DEV-08: Exploring Object-oriented Programming

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DEV-08 Exploring Object-oriented Programming

• Shelley Chase
• Development Architect Progress OpenEdge

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Todays Agenda

• Basic Principles of Object-oriented Programming
• Types, Classes, Objects, and Interfaces
• Inheritance, Polymorphism, and Delegation

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What are Objects?

• You interact with objects everyday
• A customer
• An order
• All objects contains state and behavior
• What they can do and what changes when they do
• Software objects represent these as
• Data ( like 4GL variables )
• Methods ( like 4GL procedures)

• Your car
• The telephone

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Object-oriented Programming

• Object-oriented programming is a
  method of implementation in which
  programs are organized as cooperative
  collections of objects, each of which
  represents an instance of some class...
• Grady Booch

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Object-oriented Application Development

• A way to design and build applications
• Objects bundle together data (state) and methods
  (behavior)
• Objects facilitate separating definition from
  implementation
• Much more than just syntax
• You might have already done object-oriented
  programming in the 4GL

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Designing an Object-oriented Application
Object Order
Take an Order
Object Customer
Check Credit
Create a Customer
Object beOrder
Assign Salesperson
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Basic Object-oriented Principles

• Abstraction
• Encapsulation
• Hierarchies

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Abstraction
Public View of an Object

• Abstraction is used to manage complexity
• Focus on the essential characteristics
• Eliminate the details
• Find commonalities among objects
• Defines the public contract
• Public definition for users of the object
• The Outside view
• Independent of implementation

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Abstraction - Example
What should an Order object do?
InternalOrder
Two types of Orders
ExternalOrder
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Encapsulation
Hide Implementation Details

• Encapsulation hides implementation
• Promotes modular software design data and
  methods together
• Data access always done through methods
• Often called information hiding
• Provides two kinds of protection
• State cannot be changed directly from outside
• Implementation can change without
  affecting users of the object

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Encapsulation - Example

• Implementation Outside View

Public methods of Order class
CreateOrder UpdateOrder GetOrderTotal Next
CreateOrder( ) UpdateOrder( ) GetOrderTotal(
) Next( )
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Encapsulation - Example continued
Hmm... Id like to change
CalculatePrice to CalculateTotalPrice
Object Order
orderNum AS INT custNum AS INT CalculatePrice(
)
PUBLIC CreateOrder( ) UpdateOrder(
) GetOrderTotal( ) Next( )

• GetOrderTotal calls CalculatePrice( )

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Encapsulation - Example continued
This change was easy because users of the object
will not be affected.
Object Order
orderNum AS INT custNum AS INT CalculateTotalPri
ce( )
PUBLIC CreateOrder( ) UpdateOrder(
) GetOrderTotal( ) Next( )

• GetOrderTotal now calls CalculateTotalPrice( )

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Hierarchies
Object Relationships

• Define relationships between objects
• Objects defined in terms of other objects
• Allows state and behavior to be shared and
  specialized as necessary
• Encourages code reuse
• Two important hierarchy types
• Inheritance
• Aggregation

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Hierarchies - Example
Order uses ShipInfo (Aggregation)
Order
references
is a
is a
ExternalOrder
ShipInfo
InternalOrder
InternalOrder and ExternalOrder inherit from
Order (Inheritance)
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Summary Object-oriented Principles

• Abstraction
• Break up complex problem
• Focus on public view, commonalities
• Encapsulation
• Hide implementation details
• Package data and methods together
• Hierarchies
• Build new objects by combining or extending other
  objects

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Todays Agenda

• Basic Principles of Object-oriented Programming
• Types, Classes, Objects, and Interfaces
• Inheritance, Polymorphism, and Delegation

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Type
A Type is a definition

• A Type defines the state and behavior
• Identifies inheritance relationships with other
  types
• No concern for implementation
• Enables strong-typing
• Early binding - types determined at compile time
• Type-consistency enforced at compile time and
  runtime

