Oop rosenschein

Object Oriented Programming:
Its Origins and Importance in
Computer Science
Prof. Jeffrey S. Rosenschein
School of Computer Science and Engineering
Hebrew University, Jerusalem, Israel

2
Computer Programming
 The history of computer
programming is a steady move
away from machine-oriented
views of programming towards
concepts and metaphors that
more closely reflect the way in
which we ourselves understand
the world

3
Programming progression…
 Programming has progressed through:
 machine code
 assembly language
 machine-independent programming
languages
 procedures & functions
 objects

5
Machine Language
0000 1001 1100 0110 1010 1111 0101 1000
1010 1111 0101 1000 0000 1001 1100 0110
1100 0110 1010 1111 0101 1000 0000 1001
0101 1000 0000 1001 1100 0110 1010 1111

6
Assembly Language – PDP-11

7
Assembly Language – Macro-11
GCD: TST B
BEQ SIMPLE
MOV A, R5
SXT R4
DIV B, R4
MOV B, A
MOV R5, B
CALL GCD
SIMPLE: RETURN

8
Assembly Language – Macro-11
GCD: TST B
BEQ SIMPLE
MOV A, R5
SXT R4
DIV B, R4
MOV B, A
MOV R5, B
CALL GCD
SIMPLE: RETURN
20/20

9
Machine-Independent Programming Languages
– Fortran ! This example program solves for roots of the quadratic equation,
! ax^2 +bx +c =0,for given values of a, b and c.
!
PROGRAM bisection
IMPLICIT NONE
INTEGER :: iteration
DOUBLE PRECISION :: CC, Er, xl, x0, x0_old, xr
! Set convergence criterion and guess for xl, xr.
CC = 1.d-4
xl = 8.d-1
xr = 11.d-1
! Bisection method.
Er =CC +1
iteration = 0
DO WHILE (Er > CC)
iteration = iteration + 1
! Compute x0 and the error.
x0_old = x0
x0 = (xl + xr) / 2.d0
Er = DABS((x0 - x0_old)/x0)*100.d0
WRITE (*,10) iteration, x0_old, x0, Er
10 FORMAT (1X,I4,3(2X,E10.4)) this is partial…

10
Procedures & Functions – Pascal
program ValueArg(output);
{Shows how to arrange for a procedure to have arguments.}
procedure PrintInitials(First, Last : char);
{Within this procedure, the names First and Last represent the
argument values. We’ll call write to print them.}
begin
write(‘My initials are: ’);
write(First);
writeln(Last)
end; {PrintInitials}
begin
PrintInitials (‘D’, ‘C’); {Any two characters can be arguments.}
PrintInitials (‘Q’, ‘T’); {Like strings, characters are quoted.}
PrintInitials (‘&’, ‘#’)
end. {ValueArg}

11
Objects
 (My examples will be from Java)
class Time {
private int hour, minute;
public Time (int h, int m) {
hour = h;
minute = m;
}
public void addMinutes (int m) {
int totalMinutes =
((60*hour) + minute + m) % (24*60);
if (totalMinutes<0)
totalMinutes = totalMinutes + (24*60);
hour = totalMinutes / 60;
minute = totalMinutes % 60;
}
}
this is partial…

12
“Intrinsic Power” vs. “Effective Power”
 This progression is not a matter of “intrinsic power”
 Anything you can do with a minimally capable
computer language, you can theoretically do with
any other minimally capable computer language
 But that is like saying a shovel is theoretically as
capable as a tractor. In practice, using a shovel
might make things very hard…

13
Objects
hour
minute
void addMinutes( int m )
Time
inTime
Attributes:
hour = 8
minute = 30
Methods:
void addMinutes(int m)
outTime
Attributes:
hour = 17
minute = 35
Methods:
class
objects

14
Classes and Objects
 A class is a prototype for creating
objects
 When we write a program in an
object-oriented language like Java,
we define classes, which in turn are
used to create objects
 A class has a constructor for
creating objects

15
class Time {
hour = h;
minute = m;
}
int totalMinutes =
((60*hour) + minute + m) % (24*60);
if (totalMinutes<0)
}
}
A Simple Class, called “Time” (partial)
constructor for Time

16
Definition of an “Object”
 An object is a computational entity that:
1. Encapsulates some state
2. Is able to perform actions, or methods, on this
state
3. Communicates with other objects via message
passing

17
1) Encapsulates some state
 Like a record in Pascal, it has a set of
variables (of possibly different types)
that describe an object’s state
 These variables are sometimes called
an object’s attributes (or fields, or
instance variables, or datamembers,
or …)

