ObjectOriented Concepts

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Object-Oriented Concepts
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Object-Oriented ConceptsOutlines

• The Six Basic OO Concepts
• Discovering Objects
• Discovering Attributes
• Discovering Methods
• Evaluating Candidate Objects
• Applicability of OO Concepts

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The Six Basic OO Concepts

• Object
• Class
• Instance
• Attribute
• Method
• Message

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Object

• An object represents an individual, identifiable
  item, unit, or entity, either real or abstract,
  with a well-defined role in the problem domain.
• An object can be any person, place, thing,
  event, screen, report, or concept that is
  applicable to the design of the system.

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Class

• A class is a collection of similar objects.
• A class is a general definition of the attributes
  and methods of a set of objects.
• Examples Customer, Vehicle, Account Receivables.

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Instance

• When an application is running, objects are
  instantiated from a class, and the objects become
  instances of the class
• - Joe Bair is an instance of the class of
  Customer.
• - Plymouth Voyager is an instance of the class
  of Vehicle.

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In Terms of a Relation

• A class - a relation (table)
• An object - a record
• An attribute - a data item (field)

Description
Price
On Hand
Prod
A1201
Intel PP
325.99
72
A1202
Segate 1.6G
159.45
182
A1203
Dell MD-3
1499.00
64
A1204
Tech 1.44
45.99
28
A1205
MS 3-B
34.99
267
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Attribute

• Attributes are things that an object knows.
• Attributes are properties and characteristics of
  an object.
• An attribute is basically a data item of an
  object.
• Examples
• - Account Number is an attribute of the class
  of Account.
• - Student ID is an attribute of the class of
  Student.

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Method

• Methods are things that a class or object does.
• Methods usually contain business logic employed
  in the application.
• A method is similar to a function, a procedure,
  or a subroutine in terms of structured
  programming. Examples
• - Create a New Account is a method of the
  class of Account.
• - Register for a Class is a method of the
  class of Student.

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Attributes vs. Methods

• Attributes define the data, while methods define
  the functionality.
• Attributes are defined by their names and types
  (e.g., integer, string, date).
• Methods are defined by writing the code for it.
• Attributes and methods are both shielded by the
  corresponding objects.

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Message

• When attributes and methods are encapsulated
  inside the objects, sending messages become the
  mechanism to communicate interacting objects.
• A message is composed of the name of the method
  and any necessary parameters. A general message
  format looks like this
• Object.method(parameters).
• For example, formula.square_root(100) returns a
  value of 10.

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Summary

• The OO paradigm is based on the concept that
  systems should be built out of ______.
• The objects should have both data and
  functionality. ______ define data, while ______
  define functionality.
• Similar objects are grouped into a ____, and
  objects can be created as _____of the class.
• Objects interact through sending _____.

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Summary

• The OO paradigm is based on the concept that
  systems should be built out of objects.
• The objects should have both data and
  functionality. Attributes define data, while
  methods define functionality.
• Similar objects are grouped into a class, and
  objects can be created as instances of the class.
• Objects interact through sending messages.

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Summary
Methods
Other Objects
message
Attributes
Owner Address Acct Balance Interest Rate
Tom Blake
deposit
16 Wood St.
122-467-3345
492.87
2.5
Account Object 9987554
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Discovering Objects

• There is a great deal of detail surrounding any
  OO model - attributes, methods, messages, and so
  on - but the heart of the OO model is the set of
  classes and objects.
• Discovering classes and objects is the starting
  point for all OO system development.

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Discovering Objects

• Objects can be discovered from the
• following four different perspectives
• Data
• Function
• Relationship
• Behavior

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Discovering ObjectsThe Data Perspective

• For many business applications, data is usually
  the dominant characteristic of the system.
• Business entities are usually associated with
  data.
• Discovering objects from the data perspective is
  very similar to discovering entities in
  ERD-based data modeling methodologies.

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Discovering ObjectsThe Data Perspective

• Look at events that must be recorded and
  processed by the system.
• Look at roles played by different people who
  interact with the system.
• Look at physical locations and sites that may be
  relevant to the system.

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Discovering ObjectsThe Data Perspective (contd)

• Look at organizational units that are relevant to
  the system.
• Look at physical devices that interact with the
  system.
• Look at external entities that communicate with
  the system.
• E. Yourdon
• Object-Oriented System Design An Integrated
  Approach, 1994

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Discovering ObjectsThe Data Perspective (contd)
External Entity
Event
Data
Data
Device
Organizational Unit
Data
Place
Data
Data
Player
Data
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Discovering ObjectsThe Function Perspective

• Objects exist when they function.
• Identifying responsibilities of an object in CRC
  modeling is a way to discover functions of
  objects and thereafter the objects.