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Type - Example

• Types
• Order
• InternalOrder
• Subtype of Order
• ExternalOrder
• SubType of Order

Order
is a
is a
ExternalOrder
InternalOrder
A subtype can appear anywhere a super type is
expected
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Benefits of Types (Strong-Typing)

• Compile time checking for type consistency
• myObj mySubObject. (must be subType)
• myObjmethod(). (validates signature)
• myObjdata 3. (validates data type)
• Results in safer, bug-free code because all code
  paths checked at compile time

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Class
A Class implements a Type

• A Class defines and implements a
  user-defined type
• A Class is a template (blueprint) for an object
• Data
• Methods
• Relationships to other classes

Methods Data
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Object
An Object is an instance of a Class

• An Object is created at runtime
• Maintains independent state in data members
• Code shared among object instances
• The term Object is often used to refer to both
  classes and instances

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Interface
An Interface implements a Type

• An Interface is a collection of method
  definitions for a set of behaviors a
  contract
• No implementation provided

• A Class can implement an interface
• Must implement all methods in the interface
• Behavior can be specialized
• Compiler validates implementation of interface

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Interface - Example

• Class Order implements IList interface

Interface IList
PUBLIC Next( )
Compiler checks for method definition in
implementing class
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Interface Example continued

• Write generic routine using interface
• MoveNext( listObj AS IList )
• listObj.Next( ). / Calls method
  
• in real object /
• Call with any object that implements IList
• myOrder NEW Order( ). / implements IList
  /
• MoveNext( myOrder ).
• or
• myCust NEW Customer( ). / implements IList /
• MoveNext( myCust ).

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Benefits of Interfaces

• Allows many different classes to be treated in a
  like manner
• All classes that implement an interface are
  guaranteed to have same set of methods
• Enables generic programming
• IList example allows any collection of objects to
  be navigated using Next( )
• Behavior can be specialized as needed

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Summary Object-oriented Constructs

• Type
• Enforces type consistency at compile time
• Class
• Defines type with data and methods and provides
  implementation
• Object
• Runtime instantiation of class
• Interface
• Defines type with methods no implementation
  provided

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Todays Agenda

• Basic Principles of Object-oriented Programming
• Types, Classes, Objects, and Interfaces
• Inheritance, Polymorphism, and Delegation

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Inheritance
Relationship between Classes
Super Class

• Super Class (Base class)
• Provides common functionality and data members
• Subclass (Derived class)
• Inherits public and protected members from the
  super class
• Can extend or change behavior of super class by
  overriding methods

Subclass
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Access Levels for Class Members

• PRIVATE members available
• Only within the class
• PROTECTED members available
• Within the class
• Within the class hierarchy
• PUBLIC members available
• Within the class
• Within the class hierarchy
• To users outside the class

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Inheritance Example
Class Order
PRIVATE orderNum AS INT custNum AS
INT CalculateTotalPrice( )
PROTECTED GetCredit( ) PUBLIC CreateOrder(
) UpdateOrder( ) GetOrderTotal( ) Next( )
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Inheritance Example

• Class InternalOrder inherits Order

Class Order
PRIVATE orderNum AS INT custNum AS
INT CalculateTotalPrice( )
InternalOrder
PROTECTED GetCredit( ) PUBLIC CreateOrder(
) UpdateOrder( ) GetOrderTotal( ) Next( )
PROTECTED GetCredit( ) PUBLIC CreateOrder(
) UpdateOrder( ) GetOrderTotal( ) Next( )
PROTECTED GetCredit( ) PUBLIC CreateOrder(
) UpdateOrder( ) GetOrderTotal( ) Next( )
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Inheritance and Method Overriding

• Method overriding used to specialize behavior
• Subclass may override a method in its super class
  (hierarchy)
• Method signatures must match
• Overriden method can
• Completely override behavior of super class
• Augment behavior by providing its own behavior
  and calling super class method

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Method Overriding Example 1
Order
PROTECTED GetCredit( ) credit
FindCreditScore( ).