18
Pascal Example: Represent a Time
type TimeTYPE = record
Hour: 1..23;
Minute: 0..59;
end;
var inToWork, outFromWork: TimeTYPE;

19
Java Example: Represent a Time
class Time {
hour = h;
minute = m;
}
…
}
Time inToWork = new Time(8, 30);
Time outFromWork = new Time(17, 35);
constructor for Time
attributes of Time

20
Objects
hour
minute
Time
inToWork
Attributes:
hour = 8
minute = 30
Methods:
outFromWork
Attributes:
hour = 17
minute = 35
Methods:
class
objects

21
class Time {
hour = h;
minute = m;
}
int totalMinutes =
((60*hour) + minute + m) % (24*60);
if (totalMinutes<0)
}
}
2) Is able to perform actions, or methods,
on this state
 More than a Pascal record!
 An object can also include a group of procedures/functions
that carry out actions
a method of Time

22
3) Communicates with other objects via
message passing
 Sends messages to objects, triggering
methods in those objects
void

23
Example of Object Creation
and Message Passing
bill
Attributes:
…
Methods:
…

24
and Message Passing
bill
Attributes:
…
Methods:
…
In one of bill’s methods, the following code appears:
inToWork.addMinutes(15);

25
and Message Passing
inToWork
Attributes:
hour = 8
minute = 30
Methods:
bill
Attributes:
…
Methods:
…

26
and Message Passing
inToWork
Attributes:
hour = 8
minute = 30
Methods:
inToWork.addMinutes(15)
bill
Attributes:
…
Methods:
…

27
and Message Passing
inToWork
Attributes:
hour = 8
minute = 45
Methods:
bill
Attributes:
…
Methods:
…

28
Structure of a Class Definition
class name {
declarations
constructor definition(s)
method definitions
}
attributes and
symbolic constants
how to create and
initialize objects
how to manipulate
the state of objects
These parts of a class can
actually be in any order

29
History of Object-Oriented Programming
 Started out for simulation of complex man-
machine systems, but was soon realized that
it was suitable for all complex programming
projects
 SIMULA I (1962-65) and Simula 67 (1967)
were the first two object-oriented languages
 Developed at the Norwegian Computing Center,
Oslo, Norway by Ole-Johan Dahl and Kristen
Nygaard
 Simula 67 introduced most of the key concepts of
object-oriented programming: objects and classes,
subclasses (“inheritance”), virtual procedures

30
The Ideas Spread
 Alan Kay, Adele Goldberg and colleagues at
Xerox PARC extend the ideas of Simula in
developing Smalltalk (1970’s)
 Kay coins the term “object oriented”
 Smalltalk is first fully object oriented language
 Grasps that this is a new programming paradigm
 Integration of graphical user interfaces and
interactive program execution
 Bjarne Stroustrup develops C++ (1980’s)
 Brings object oriented concepts into the C
programming language

31
Other Object Oriented Languages
 Eiffel (B. Meyer)
 CLOS (D. Bobrow, G. Kiczales)
 SELF (D. Ungar et al.)
 Java (J. Gosling et al.)
 BETA (B. Bruun-Kristensen, O. Lehrmann
Madsen, B. Møller-Pedersen, K. Nygaard)
 Other languages add object dialects, such as
TurboPascal
 …

32
REVIEW: Definition of an “Object”
 An object is a computational entity that:
 Encapsulates some state
 Is able to perform actions, or methods, on this
state
 Communicates with other objects via message
passing

33
Encapsulation
 The main point is that by thinking of the
system as composed of independent objects,
we keep sub-parts really independent
 They communicate only through well-defined
message passing
 Different groups of programmers can work on
different parts of the project, just making sure
they comply with an interface
 It is possible to build larger systems with less
effort

34
Advantages
Building the system as a group of interacting
objects:
 Allows extreme modularity between
pieces of the system
 May better match the way we (humans)
think about the problem
 Avoids recoding, increases code-reuse

35
But there’s more…
 Classes can be arranged in a hierarchy
 Subclasses inherit attributes and
methods from their parent classes
 This allows us to organize classes, and
to avoid rewriting code – new classes
extend old classes, with little extra work!
 Allows for large, structured definitions

36
Example of Class Inheritance
extends
extends
toString( )
equals( Object obj )
getClass( )
Shape
Rectangle
Time
extends
extends
Objects made from this
class, for example, have
all the attributes and
methods of the classes
above them, all the way
up the tree
Object
Circle Triangle
extends
hour
minute
color
borderWidth
Color getColor( )
void setBorderWidth( int m )
int computeArea( )
length
width
int computeArea( )
radius base
height
int computeArea( )

