1. 6001 structure & interpretation of computer programs recitation 13/ november 6, 1997

2. topics object oriented design objects and classes properties
subclassing
object oriented coding
methods & members
inheritance
1/17/2019 daniel jackson

3. modelling the world with objects
idea
a good way to structure a program is to mirror the structure of the “real world”
first model the real world, then translate into object oriented program
elements
an object is some entity that has an identity
examples: a person, a car, a segment of a road, a file
non-examples: an integer, the weather, a friendship, an airline flight
a class is a collection of similar objects
examples: Person, the class of all persons; Car, the class of all cars
1/17/2019 daniel jackson

4. properties local properties there are properties of individual objects
a person has an age, a social security number
a car has a mileage, a speed
a file has a size
associations
and properties of more than one object, sometimes called associations
a student is majoring in a course
a car is driving on a road segment
a file is in a directory
1/17/2019 daniel jackson

5. subclassing definition
if every object of a class C can also be regarded as an object of class D, then C is said to be a subclass of D
examples
Person is a subclass of Animal
Car is a subclass of Vehicle
Square is a subclass of Rectangle
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6. object models idea can express associations and subclasses graphically
use arrow for subclassing and plain line for association
drives
Vehicle
Person
Car
Truck
TruckDriver
1/17/2019 daniel jackson

7. exercise draw an object model for a drawing program
supports variety of lines and shapes, grouping, layers
1/17/2019 daniel jackson

8. object-oriented programming in Java
object has
members or instance variables for holding local state
methods for reading/writing members
class defines
methods and members for all objects of the class
a constructor for making objects of the class
computation model
“sending a message to o”:
executing o.m (a1, a2, … )
causes object o to execute method m on its members with arguments a1, a2, …
this is how other objects access the local state of o
inheritance
can declare one class to be a subclass of another
subclass gets all the methods and members of the superclass, but may redefine them
properties
are coded as members
with associations, have to decide where the state goes
1/17/2019 daniel jackson

9. object-oriented programming in Scheme
OO constructs not built-in, so
have to simulate them
make inherited members a separate data structure (called a part in lecture)
Scheme system doesn’t “understand” objects, so only find errors when the program runs
1/17/2019 daniel jackson

10. get_radius : –> int set_radius : int –> undef
example
Shape
color : Color area : int
draw : Screen –>undef get_color : –> Color set_color : Color –> undef
Shape class
members
color, area, position, boundary
methods
draw, get_color, set_color
Circle class
radius
set_radius, get_radius
Line class
length
arrow_style
move_tail
Circle
radius : int
get_radius : –> int set_radius : int –> undef
1/17/2019 daniel jackson

11. exercise sketch the classes for the drawing program for each class:
members
methods
1/17/2019 daniel jackson