• Class InternalOrder inherits Order

InternalOrder
PROTECTED GetCredit ( ) credit -1.
/unlimited/
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Method Overriding Example 2
Order
PROTECTED GetCredit( ) credit
CalculateCredit( ).

• Class ExternalOrder inherits Order

ExternalOrder
PROTECTED GetCredit( ) credit
SUPERGetCredit( )
extraMoney.
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Benefits of Inheritance and Overriding

• Inheritance supports modular design
• Common behavior put in super class and used by
  subclass
• Subclass can override to specialize behavior
• Inheritance is strongly-typed
• InternalOrder myOrder NEW InternalOrder.
• myOrder.GetCredit( ).

myOrder knows it is an InternalOrder
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Polymorphism
One interface, many implementations

• Execution of an overridden method in a subclass
  from a reference to a super class
• superclassmethod( )
• Code written using super class
• Tightly coupled to inheritance and overriding
• Super class used at compile time, subclass
  assigned at runtime
• Method call on super class dispatched to
  subclass method at runtime

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Polymorphism Example
Order
PROTECTED GetCredit( ) credit
CalculateCredit( ).
InternalOrder
ExternalOrder
PROTECTED GetCredit ( ) credit -1.
/unlimited/
PROTECTED GetCredit( ) credit
SUPERGetCredit( )
extraCreditPoints.
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Polymorphism Example continued
DEFINE myOrder AS Order. if (bInternalCust
TRUE) myOrder NEW InternalOrder( ). else
myOrder NEW ExternalOrder( ). myOrderGetCred
it( ).
Super Class reference
Calls InternalOrderGetCredit( ) or
ExternalOrderGetCredit( )
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Benefits of Polymorphism

• Supports generic programming using super class or
  interface
• Type used at compile time is super class or
  interface
• Specialized behavior is called at runtime
  automatically
• Built on inheritance and overriding

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Delegation

• Delegation is the use of other objects
  within a class
• Class forwards method calls to the
  contained object
• Class wraps the delegate object
• Creates an instance of the object
• Defines a stub method for any referenced
  methods that should be public
• No access to protected or private members

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Delegation Example

• Class Order references a ShipInfo object

ShipInfo
PRIVATE id shipdate promisedate PUBLIC SetDate
( ) GetDate( )
calls
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Benefits of Delegation

• Delegation supports modular design
• Purposed class does work
• Class uses delegate to provide needed
  functionality
• Class can determine what to put in API
• With inheritance super class dictates API with
  delegation wrapper class decides what to expose

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Thats Object-oriented ProgrammingWhat did we
learn
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Terminology / Concept Review

• Abstraction Public API
• Encapsulation Hide implementation details
• Hierarchy Relationships between classes
• Strong-typing Type consistency enforced
• Class Data members and methods
• Object Runtime instance of a class
• Interface Set of method definitions contract
• Inheritance Inherit/specialize from super
  class
• Polymorphism Most-derived method called
  from super class reference
• Delegation Other objects do the work

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Benefits of OO Programming

• Promotes modular design
• Data and methods that operate on that data are
  contained in one place
• Commonalities put into super classes
• Code reuse through hierarchies
• Inheritance and delegation
• Strong-typing
• Compile-time type checking
• Runtime type checking

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Recommended OO Books

• Object-Oriented Analysis and Design with
  Applications (2nd Edition) by Grady Booch
• Object-Oriented Modeling and Design by James R
  Rumbaugh
• Design Patterns, Elements of Reusable
  Object-oriented Software by Erich Gamma

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In Summary

• Object-oriented programming is more than syntax
  must be part of design
• Many benefits in OO Programming
• Can be combined with procedural programming, not
  all or nothing
• OpenEdge 10.1 Language enhancements support
  object-oriented programming naturally
• Attend Session DEV-10 for details

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Questions?
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Thank you for your time!
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(No Transcript)