38
Java Class Hierarchy, another view

39
Polymorphism
 An object has “multiple identities”, based on
its class inheritance tree
 It can be used in different ways

40
Polymorphism
 A Circle is-a Shape is-a Object
toString( )
getClass( )
Shape
extends
extends
Object
Circle
color
borderWidth
Color getColor( )
int computeArea( )
radius

41
Polymorphism
 A Circle is-a Shape is-a Object
toString( )
getClass( )
Shape
extends
extends
Object
Circle
color
borderWidth
Color getColor( )
int computeArea( )
radius
Shape
Circle
Object
A Circle object really has 3 parts

42
How Objects are Created
Circle c = new Circle( );

43
c
Shape
Circle
Object
1.1.1.1.
Execution Time

44
c
Shape
Circle
Object
c
Shape
Circle
Object
1.1.1.1. 2.2.2.2.
Execution Time

45
c
Shape
Circle
Object
c
Shape
Circle
Object
c
Shape
Circle
Object
1.1.1.1. 2.2.2.2. 3.3.3.3.
Execution Time

46
Three Common Uses for Polymorphism
1. Using Polymorphism in Arrays
2. Using Polymorphism for Method
Arguments
3. Using Polymorphism for Method
Return Type

47
1) Using Polymorphism in Arrays
 We can declare an array to be filled with “Shape”
objects, then put in Rectangles, Circles, or
Triangles
extends
toString( )
getClass( )
Shape
Rectangle
extends
extends
Object
Circle Triangle
extends
color
borderWidth
Color getColor( )
int computeArea( )
length
width
int computeArea( )
radius base
height
int computeArea( )

48
Triangles
samples
(an array
of Shape
objects)
[0] [1] [2]

49
Triangles
[0] [1] [2]
[2]
firstShape
Attributes:
length = 17
width = 35
Methods:
int
computeArea( )
secondShape
Attributes:
radius = 11
Methods:
int computeArea( )
thirdShape
Attributes:
base = 15
height = 7
Methods:
int computeArea( )
samples
(an array
of Shape
objects)

50
Triangles
[0] [1] [2]
[2]
firstShape
Attributes:
length = 17
width = 35
Methods:
int
computeArea( )
secondShape
Attributes:
radius = 11
Methods:
int computeArea( )
thirdShape
Attributes:
base = 15
height = 7
Methods:
int computeArea( )Rectangle
Circle
Trianglesamples
(an array
of Shape
objects)

51
2) Using Polymorphism for Method
Arguments
 We can create a procedure that has Shape as
the type of its argument, then use it for objects
of type Rectangle, Circle, and Triangle
public int calculatePaint (Shape myFigure) {
final int PRICE = 5;
int totalCost = PRICE * myFigure.computeArea( );
return totalCost;
}
The actual definition of computeArea( ) is known only at
runtime, not compile time – this is “dynamic binding”

52
Arguments
 Polymorphism give us a powerful way of
writing code that can handle multiple types
of objects, in a unified way
public int calculatePaint (Shape myFigure) {
final int PRICE = 5;
int totalCost = PRICE * myFigure.computeArea( );
return totalCost;
}
To do this, we need to declare in Shape’s class definition
that its subclasses will define the method computeArea( )

53
Return Type
 We can write general code, leaving the type
of object to be decided at runtime
public Shape createPicture ( ) {
/* Read in choice from user */
System.out.println(“1 for rectangle, ” +
“2 for circle, 3 for triangle:”);
SimpleInput sp = new SimpleInput(System.in);
int i = sp.readInt( );
if ( i == 1 ) return new Rectangle(17, 35);
if ( i == 2 ) return new Circle(11);
if ( i == 3 ) return new Triangle(15, 7);
}

54
Java Convinced Me to Start Teaching
Object-Oriented in Intro to CS
 Java is Object-Oriented from the Ground Up
 Java has the elegance that comes from being
designed after other OO languages had been in use
for many years
 Java has strong type checking
 Java handles its own memory allocation
 Java’s syntax is “standard” (similar to C and C++)
 Java is a good teaching language, but it (or
something close) will also be seen by students in
industry

55
Object-Oriented Programming in Industry
 Large projects are routinely programmed
using object-oriented languages
nowadays
 MS-Windows and applications in MS-
Office – all developed using object-
oriented languages
 This is the world into which our students
are graduating…

56
Conclusions
 Object-oriented programming provides a superior
way of organizing programming projects
 It encourages a high degree of modularity in
programming, making large projects easier to
implement
 It provides powerful techniques like inheritance and
polymorphism to help organize and reuse code
 Object-oriented languages like Java both provide
a good environment for beginning students to
learn programming, and match real-world
developments in computer programming

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