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Adding Chapter Responsibilities to Problem Domain Classes
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Objectives
In this chapter, you will:
Write a new problem domain class definition
Create custom methods Write class variables and methods
Write overloaded methods Work with exceptions
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Writing a New Problem Domain Class Definition
Slip class
Attributes
slipId slipWidth slipLength
Custom method
LeaseSlip
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Writing a New Problem Domain Class Definition
Class definition of Slip
Class header
Public Class Slip
Attribute definition statements
Private slipId As Integer Private slipWidth As Integer Private slipLength As Integer
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Writing a New Problem Domain Class Definition
Class definition of Slip (Continued)
Parameterized constructor
Enables attributes to be automatically populated whenever an instance is created
Public Sub New(ByVal aSlipId As Integer, _ ByVal aSlipWidth As Integer, ByVal aSlipLength As Integer)
Argument data types must be assignment compatible with parameter data types
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Writing a New Problem Domain Class Definition
Class definition of Slip (Continued)
Statements to populate the Slip attributes
To avoid redundant code, setter methods are invoked instead of directly assigning the values
'invoke setter methods to populate attributes SetSlipId(aSlipId) SetSlipWidth(aSlipWidth) SetSlipLength(aSlipLength)
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Writing a New Problem Domain Class Definition
Class definition of Slip (Continued)
TellAboutSelf method
Invokes the three getters Concatenates the returned values Returns the result
Public Function TellAboutSelf() As String Dim info As String info = "Slip: Id = " & GetSlipId() & ", Width = " & GetSlipWidth() _
& ", Length = " & GetSlipLength() Return info End Function
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Writing a New Problem Domain Class Definition
Class definition of Slip (Continued)
TellAboutSelf method
Polymorphic methods
Two methods with the same name, residing in different classes, that behave differently Example: TellAboutSelf methods for Customer and Slip classes
Setter and getter methods
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Creating Custom Methods
Custom methods
Process data
Accessor methods
Store and retrieve attribute values
LeaseSlip method
A custom method Computes the lease fee for a slip
Fees range from $800 to $1,500 depending on the slip's width
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Creating Custom Methods
LeaseSlip method
A Function procedure
Will return a value: the lease fee amount
Has Public accessibility
Can be invoked by any object
Return data type: Single
Will compute and return the lease fee, a dollar amount
Method header:
Public Function LeaseSlip() As Single
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Creating Custom Methods
Select statement for computing lease fee:
Dim fee As Single Select Case slipWidth Case 10 fee = 800 Case 12 fee = 900 Case 14 fee = 1100 Case 16 fee = 1500 Case Else fee = 0 End Select Return fee
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Writing Class Variables and Methods
Instance variables and methods
Each instance has a copy
Class variables and methods
Shared by all instances of the class Each instance does not have its own copy Declared using the keyword Shared
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Writing Class Variables and Methods
Variable numberOfSlips can be used to track the total number of slips numberOfSlips variable
Initialized to 0 Private accessibility
Will be accessed only by methods within the Slip class Private Shared numberOfSlips As Integer = 0
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Writing Class Variables and Methods
Code added to the Slip constructor for the numberOfSlips variable
'increment shared attribute numberOfSlips += 1
Increments numberOfSlips each time a slip instance is created
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Writing Class Variables and Methods
GetNumberOfSlips() method Returns the contents of numberOfSlips Written as a Function procedure
Returns a value
Written as a shared method
Not associated with any individual instance
Has Public accessibility
Will be invoked by other classes
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GetNumberOfSlips() method
'shared (class) method Public Shared Function GetNumberOfSlips() As Integer Return numberOfSlips End Function
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Writing Overloaded Methods
Method signature: Name and parameter list A class definition can contain several methods with the same name, as long as their parameter lists differ
VB .NET identifies a method by its signature, not only by its name
Overloaded method: a method that has the same name as another method in the same class, but a different parameter list
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Overloading a Constructor
Problem
Most slips at Bradshaw Marina are 12 feet wide and 25 feet long, but a few have different widths and lengths Current Slip constructor requires three arguments: slipId, slipWidth, and slipLength
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Overloading a Constructor
Solution
Write a second Slip constructor that has only a single parameter for slipId Include statements to assign the default values of 12 and 25 to slipWidth and slipLength, respectively
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Overloading a Constructor
Constants can be used for the default width and length values:
'constants Private Const DEFAULT_SLIP_WIDTH As Integer = 12 Private Const DEFAULT_SLIP_LENGTH As Integer = 25
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Overloading a Constructor
Code for second constructor:
'1-parameter constructor 'Overloads keyword not used with constructors Public Sub New(ByVal aSlipId As Integer) 'invoke 3-parameter constructor, passing default values Me.New(aSlipId, DEFAULT_SLIP_WIDTH, DEFAULT_SLIP_LENGTH) End Sub
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Overloading a Constructor
VB .NET determines which constructor to invoke by the argument list
If the argument consists of three values
Original constructor with three parameters is executed
If the argument consists of a single value
New constructor with one parameter is invoked
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Overloading Custom a Method
