Chapter 11: Implementing Inheritance and Association Visual Basic.NET Programming: From Problem Analysis to Program Design

Visual Basic.NET Programming: From Problem Analysis to Program Design2 Objectives Explore inheritance Create subclasses Write abstract classes and methods Define interfaces Create association links

Visual Basic.NET Programming: From Problem Analysis to Program Design3 Exploring Inheritance Base class –Serves as base for subclasses Derived class –Also called subclass –Derive some attributes and behavior from superclass When you instantiate subclass –Actually instantiate inheritance hierarchy

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5 Writing a Superclass Definition Overridable keyword –Tells VB.NET that overriding method is permitted Override superclass method –Write method with same signature in subclass

Visual Basic.NET Programming: From Problem Analysis to Program Design6 Instantiating Superclasses From Example 11-2: 5. Dim person1, person2, person3 As Person 6. ' create 3 instances of Person 7. person1 = New Person(“Eleanor”, “Atlanta”) 8. person2 = New Person(“Emily”, “St. Louis”) 9. person3 = New Person(“Graham”, “Marietta”)

Visual Basic.NET Programming: From Problem Analysis to Program Design7 Creating Subclasses Similar to writing any class definition Subclass must perform three tasks: –Specify superclass using keyword Inherits –Invoke superclass constructor as first statement in its constructor –Add keyword Overrides to methods overriding superclass methods MyBase keyword –Refers to superclass

Visual Basic.NET Programming: From Problem Analysis to Program Design8 Example 11-3: Student Class Definition (excerpt) 1. Public Class Student 2. Inherits Person 6. Public Sub New(ByVal aName As String, ByVal anAddress As String, ByVal anId As Integer, ByVal aMajor As String) 7. ' invoke superclass constructor to populate name & address attributes 8. MyBase.New(aName, anAddress)

Visual Basic.NET Programming: From Problem Analysis to Program Design9 Example 11-3: Student Class Definition (excerpt) 27. Public Overrides Function TellAboutSelf() As String 28. Dim info As String 29. info = MyBase.TellAboutSelf() & ", " & GetId() & ", " & GetMajor() 30. Return info 31. End Function

Visual Basic.NET Programming: From Problem Analysis to Program Design10 Instantiating an Inheritance Hierarchy When instantiating subclass –Create composite instance containing attributes and methods of both superclass and subclass –Client does not know about superclass

Visual Basic.NET Programming: From Problem Analysis to Program Design11 Understanding Private versus Protected Access Private accessibility –No other object may access attributes directly –Includes subclasses Protected accessibility –Provides access to subclasses

Visual Basic.NET Programming: From Problem Analysis to Program Design12 Example 11-5: Person with Protected Methods (excerpt) 5. Protected Sub New(ByVal aName As String, ByVal anAddress As String) 6. SetName(aName) 7. SetAddress(anAddress) 8. End Sub

Visual Basic.NET Programming: From Problem Analysis to Program Design13 Adding a Second Subclass Can add more than one subclass to superclass Superclass does not know subclasses exist

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Visual Basic.NET Programming: From Problem Analysis to Program Design15 Writing Abstract Classes and Methods Abstract class –Class that cannot be instantiated –Includes keyword MustInherit in header Concrete class –Class that can be instantiated

Visual Basic.NET Programming: From Problem Analysis to Program Design16 Writing Abstract Classes and Methods (continued) Abstract method –Method that consists only of signature –Has no code –Require all subclasses to implement method –Method signature contains keyword MustOverride

Visual Basic.NET Programming: From Problem Analysis to Program Design17 Example 11-9: Abstract Person Class with Abstract Method (excerpt) 1. Public MustInherit Class Person 23. Public MustOverride Function TellAboutSelf() As String

Visual Basic.NET Programming: From Problem Analysis to Program Design18 Defining Interfaces Adding abstract method to class –Requires that subclasses override method –Assures and enforces standardization –Another way to require that methods be included in classes is to define an interface Interface –VB.NET component –Defines abstract methods that must be implemented

Visual Basic.NET Programming: From Problem Analysis to Program Design19 Defining Interfaces (continued) Define interface containing abstract methods Require that classes implement methods in interface

Visual Basic.NET Programming: From Problem Analysis to Program Design20 Example 11-10: IPersonInterface Public Interface IPersonInterface Function TellAboutSelf() As String End Interface

