS 200 Spring 2002

7. 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
8 March 2002
CS 200 Spring 2002

8. 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
8 March 2002
CS 200 Spring 2002

9. 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.)
8 March 2002
CS 200 Spring 2002

10. Inheritance
8 March 2002
CS 200 Spring 2002

11. 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))))))
8 March 2002
CS 200 Spring 2002

12. 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
8 March 2002
CS 200 Spring 2002

13. > (ask half 'add half) | (eq? add value) | #f
> (trace ask)
> (trace eq?)
> (ask half 'add half)
|(ask #<procedure> add #<procedure>)
| (eq? add value)
| #f
| (eq? add get-denominator)
| (eq? add get-numerator)
| (eq? add add)
| #t
| (ask #<procedure> value)
| |(eq? value value)
| |#t
| 1/2 |1 1
8 March 2002
CS 200 Spring 2002

14. make-number make-fraction > (trace ask) > (trace eq?)
> (ask half 'add half)
|(ask #<procedure> add #<procedure>)
| (eq? add value)
| #f
| (eq? add get-denominator)
| (eq? add get-numerator)
| (eq? add add)
| #t
| (ask #<procedure> value)
| |(eq? value value)
| |#t
| 1/2 |1 1
8 March 2002
CS 200 Spring 2002

15. Inheritance
Inheritance is using the definition of one class to make another class
make-fraction uses make-number to inherit the behaviors of number
8 March 2002
CS 200 Spring 2002

16. English C++ Java Eiffel Beta Smalltalk (72)
A Fraction is a special 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!
Number
Fraction
Note: people draw this different ways (some times the arrow points the opposite way)
8 March 2002
CS 200 Spring 2002

17. CS 200 Fraction inherits from Number.
Fraction is a subclass of Number.
The superclass of Fraction is Number.
Number
Fraction
Note: people draw this different ways (some times the arrow points the opposite way)
8 March 2002
CS 200 Spring 2002

18. 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
We will cover types and subtyping after Spring Break, just warning you now
8 March 2002
CS 200 Spring 2002

19. PS6 Make an adventure game programming with objects.
8 March 2002
CS 200 Spring 2002

20. PS6 Classes object physical-object place mobile-object thing person
make-class is the
procedure for constructing
objects in the class class
mobile-object
thing
person
student
police-officer
make-student constructs a student. The student class inherits from person,
which inherits from mobile-object, which inherits from physical-object, which
inherits from object.

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

22. Are there class hierarchies like this in the real world or just in fictional worlds like Charlatansville?
8 March 2002
CS 200 Spring 2002

23. Microsoft Foundation Classes
CButton inherits from CWnd inherits from CObject
“A button is a kind of window is a kind of object”
8 March 2002
CS 200 Spring 2002

24. Not at all uncommon to have class hierarchies like this!
RotationPathInterpolator
PathInterpolator
Interpolator
Node
Selector
Leaf
SceneGraphObject
Not at all uncommon to have
class hierarchies like this!
Java 3D Class Hierarchy Diagram
8 March 2002
CS 200 Spring 2002

25. Charge PS6 Spring Break Programming with Objects
Read rest of GEB, Part I
Reading questions on the Notes today
8 March 2002
CS 200 Spring 2002