Implementing the Methods of a Class
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Implementing the Methods of a Class |
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Details on Implementing Methods |
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The Case of Function Methods |
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So far, we have seen how to create sub procedure in a class, which is a method that does not return a value. Like a normal procedure, a method can be made to return a value, in which case it would be a function. To create a method as a function, use the same techniques we have used so far to create a function. Here is an example: |
Friend Class Rectangle
Public Length As Double
Public Height As Double
Function Assign() As Double
End Function
End Class
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After declaring a procedure, in its body, you can implement the expected behavior. As seen for a sub procedure, when a method has been created, it has access to all of the other members of the same class. This means that you don't have to re-declare a member of a class to access it in a method. Based on this, you can manipulate any member variable of the same class as you wish. This means that, naturally, you do not have to create a function method to change the value of a member variable. A normal sub procedure can take care of this. Instead, you would create a method if you need to perform a calculation and get a value from it instead of storing that value in one of the member variables. Here is an example: |
Friend Class Rectangle
Public Length As Double
Public Height As Double
Function Assign() As Double
End Function
Function Perimeter() As Double
End Function
End Class
Therefore, in the body of the function, you can access a member variable, you can call another method of the same class, or you can use an external value as you see fit. When the function exits, make sure it returns the appropriate value based on its type. Like the member variables, the methods can be accessed outside of the class using the period operator. Here is an example: Public Module Exercise
Friend Class Rectangle
Public Length As Double
Public Height As Double
Function Perimeter() As Double
Return (Length + Height) * 2
End Function
Function Area#()
Return Length * Height
End Function
End Class
Public Function Main() As Integer
Dim Recto As Rectangle
Recto = New Rectangle
Recto.Length = 42.58 : Recto.Height = 28.08
MsgBox("=-= Rectangle Characteristics =-=" & vbCrLf & _
"Length: " & vbTab & vbTab & Recto.Length & vbCrLf & _
"Height: " & vbTab & vbTab & Recto.Height & vbCrLf & _
"Perimeter: " & vbTab & Recto.Perimeter() & vbCrLf & _
"Area: " & vbTab & vbTab & Recto.Area())
Return 0
End Function
End Module
This would produce:
Like a sub procedure that is created as a method, a member function of a class can be made private, public, or friendly. It follows the exact same rules we reviewed early.
Like regular procedures we have used so far, methods can have arguments. The rules are the same as we have applied them so far. When you create a method, it has direct access to all regular members of its class. This means that you don't have to create an argument for a member variable of the same class. You would need an argument only if an external value would be passed to the method.
Just as done for regular procedures, a method of a class can be overloaded. To overload a method, create more than one method with the same name. The methods with same names must have different rules in their arguments:
Any or both of these two rules must be respected.
When you declare a variable of a class, you are said to have created an instance of the class. This makes an object available so you can initialize and use its member variables and methods as you see fit. Just as you can declare one instance of a class, in the same way, you can declare as many instances as you judge it necessary. After declaring various variables of the same class and initializing them, each variable has and keeps its own values. Here is an example: Public Module Exercise
Friend Class Rectangle
Public Length As Double
Public Height As Double
Public Function CalculatePerimeter#()
Return (Length + Height) * 2
End Function
Public Function CalcaulteArea#()
Return Length * Height
End Function
End Class
Public Function Main() As Integer
Dim RegistrationCard As New Rectangle
Dim Invoice As New Rectangle
RegistrationCard.Length = 24.55 : RegistrationCard.Height = 20.68
Invoice.Length = 8.5 : Invoice.Height = 11.65
MsgBox("==========================" & vbCrLf & _
"=-= Paper Registration Characteristics =-=" & vbCrLf & _
"Length: " & vbTab & vbTab & RegistrationCard.Length & vbCrLf & _
"Height: " & vbTab & vbTab & RegistrationCard.Height & vbCrLf & _
"Perimeter: " & vbTab & _
RegistrationCard.CalculatePerimeter#() & vbCrLf & _
"Area: " & vbTab & vbTab & _
RegistrationCard.CalcaulteArea#() & vbCrLf & _