Problem Bradshaw Marina permits a discounted lease fee under certain conditions Solution A second version of the LeaseSlip method that accepts a value for the percentage discount
Will overload the original LeaseSlip method
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Overloading a Custom Method
Two LeaseSlip methods in Slip, each with a different signature Original LeaseSlip has an empty parameter list Second LeaseSlip has a parameter variable named aDiscountPercent
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Overloading a Custom Method
Header for new method
Must contain the keyword Overloads
Public Overloads Function LeaseSlip(ByVal aDiscountPercent As Single) _ As Single
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Overloading a Custom Method
New method:
'overloaded custom method LeaseSlip if discount requested Public Overloads Function LeaseSlip(ByVal aDiscountPercent As Single) _ As Single 'invoke LeaseSlip() to get fee Dim fee As Single = Me.LeaseSlip() 'calculate and return discount fee Dim discountFee As Single = fee * (100 - aDiscountPercent) / 100 Return discountFee End Function
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Overloading a Custom Method
VB .NET will execute the appropriate method depending on the argument list
If no argument is passed
Original LeaseSlip method is invoked
If an argument is coded
Overridden method is executed
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Working with Exceptions
An exception
Used to notify the programmer of errors, problems, and other unusual conditions that may occur while the system is running An instance of the Exception class or one of its subclasses
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Working with Exceptions
VB .NET uses five keywords to deal with exceptions
Try
Used by the client
Catch
Used by the client
Finally
Used by the client
End Try
Used by the client
Throw
Used by the server
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Working with Exceptions
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Data Validation for slipId
slipId value should be within the range of 1 through 50 SetSlipId method
Data validation logic used to verify that slipId values are in the range of 1 through 50 Create and throw an exception instance if a value is outside the valid range
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Data Validation for slipId
Constant MAXIMUM_NUMBER_OF_SLIPS
Simplifies future maintenance: should Bradshaw Marina decide to have docks with more than 50 slips
Private Const MAXIMUM_NUMBER_OF_SLIPS As Integer = 50
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Data Validation for slipId
If statement
Determines if aSlipId is within valid range If a value is outside acceptable range
An instance of Exception class is created and thrown
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Data Validation for slipId
Public Sub SetSlipId(ByVal aSlipId As Integer) 'reject slipId if < 0 or > maximum If aSlipId < 1 Or aSlipId > MAXIMUM_NUMBER_OF_SLIPS Then Throw New Exception("Slip Id not between 1 and " _ & MAXIMUM_NUMBER_OF_SLIPS) Else slipId = aSlipId End If End Sub
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Data Validation for slipWidth
Code must be added to the SetSlipWidth method to verify that the width parameter is a valid width value: 10, 12, 14, or 16 VALID_SLIP_WIDTHS
An integer array Stores valid width values
Private Shared ReadOnly VALID_SLIP_WIDTHS As Integer() = {10, 12, 14, 16}
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Data Validation for slipWidth
Validation logic
Will iterate the array, seeking a match between the parameter value received (aSlipWidth) and an array value If a matching value is found
Parameter is valid
If the end of the array is reached without finding a matching value
Parameter contains an invalid value An exception is created and thrown
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Data Validation for slipWidth
Validation logic:
Dim validWidth As Boolean = False 'search for a valid width Dim i As Integer For i = 0 To VALID_SLIP_WIDTHS.Length - 1 If aSlipWidth = VALID_SLIP_WIDTHS(i) Then validWidth = True Next i
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Data Validation for slipWidth
If statement to test validWidth
If true
Width attribute is populated with parameter value
If not true
An instance of Exception is created and thrown 'if a valid width found, set value If validWidth Then slipWidth = aSlipWidth Else 'else throw exception Throw New Exception("Invalid Slip Width") End If
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Catching Exceptions
If a method that might throw an exception is invoked, the invoking code must be prepared to catch the exception
.NET CLR will terminate processing if an exception is thrown and not caught
Try block structure
Keyword Try Code containing invoking statement or statements Keyword End Try
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Catching Exceptions
Catch block
Receives and deals with an exception Must specify a parameter variable to receive a reference to the exception instance
Dim aSlip As Slip Try 'force an exception with invalid slipID (150) aSlip = New Slip(150, 10, 25) Console.WriteLine(aSlip.TellAboutSelf()) Catch theException As Exception Console.WriteLine("An exception was caught: " & _ theException.ToString())
End Try
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Catching Exceptions
Finally block
Used to add statements that are to be executed whether or not an exception was caught
Finally Console.WriteLine("Finally block always executes")
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Summary
Custom methods: methods which process data Class variables and methods are associated with the class instead of individual instances A new instance is given a copy of all instance variables and access to instance methods A new instance does not get a copy of class variables and methods A method signature consists of the method name and its parameter list VB .NET identifies a method by its signature
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Summary
An overloaded method has the same name as another method in the same class, but a different signature Exceptions notify the programmer of errors, problems, and other unusual conditions that may occur while the system is running Keywords dealing with exceptions: Try, Catch, Finally, End Try, and Throw CLR terminates processing if an exception is thrown and not caught
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