Object-Oriented Programming

Object-Oriented Notes

Approach

The focus of attention in object-oriented programming is on objects rather than methods.

The nouns in a system requirements document help the VB programmer determine an initial set of classes with which to begin the design process.

These classes are then used to instantiate objects that will work together to implement the system.

Class Scope

A class's instance variables and methods belong to that class's scope.

Within a class's scope, class members are immediately accessible by all of that class's methods and can be referenced simply by name.

If a method defines a variable with the same name as a variable with class scope (i.e., an instance variable), the class-scope variable is hidden by the method-scope variable in the method scope.

A hidden instance variable can be accessed in the method by preceding its name with the keyword Me and the dot operator as in Me.var1.

Outside a class's scope, class members cannot be referenced directly by name.

Those class members (such as Public members) that are visible can only be accessed off a “handle.”

Primitive data type members can be referred to by objectReferenceName.primitive_VariableName

Object members can be referenced by objectReferenceName.objectMemberName.

example: t1.setTime( 27, 74, 99 )

Overloaded Constructors

A method's signature refers to the number, types and order of the arguments in the parameter list.

Methods of a class can be overloaded, but only by other methods of the same class.

To overload a method of a class, simply provide an additional method definition with the same name as the original method, but with a different parameter list.

The appropriate constructor is determined by checking the method signature specified in the constructor call with the signature specified in each method definition.

If a redefinition of a method has a different signature than the original method, it is method overloading .

Constructors can be overloaded to initialize one, some, or all of an object's instance variables.

The next example overloads the New constructor method to provide a convenient variety of ways to initialize clsTime objects.

' No-argument constructor initializes each instance
' variable to zero. Ensures that clsTime object starts in
' a consistent state.
Public Sub New( )
     setTime( 0, 0, 0 )
End Sub

' One-argument constructor: hour supplied, minute
' and second default to 0.
Public Sub New( ByVal h As Integer )
     setTime( h, 0, 0 )
End Sub

' Two-argument constructor: hour and minute supplied, second
' defaults to 0.
Public Sub New( ByVal h As Integer, ByVal m As Integer )
     setTime( h, m, 0 )
End Sub

' Three-argument constructor: hour, minute and second supplied.
Public Sub New( ByVal h As Integer, ByVal m As Integer, ByVal s As Integer )
     setTime( h, m, s )
End Sub

Dim t1 As clsTime = New clsTime ( )
Dim t2 As clsTime = New clsTime ( 2 )

Dim t3 As clsTime = New clsTime ( 21, 34 )

Dim t4 As clsTime = New clsTime ( 12, 25, 42 )

Dim t5 As clsTime = New clsTime ( 27, 74, 99 )

The appropriate constructor is determined by checking the number, types and order of the arguments specified in the constructor call with the number, types and order of the parameters specified in each method definition.

The matching constructor is called automatically.