"----------------------------------------------------" & vbCrLf & _
"=-= Paper Registration Characteristics =-=" & vbCrLf & _
"Length: " & vbTab & vbTab & Invoice.Length & vbCrLf & _
"Height: " & vbTab & vbTab & Invoice.Height & vbCrLf & _
"Perimeter: " & vbTab & Invoice.CalculatePerimeter#() & vbCrLf & _
"Area: " & vbTab & vbTab & Invoice.CalcaulteArea#() & vbCrLf & _
"==========================")
Return 0
End Function
End Module
This would produce:
In order to access the member variables of the above Rectangle class, you must declare a variable of the class. The Visual Basic language provides an alternative. You can declare a member variable so that you can access it from anywhere in your code without primarily declaring a variable of the class. In order to have such a member variable, you must explicitly create it as shared. To create a shared member variable, type the Shared keyword on its left when declaring it. After the shared member variable has been declared, you can use it like any other member variable of that class, except that you don't need to declare an instance of that class when you want to access that member variable. Here is an example: Public Module Exercise
Friend Class Rectangle
Public Length As Double
Public Shared Height As Double
Public Function CalculatePerimeter#()
Return (Length + Height) * 2
End Function
Public Function CalcaulteArea#()
Return Length * Height
End Function
End Class
Public Function Main() As Integer
REM Notice that a Rectangle variable is not declared
Rectangle.Height = 20.68
MsgBox("Rectangle Characteristics" & vbCrLf & _
"Height:" & vbTab & vbTab & Rectangle.Height)
Return 0
End Function
End Module
This would produce:
Based on this, when creating a class, you will decide whether you want a particular member variable to be shared or not. You can have only one, two, more, or all member variables shared in a class. Experience and your own goal will guide you.
Consider the following class: Public Module Exercise
Friend Class Rectangle
Public Shared Length As Double
Public Shared Height As Double
Public Function CalculatePerimeter#()
Return (Length + Height) * 2
End Function
Public Function CalcaulteArea#()
Return Length * Height
End Function
End Class
Public Function Main() As Integer
REM Notice that a Rectangle variable is not declared
Rectangle.Height = 20.68
Rectangle.Length = 32.47
MsgBox("Rectangle Characteristics" & vbCrLf & _
"Length:" & vbTab & Rectangle.Length & vbCrLf & _
"Height:" & vbTab & Rectangle.Height)
Return 0
End Function
End Module
This would produce:
Like member variables, a method can be shared among classes. In some cases, shared methods are more used than shared member variables because a shared method allows performing an action on a class without declaring an instance of that class. To create a shared method, type the Shared keyword on the left of the Sub or the Function keyword. Here is an example: Here is an example: Friend Class Rectangle
Shared Function CalculatePerimeter#()
Return (Length + Height) * 2
End Function
End Class
You can apply the access modifier on the method as we have done so far. Here are examples: Friend Class Rectangle
Public Shared Function CalculatePerimeter#()
End Function
Public Shared Function CalcaulteArea#()
End Function
End Class
Like a shared member variable, once a method has been created as shared, it can be accessed directly from anywhere. Remember that you would need to type the name of the class before accessing the method. The name of the class allows you to "qualify" the method. Here is an example: Public Module Exercise
Friend Class Rectangle
Public Shared Length As Double
Public Shared Height As Double
Public Shared Function CalculatePerimeter#()
Return (Length + Height) * 2
End Function
Public Shared Function CalcaulteArea#()
Return Length * Height
End Function
End Class
Public Function Main() As Integer
REM Notice that a Rectangle variable is not declared
Rectangle.Height = 20.68
Rectangle.Length = 32.47
MsgBox("Rectangle Characteristics" & vbCrLf & _
"Length:" & vbTab & vbTab & Rectangle.Length & vbCrLf & _
"Height:" & vbTab & vbTab & Rectangle.Height & vbCrLf & _
"Perimeter: " & vbTab & Rectangle.CalculatePerimeter#() & vbCrLf & _
"Area: " & vbTab & vbTab & Rectangle.CalcaulteArea#())
Return 0
End Function
End Module
We have mentioned two techniques of accessing the members of a class, one consisted of declaring a variable of the class, the other had to do with Shared members. We know already that the members of a class are made available to all other members of the same class without being declared or qualified. Consider the following class: Public Class Triangle
Public Base As Double
Public Height As Double
Public Area As Double
Public Sub Display()
Dim Area As Double
Area = Base * Height / 2
End Sub
End Class
