Transcript Lecture 1 part a - School of Computing

Object-Oriented Programming
OOP
John Gilligan
School of Computing DIT Kevin St
What is a program?



“A program is a text which evokes
computation”
Edsgar Dijkstra
As we will see there are many kinds and
styles of program.
What does it mean to evoke
computation?



Well there is a program and something is
computed when the program is executed.
Ideally the computation should achieve
something that we want when the program
is run. The executed program should meet
our requirements.
There may be an informal or formal
specification of these requirements
Clearly there is a relationship
between specification and program
Spec
Program
From Specification to Program




Different Approaches to Programming vary in
how they consider this relationship.
Typically the specification describes the problem
in terms of inputs and outputs.
That is the input conditions or preconditions
describe conditions or the state that holds now
The output conditions or postcondition describes
the desired conditions or result that will hold after
the problem has been solved or the program
executed.
Verification



The program must be able to match the
specification , that is do what its supposed
to.
To prove that it does, it must be shown that
the output state is reachable from the input
state given the program.
This is called program verification.
There are many ways of developing
programs



Traditional programming, called imperative
programming, produces a sequence of statements
which bring you from input to output conditions.
There are many formal verification techniques
such as Dijkstras weakest precondition semantics.
Imperative programs tell you how to achieve a
solution. They implement an algorithm .i.e. a
sequence of steps which yield a solution.
Declarative Programming




The major alternative to imperative programming
is declarative programming which says what is to
be done not how to do it.
Examples of declarative programming include
functional and logic programming such as Prolog.
In logic programming computation is produced as
a by-product of proving that the output can be
derived from the input.
In a pure functional language, such as Haskell, all
functions are without side effects, and no explicit
state or state changes exist at all.
So how does Object Oriented
Programming fit into this



Object-oriented programming (OOP) is a
programming paradigm that uses "objects" and
their interactions to design applications and
computer programs.
Programming techniques may include features
such as encapsulation, modularity, polymorphism,
and inheritance.
Many modern programming languages now
support OOP.
Imperative Versus OOP

The big 2 approaches to Programming are
Imperative and Object Oriented

Let us look at these in more detail.
History of Imperative programming






The earliest imperative languages were the machine languages of the
original computers.
FORTRAN was a compiled language that allowed named variables,
complex expressions, subprograms, and many other features now
common in imperative languages.
In the late 1950s and 1960s, ALGOL was developed in order to allow
mathematical algorithms to be more easily expressed, and even served
as the operating system's target language for some computers.
COBOL (1960) and BASIC (1964) were both attempts to make
programming syntax look more like English.
In the 1970s, Pascal was developed by Niklaus Wirth
C was created by Dennis Ritchie while he was working at Bell
Laboratories. Wirth went on to design Modula-2, and Oberon.
Features of Imperative Languages





Variables and Constant Declarations
Assignment Statements
If Statements
Loops
Arrays
Example of imperative program
piece

int is_leap_year(int year)
{
int result;
if ( (year%4) != 0 )
// or: if ( year%4 )
result = FALSE;
// means: if year is not divisible by 4
else if ( (year%400) == 0 ) // or: if ( !(year%400) )
result = TRUE;
// means: if year is divisible by 400
else if ( (year%100) == 0 ) // or: if ( !(year%100) )
result = FALSE;
// means: if year is divisible by 100
else
// (but not by 400, since that case
result = TRUE;
// considered already)
return ( result );
}
And another


m_fMin = 10000; m_fMax = 0;
int i;
for ( i = 0; i < ARRAY_SIZE; i++ ) {
m_fResultArray[i] = sqrt(m_fInitialArray[i] * 2.8f);
if ( m_fResultArray[i] < m_fMin )
m_fMin = m_fResultArray[i];
if ( m_fResultArray[i] > m_fMax )
m_fMax = m_fResultArray[i]; }}
Advantages of Imperative
Programming






Works
Follows intuitive algorithmic reasoning
Can be made modular
Can be documented
Good proof techniques
Widely accepted
Disadvantages of imperative
programming






Can be messy
Poor for re-use
Data types buried in code (millenium Bug)
Arbitrary variable names
Allows bad programmers to have long
unwieldy programs
Maintenance a high cost
As an Alternative


Consider Object Oriented Programming
The term “object-oriented” was coined by
Alan Kay in 1967
Roots




Object-oriented programming can trace its roots to the
1960s.
As hardware and software became increasingly complex,
quality was often compromised.
Researchers studied ways in which software quality could
be maintained. Object-oriented programming was
deployed in part as an attempt to address this problem by
strongly emphasizing discrete units of programming logic
and re-usability in software.
Computer programming methodology focuses on data
rather than processes, with programs composed of selfsufficient modules (objects) containing all the information
needed within its own data structure for manipulation.
OOP



Object-oriented programming may be seen as a collection of
cooperating objects, as opposed to a traditional view in which a
program may be seen as a group of tasks to compute ("subroutines").
In OOP, each object is capable of receiving messages, processing data,
and sending messages to other objects.
Each object can be viewed as an independent little machine with a
distinct role or responsibility. The actions or "operators" on the objects
are closely associated with the object.
For example, in object oriented programming, the data structures tend
to carry their own operators around with them (or at least "inherit"
them from a similar object or "class"). The traditional approach tends
to view and consider data and behavior separately.
At First

The Simula programming language was the first to
introduce the concepts underlying object-oriented
programming (objects, classes, subclasses, virtual
methods, coroutines, garbage collection, and
discrete event simulation) as a superset of Algol.
Simula was used for physical modeling, such as
models to study and improve the movement of
ships and their content through cargo ports.
Smalltalk was the first programming language to
be called "object-oriented".
History

The 1980s saw a rapid growth in interest in object-oriented programming.

These languages were imperative in style, but added features to support objects.
The last two decades of the 20th century saw the development of a considerable number
of such programming languages.
Smalltalk, originally conceived by Alan Kay in 1969, was released in 1980 by the Xerox
Palo Alto Research Center.
Drawing from concepts in another object-oriented language — Simula (which is
considered to be the world's first object-oriented programming language, developed in
the late 1960s) — Bjarne Stroustrup designed C++, an object-oriented language based
on C. C++was first implemented in 1985.
In the late 1980s and 1990s, the notable imperative languages drawing on objectoriented concepts were Perl, released by Larry Wall in 1987; Python, released by Guido
van Rossum in 1990; PHP, released by Rasmus Lerdorf in 1994; Java, first released by
Sun Microsystems in 1994 and Ruby, released in 1995 by Yukihiro “matz” Matsumoto.




JAVA

In the past decade Java has emerged in wide
use partially because of its similarity to C
and to C++, but perhaps more importantly
because of its implementation using a
virtual machine that is intended to run code
unchanged on many different platforms.