Visual Basic.NET Programming: From Problem Analysis to Program Design21 Example 11-10: IPersonInterface (continued) Interface –Begins with header Public Interface –Ends with the key words End Interface –Naming convention: Name begins with capital letter “I” All functionality must be defined by class that implements interface

Visual Basic.NET Programming: From Problem Analysis to Program Design22 Example 11-10: IPersonInterface (continued) VB.NET class –Can inherit from only one superclass –Can also implement one or more interfaces

Visual Basic.NET Programming: From Problem Analysis to Program Design23 Example 11-11: Student and Professor Implementing IPersonInterface (excerpt) 1. Public Class Student 2. Inherits Person 3. Implements IPersonInterface

Visual Basic.NET Programming: From Problem Analysis to Program Design24 Example 11-11: Student and Professor Implementing IPersonInterface (continued) 12. Public Function TellAboutSelf() As String Implements IPersonInterface.TellAboutSelf 13. Dim info As String 14. info = GetName() & ", " & GetAddress() & ", " & GetId() & ", " & GetMajor() 15. Return info 16. End Function

Visual Basic.NET Programming: From Problem Analysis to Program Design25 Example 11-11: Student and Professor Implementing IPersonInterface (continued) 1. Public Class Professor 2. Inherits Person 3. Implements IPersonInterface

Visual Basic.NET Programming: From Problem Analysis to Program Design26 Example 11-11: Student and Professor Implementing IPersonInterface (continued) 12. Public Function TellAboutSelf() As String Implements IPersonInterface.TellAboutSelf 13. Dim info As String 14. info = GetName() & ", " & GetAddress() & ", " & GetRank() & ", " & GetDepartment() 15. Return info 16. End Function

Visual Basic.NET Programming: From Problem Analysis to Program Design27 Creating Association Links Two or more classes often have association relationships Implement association relationships by creating links between instances of classes

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Visual Basic.NET Programming: From Problem Analysis to Program Design29 Implementing One-to-One Association One-to-one association example –Each person has one phone –Each phone is owned by one person One-to-one association –Each class has attribute referencing other class

Visual Basic.NET Programming: From Problem Analysis to Program Design30 Example 11-12: Person Class Definition with Phone Association (excerpt) 1. Public MustInherit Class Person 2. Private name As String 3. Private address As String 4. Private phone As Phone

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Visual Basic.NET Programming: From Problem Analysis to Program Design32 Implementing One-to-Many Associations Association relationships include –One-to-one –One-to-many –Many-to-many Often implemented using: –Arrays –ArrayLists

Visual Basic.NET Programming: From Problem Analysis to Program Design33 Implementing One-to-Many Associations (continued) Implement association relationships by –Connecting instances of classes having association relationships Connections are accomplished by creating and populating attribute reference variables

Visual Basic.NET Programming: From Problem Analysis to Program Design34 Example 11-14: Person Class Definition with Multiple Phones (excerpt) 1. Public MustInherit Class Person 2. Private name As String 3. Private address As String 4. Private phones As ArrayList

Visual Basic.NET Programming: From Problem Analysis to Program Design35 Example 11-15: Creating Student and Professor with Multiple Phones (excerpt) 3. Module Client 4. Sub Main() 5. Dim student1 As Student, professor1 As Professor 6. Dim phoneInfo() As String, i As Integer

Visual Basic.NET Programming: From Problem Analysis to Program Design36 Programming Example: Student Enrolls In Course Demonstrates implementation of inheritance and many-to-many association involving subclass

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Visual Basic.NET Programming: From Problem Analysis to Program Design38 Summary Inheritance –Superclass specifies general characteristics of object –Subclass inherits characteristics from superclass and adds more specialized attributes and methods Writing definition for subclass is similar to writing any class

Visual Basic.NET Programming: From Problem Analysis to Program Design39 Summary (continued) Use keyword Overridable to tell VB.NET that overriding method is permitted Private accessibility –Can only be accessed within class Protected accessibility –Can be accessed within class and subclasses Abstract class –Class that you do not instantiate

Visual Basic.NET Programming: From Problem Analysis to Program Design40 Summary (continued) Abstract method –Method that consists only of signature Interface –Defines abstract methods that must be implemented Association relationships: –One-to-one –One-to-many –Many-to-many