When the Area variable is used in the Display() method, there are two variables available and named Area. It makes it confusing to know what particular variable is being accessed. You can use a special member of a class that allows you to specify the member of a class when accessing it. This member is called Me. When using Me, you can access any member of a class within any method of the same class. Here is an example: Public Module Exercise
Public Class Triangle
Public Base As Double
Public Height As Double
Public Area As Double
Public Sub Display()
Dim Area As Double
' Using "this" to access the members of this class
Me.Base = 24.55
Me.Height = 20.75
' You cannot use this to access Area because Area
' is not a member of this class
Area = Me.Base * Me.Height / 2
MsgBox("Triangle Characteristics" & vbCrLf & _
"Base:" & vbTab & Me.Base & vbCrLf & _
"Height:" & vbTab & Me.Height & vbCrLf & _
"Area: " & vbTab & Area) ' Area is not a member of the Exercise class
End Sub
End Class
Public Function Main() As Integer
Dim tri As Triangle = New Triangle
tri.Display()
Return 0
End Function
End Module
This would produce:
There are rules you must follow when using Me:
In Lesson 2, we saw that when you declare a variable, the compiler initializes its allocated memory with a default value. Consider the following program: Public Module Exercise
Private Class Square
Private Side As Double
Function CalculatePerimeter() As Double
Return Side * 4
End Function
Function CalculateArea() As Double
Return Side * Side
End Function
Public Sub ShowMessage()
MsgBox("Square Characteristics" & vbCrLf & _
"Side:" & vbTab & vbTab & FormatNumber(Side) & vbCrLf & _
"Perimeter:" & vbTab & _
FormatNumber(CalculatePerimeter()) & vbCrLf & _
"Area:" & vbTab & vbTab & FormatNumber(CalculateArea()))
End Sub
End Class
Public Function Main() As Integer
Dim sqr As Square = New Square
sqr.ShowMessage()
Return 0
End Function
End Module
This would produce:
Notice that this program indicates that the Side member variable of the Square class was initialized with 0. This means that, like the regular variables, the member variables of a class are initialized by the compiler with default values that depend on the type of the variable. For example, a numeric member variable is initialized with 0 while a string-based variable is initialized with an empty string. After adding a member variable to a class, instead of relying on the default value assigned by the compiler, you can initialized it with a value of your choice, depending on the type of the variable. You have various alternatives.
One way you can initialize a member variable is to assign it the desired value when declaring it. Here is an example: Public Module Exercise
Private Class Square
Private Side As Double = 48.25
Function CalculatePerimeter() As Double
Return Side * 4
End Function
Function CalculateArea() As Double
Return Side * Side
End Function
Public Sub Message()
MsgBox("Square Characteristics" & vbCrLf & _
"Side:" & vbTab & vbTab & FormatNumber(Side) & vbCrLf & _
"Perimeter:" & vbTab & _
FormatNumber(CalculatePerimeter()) & vbCrLf & _
"Area:" & vbTab & vbTab & FormatNumber(CalculateArea()))
End Sub
End Class
Public Function Main() As Integer
Dim sqr As Square = New Square
sqr.Message()
Return 0
End Function
End Module
This would produce:
Notice that, this time, the default value assigned to the member variable applies. Instead of initializing a member variable when declaring it, you can create a method that would be used to do this. Here is an example: Public Class Square
Private side As Double
Public Sub SetSide()
side = 48.25
End Sub
End Class
Such a method can also be used to initialize more than one value. When you create a method used to initialize one or more member variables of a class, if you want the initialization to apply, you must make sure that you call that method first before calling other methods of the class. Just as you can create a method to initialize the member(s) of a class, you can overload that method with different versions to perform different initializations. Here are examples: Public Module Exercise
Private Class Square
Private side As Double
Public Sub SetSide()
side = 48.25
End Sub
Public Sub SetSide(ByVal sd As Double)
side = sd
End Sub
Function CalculatePerimeter() As Double
Return Side * 4
End Function
Function CalculateArea() As Double
Return Side * Side
End Function
Public Sub Message()
MsgBox("Square Characteristics" & vbCrLf & _
"Side:" & vbTab & vbTab & FormatNumber(Side) & vbCrLf & _
"Perimeter:" & vbTab & _
FormatNumber(CalculatePerimeter()) & vbCrLf & _
"Area:" & vbTab & vbTab & FormatNumber(CalculateArea()))
End Sub
End Class
Public Function Main() As Integer
Dim sqr As Square = New Square
sqr.SetSide()
sqr.Message()
Dim sd As Double
sd = CDbl(InputBox("Enter Square Side: "))
sqr.SetSide(sd)
sqr.Message()
Return 0
End Function
End Module
Here an example of running the program:
Notice that, although the Square.Side member variable is private, you cab call the SetSide() public method to initialize it before displaying the characteristics of a square.