Object C
Object A
Functions
Object B
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Discovering ObjectsThe Function Perspective
Object Types

• ? Actor objects. Actors are people and/or
• organizations in the real world that
  interact with the
• system.
• ? Business objects. Business classes are places,
• things, concepts, and events that describe
  that the
• business is all about.
• ? Interface objects. Interface classes are input
• screens and menus that make up the user
  interface of
• the system.
• ? Report objects. Report classes are created for
  any
• printed or electronic reports or pieces of
  output.

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Discovering ObjectsThe Relationship Perspective

• System objects rarely stand alone, and they often
  have relationships with other objects.
• A relationship establishes a logical connection
  among objects.
• Relationships connect one object with another,
  similar to the way verbs and verb phrases relate
  one noun or noun phrase to another. For example,
  Joe is an employee of the Information System
  Department.

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Discovering ObjectsThe Relationship Perspective
(contd)

• There are two fundamental sets of object
  relationship
• (1) Generalization the IS-A relationship
  (Class Instantiation).
• (2) Association the IS-PART-OF relationship
  (Inheritance and Aggregation).

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Discovering ObjectsThe Relationship Perspective
(contd)
Student
Person
IS-A
Employee
Department
IS-PART-OF
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Discovering ObjectsThe Behavior Perspective

• Objects have particular behavior, and behavior
  has three basic components
• (1) State An objects status, phase, or
  situation.
• (2) Condition A cause for an object to make
  the transition from one state to another.
• (3) Action An act of an object in a particular
  state or transition.

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Discovering ObjectsThe Behavior Perspective
(contd)

• A closer look at the behavior of objects
  typically generates the well-known
  state-transition diagram.
• By tracing the analyzing the state transitions,
  more related objects can be found.

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Discovering ObjectsThe Behavior Perspective
(contd)
Object A
Object D
State 1
x
receive message x send message a
y
b
a
State 2
Object B
Object C
receive message y send message b
State 3
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Discovering ObjectsThe Behavior Perspective
(contd)
Object A
Object D
State 1
x
receive message x send message a
y
b
a
State 2
Object B
Object C
receive message y send message b
State 3
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Discovering ObjectsOther Tips

• Look for anything that interacts with the
• system, or that is part of the system.
• Ask yourself if there is a customer.
• Customer is a very common class.
• Follow the monetary transaction.
• Who, where, and how the money is spent.

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Discovering Attributes

• Analysis attributes. During the OO analysis
  stage, these attributes become apparent.
• Design attributes. These are attributes (e.g.,
  print device) that do not surface until the
  design stage.
• Relevant attributes. List only attributes that
  exist in the application domain.
• Service attributes. List attributes that are used
  by the methods of the object.

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Discovering Methods

• There are two kinds of methods private and
  public.
• Private methods are automatically activated by
  the system when an object is accessed. They are
  private because they are not seen and cannot be
  used by the users.

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Discovering Methods (contd)

• Private methods can be defined at the class
  level. For example, all employees salary are
  within a range and can be enforced by a private
  method.
• Private methods can also be defined at the
  instance level. For example, a private method for
  editing a particular employees salary with a
  unique range can be defined at the object level.

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Discovering Methods (contd)

• Public methods contain business logic and can be
  invoked by the receipt of a message sent from
  another class or object instance.
• For example, the public method Calculate Salary
  for the calculation of an employers salary is
  defined and stored in the Employee class. The
  method can be called to calculate the salary of a
  particular employee.

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Discovering Methods (contd)
Object A
Object B
Object B
Public
Private
Method
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Evaluating Candidate Objects

• More objects does not necessarily mean better.
• The object structure and class hierarchy should
  be manageable.
• Candidate objects have to be evaluated before
  they become permanent.

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Evaluating Candidate ObjectsCriteria

• Object independence. If an independent existence
  of an object is important, then it may well be an
  object.
• Necessary remembrance. The proposed object should
  have some data that it must remember.
• More than one attribute. Most of the objects
  have at least one attribute.

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Evaluating Candidate ObjectsCriteria (contd)

• Needed functionality. It is possible for an
  object to have no attributes, but it is highly
  implausible for an object to have no methods or
  services.
• Essential functionality. The proposed object
  should have methods that are relevant and
  necessary for the system.

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Evaluating Candidate ObjectsCriteria (contd)

• Common attributes. All the attributes of the
  proposed class should apply to each instance of
  the class.
• Common functionality. All the methods of the
  proposed class should apply to each instance of
  the class.
• J. Rumbaugh et al.
• Object-Oriented Modeling and Design, 1991
• M. Coad E. Yourdon
• Object-Oriented Analysis, 1991

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Applicability of OO Concepts
Object-oriented technology is applicable to the
analysis and design of any system of any size in
virtually any business area.
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Applicability of OO Concepts

• Abstract data typing (Modula-2)
• Generic processing (LISP)
• Semantic networks (AI)
• Simulation (SIMULA)
• Interface-based programming (Ada)