Introduction%20to%20Objects

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Introduction to Objects

• Adapted from TEN STEPS TO OBJECT-SPEAK
• a CPT Tech Talk by
• Joy Starks
• September 17, 1999

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OBJECTIVES

• Discuss the history of Object Oriented
  Technology (OT)
• Describe basic concepts of OT
• Define terms in Object Oriented Analysis Design
  (OAD) and Programming (OOP)
• Relate definitions to real-world examples
• Mention some benefits of OT
• Practice by Example

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HISTORY OF OT

• 1969
• Dr. Kristin Nygaard of Norway
• Model fjord and movement of ships passing through
  it

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HISTORY OF OT

• The Problem
• structured programming separates data from
  procedures
• The Solution
• Object Oriented Programming
• model each component (data and procedure) as a
  single unit
• model relationships between components

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OT BASIC CONCEPTS

• Objects and Classes
• Operations
• Requests
• Attributes

• Inheritance
• Encapsulation
• Polymorphism

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WHATS AN OBJECT?
NOUN

• Anything, real or abstract, about which we store
  data
• Dr. Nygaards objects
• mathematical models of boats
• physical aspects of the fjord being analyzed
• Other Examples
• an invoice, an organization, a screen with which
  a user interacts, a drawing, an airplane, an
  order-filling process...

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WHATS AN OPERATION?
VERB

• An activity that reads or manipulates data of an
  object
• Dr. Nygaards operations
• boats float, sink, move etc.
• Other Examples
• calculating a total, checking a balance, adding a
  new employee, changing an address, deleting a
  customer ...

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HOW OBJECTS COMMUNICATE
Adverb

• An object is sent a message, which in turn causes
  an operation to be invoked
• Sometimes the operation returns a response

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WHATS AN ATTRIBUTE?
Adjective

• Characteristics that add detail to an object
• Dr. Nygaards attributes
• color, weight, size, etc.
• Other Examples
• someones name or address, an employees title, a
  books author , a part number, a rooms dimensions

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INHERITING ATTRIBUTES

• Objects can be decomposed into other objects
• Generalization hierarchy
• Supertype versus subtype
• An object inherits the attributes in its parent
  class

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SAMPLE GENERALIZATION HIERARCHY
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WHAT ARE CLASSES?

• An object type or object class is a category of
  an object that has similar characteristics and
  behavior
• Example employee or student
• A class acts as a template or blueprint for
  object instances

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OBJECT INSTANCES
Proper Noun

• An object instance is a specific example of an
  object type
• Examples
• John P. Smith, Invoice 12356
• And just to make it all really confusing, object
  instances are sometimes called objects!

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ENCAPSULATION

• The process of making implementation details of
  an object transparent to a user
• Packaging data and operations together
• Allows for information hiding
• The black box approach

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POLY-WHAT?

• Polymorphism
• the ability of two or objects to respond to the
  same message, each in its own way
• an instruction is given using a generalized,
  rather than specific, detailed command
• while specific actions would be different,
  results are the same
• Example
• Animal Speak
• Dogs bark cats meow

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SOME BENEFITS OF OBJECTS

• Reusability
• classes can be reused or inherited
• Stability
• over time, classes become more stable
• Easier Design
• black box concept
• Faster Design
• create applications from existing components

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Practicing

• A mammal is an object
• Bear, buffalo, whale, and dolphin are subtypes
• Yogi, Wilbur, and Smokey are instances of the
  Bear subtype
• Eye color, ear size, and weight are attributes

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Your Turn
Match each VCR example on the left to the
object-oriented term on the right.

• VCR
• A Sony VCR
• My VCR, Serial 9234
• Playback, record, and audio dubbing
• the VCR contains complex components that we
  assume work
• Use the remote control

• object subtype
• messages
• object instance
• operations
• object
• encapsulation

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Your Turn Matching

• VCR IS an object
• A Sony VCR is an object subtype
• Serial 9234 of Sony VCR is an object instance
• Playback, record, and audio dubbing are examples
  of VCR operations
• The concept that the VCR contains complex
  components you assume work is encapsulation
• When you use a remote control, you are sending
  requests to the VCR

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Objects are Models

• A MODEL . . .
• represents an aspect of reality
• helps us to understand reality
• assists us in inventing systems or redesigning
  business areas
• should be simpler than reality
• for example, model cars are simpler thana real
  car

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OBJECT STRUCTURE DIAGRAM

• Large rectangle with two horizontal dividing
  lines
• Top third
• object name
• Middle third
• object attributes
• Bottom third
• object methods

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SAMPLE OBJECT STRUCTURE DIAGRAM
TRAFFIC LIGHT Color Turn Red Turn Yellow Turn
Green
The operations are described in terms of
what they do -- not how they do it.
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YOU TRY ONE ...

• Draw an object structure diagram for the VCR
  object

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Summary of Terminology

• Objects, Classes, Operations, Attributes
• Inheritance, Encapsulation, Polymorphism
• Generalization Hierarchies
• Object Structure Diagrams
• Event Diagrams

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The End