SE_Lec 06_Object Oriented Analysis and Design

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 System development refers to all activities that go into
producing information system solution.
 System development activities consist of system
analysis, modeling, design, implementation, testing and
maintenance.
 Object-Oriented (OO) systems development is a way to
develop software by building self-contained modules
that can be more easily:
 Replaced
 Modified
 and Reused.

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 A software system is a set of mechanism for performing
certain action on certain data.
Algorithm + Data structure = Program

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 The code is subdivided into modules
 Each module is made of procedures and data structures
 Procedures operate on data, e.g. modifying them
 e.g. the “C” programming language
 There is no strong connection between procedures
 Every procedure may possibly access any data and modify
it and data

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 OO approach is more like creating a lot of helpers that
take on an active role, a spirit, that form a community
whose interactions become the application.
 Models the problem to be solved as a set of interacting
objects, each carrying its own state and exhibiting its
own behavior
 By separating concerns, this approach has proved to be
robust and useful to solve more complex programming
problems.

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 The physical world is made of a set of objects that can
interact
 Each of them presents
 An identity (objects can be recognized)
 A state (objects remember their history)
 A behavior (objects react to external stimuli in a
predictable way)

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 Faster development,
 Increased quality
 Raising the level of abstraction.
 It adapts to
 Changing requirements
 Easier to maintain
 More robust
 Promote greater design
 Code reuse

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 Goals:
 Define Objects and classes
 Describe objects‘ methods, attributes and how objects
respond to messages,
 Define Polymorphism, Inheritance, data abstraction,
encapsulation, and protocol,
 Describe objects relationships,
 Describe object persistence.
 Understand meta-classes.

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 The term object was first formally utilized in the Similar
language to simulate some aspect of reality.
 An object is an entity.
 It knows things (has attributes)
 It does things (provides services or has methods)
 Note:
 Attributes or properties represented by data type and
describe object‘s state (data)
 Methods define objects behavior and specify the way in
which an Object‘s data are manipulated.

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 It Knows things (attributes)
 I am an Employee.
 I know my name, social security number and my address.
 Attributes
 I am a Car.
 I know my color,
 manufacturer, cost,
 owner and model.
 It does things (methods)
 I know how to compute my payroll.

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 In an object-oriented system, everything is an object:
numbers, arrays, records, fields, files, forms, an
invoice, etc.
 An Object is anything, real or abstract, about which we
store data and those methods that manipulate the data.
 Conceptually, each object is responsible for itself.
 A window object is responsible for things like opening,
sizing, and closing itself.

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 When developing an O-O application, two basic
questions always arise.
 What objects does the application need?
 What functionality should those objects have?

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 Simple Name (written as UpperCase-first Noun)
 Path Name

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 Represent named properties of a UML class
 UML class can have many attributes of different names
 Attribute name is generally a short noun or a noun
phrase written in lowerCase-first text
 Attribute declaration may include visibility, type and
initial value: +attributeName : type = initial-value

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 Represent named services provided by a UML class
 UML class can have many operations of different
names
 Operation name is generally a short verb or a verb
phrase written in lowerCase-first text
 Operation may include visibility, parameters, and
return type: +opName(param1 : type = initial_value)
: return-type

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 Three levels of class, attribute and operation visibility:
• private (-), available only to the current class
• protected (#), available to the current and inherited
classes
• public (+), available to the current and other classes

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 Each class represents a set of objects that share the
same attributes, operations, relationships, and
semantics
 For each of the class attributes, objects can have
specific attribute values
 For each of the class operations, objects may have
different implementations

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 Represent a relation between a parent (a more abstract
class) and a child (a more specific class)
 Generally referred to as a “is-a-kind-of” relationship
 Child objects may be used instead of parent objects
since they share attributes and operations; the opposite
is not true

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 An object-oriented system organizes classes into
subclass-super hierarchy.
 At the top of the hierarchy are the most general classes
and at the bottom are the most specific
 A subclass inherits all of the properties and methods
(procedures) defined in its super class.

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 Inheritance is a relationship between classes where one
class is the parent class of another (derived) class.
 Inheritance allows classes to share and reuse behaviors
and attributes.
 The real advantage of inheritance is that we can build
upon what we already have and,
 Reuse what we already have.

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 OO systems permit a class to inherit from more than
one superclass.

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 Information hiding is a principle of hiding internal data
and procedures of an object.
 By providing an interface to each object in such a way
as to reveal as little as possible about its inner
workings.
 Encapsulation protects the data from corruption.

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 Objects perform operations in response to messages.
For example, you may communicate with your
computer by sending it a message from hand-help
controller.

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 Consider a payroll program that processes employee
records at a small manufacturing firm. This company
has three types of employees:
 Managers: Receive a regular salary.
 Office Workers: Receive an hourly wage and are eligible
for overtime after 40 hours.
 Production Workers: Are paid according to a piece rate.

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 Structured Approach
FOR EVERY EMPLOYEE DO
BEGIN
IF employee = manager THEN
CALL computeManagerSalary
IF employee = office worker THEN
CALL computeOfficeWorkerSalary
IF employee = production worker THEN
CALL computeProductionWorkerSalary
END

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 OO Approach
FOR EVERY EMPLOYEE DO
BEGIN
employee computePayroll
END

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 Polymorphism means that the same operation may
behave differently on different classes.
 Example: computePayroll

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 Link: model of logical or physical link between objects

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 Relationship: descriptor of links

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 Multiplicity: constraint on the min/max number of links
connected to an object

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 The concept of association represents relationships
between objects and classes. For example a pilot can
fly planes
 Association can be binary, between two classes, or n-
ary, among more than two classes
 Can include association name, direction, role names,
multiplicity, and aggregation type

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 Represent a specific, whole/part structural relationship
between class objects
 Composition [is entirely made of]: (closed diamond)
represents exclusive relationship between two class
objects (e.g., a faculty cannot exist without nor be a part
of more than one university)
 Aggregation [is part of]: (open diamond) represents
nonexclusive relationship between two class objects (e.g.,
a student is a part of one or more faculties)

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 Objects have a lifetime. An object can persist beyond
application session boundaries, during which the object
is stored in a file or a database, in some file or database
form.

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 Everything is an object.
 How about a class?
 Is a class an object?
 Yes, a class is an object! So, if it is an object, it must
belong to a class.
 Indeed, class belongs to a class called a Meta-Class or
a class' class.
 Meta-class used by the compiler. For example, the
meta-classes handle messages to classes, such as
constructors and "new.“

SE_Lec 06_Object Oriented Analysis and Design

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SE_Lec 06_Object Oriented Analysis and Design