When you declare a variable of a class, a special method must be called to initialize the members of that class. This method is automatically provided for every class and it is called a constructor. Whenever you create a new class, a constructor is automatically provided to it. This particular constructor is called the default constructor. You have the option of creating it or not. Although a constructor is created for your class, you can customize its behavior or change it as you see fit. The constructor of a class is called New and it is created as a sub procedure. Here is an example: Public Class Square
Public Sub New()
End Sub
End Class
Like every method, a constructor is equipped with a body. In this body, you can access any of the member variables (or method(s)) of the same class. Consider the following program: Public Module Exercise
Private Class Square
Public Sub New()
MsgBox("Square Builder")
End Sub
End Class
Public Function Main() As Integer
Dim sq As Square = New Square
Return 0
End Function
End Module
When executed, it would produce:
This shows that, when a class has been instantiated, its constructor is the first method to be called. For this reason, you can use a constructor to initialize a class, that is, to assign default values to its member variables. Based on this, instead of initializing the member variable(s) of a class when initializing it or them, or instead of creating a special method used to initialize the member variable(s) of a class, you can use a constructor to do this. The advantage of a constructor is that it doesn't need to be called: it is automatically available whenever the class is instantiated.
In the previous section, we saw that there was always a default constructor for a new class that you create; you just havce the option of explicitly creating one or not. The default constructor as we saw it doesn't take arguments: this is not a rule, it is simply assumed. Instead of a default constructor, you may want to create a constructor that takes an argument. Here is an example: Private Class Square
Public Sub New(ByVal sd As Double)
End Sub
End Class
With this type of constructor, when you declare an instance of the class, you can use this new constructor to initialize the class. Here is an example: Public Module Exercise
Private Class Square
Public Sub New(ByVal sd As Double)
MsgBox("Square Builder")
End Sub
End Class
Public Function Main() As Integer
Dim sqr As Square = New Square(38.64)
Return 0
End Function
End Module
If you create one constructor for your class and pass at least one argument to that constructor, the automatic default constructor created by the compiler disappears. This implies that if you declare an instance of the class and use the default constructor to initialize it, you would receive an error when you compile the program. Based on this, the following program will produce an error: Public Module Exercise
Private Class Square
Public Sub New(ByVal sd As Double)
MsgBox("Square Builder")
End Sub
End Class
Public Function Main() As Integer
Dim sqr As Square = New Square ' The default constructor is not available
Return 0
End Function
End Module
If you still want to use the default constructor in a class after creating a constructor that takes at least one argument, you must explicitly create that default constructor.
A constructor is the primary method of a class. It allows the programmer to initialize a variable of a class when the class is instantiated. A constructor that plays this role of initializing an instance of a class is also called an instance constructor. Most of the time, you don't need to create a constructor, since one is automatically provided to any class you create. Sometimes though, as we have seen in some classes, you need to create your own constructor as you judge it necessary and sometimes, a single constructor may not be sufficient. For example, when creating a class, you may decide, or find out, that there must be more than one way for a user to initialize a variable. Like any other method, a constructor can be overloaded. In other words, you can create a class and give it more than one constructor. The same rules used on overloading regular methods also apply to constructors: the different constructors must have different number of arguments or different types of arguments.
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Practical
Learning: Implementing the Methods of a Class
