Interfaces

Introduction We described abstract classes as those intended for inheritance. Another type of class designed for this purpose is referred to as an interface. An interface is a class that creates a foundation that new derived classes can use. As done for an abstract class, you can create original behavior that the deriving classes would use.  Unlike abstract classes, you don't implement the members (properties and procedures) of the interface. The general idea of an interface is only to lay a foundation, as creating a structural base, that the new classes would follow, although they can customize the behavior(s) of the parent interface but the parent interface does not "decide" what the behavior of a member would be.  Interface Creation To create an interface, you start with the Interface keyword followed by the name of the interface. You end the interface definition with an End Interface line. By tradition or good habit, the name of an interface starts with I. Here is an example of a starting interface: File: RegularTriangle.vb Public Interface ITriangle End Interface After creating an interface, you can derive a class from it. When deriving from an interface, instead of the Inherits keyword, you use Implements followed by the name of the interface. Here is an example of a new class named Regular and that is based on the above ITriangle interface: File: RegularTriangle.vb Public Interface ITriangle End Interface Public Class RegularTriangle Implements ITriangle End Class As with other classes, once you have derived the class, you can create objects from it and instantiate it using the New operator. Here is an example: File: Exercise.vb Module Exercise Public Function Main() As Integer Dim reg As RegularTriangle reg = New RegularTriangle Return 0 End Function End Module   As mentioned earlier, the purpose of having an interface is to create a skeleton that derived classes would follow. To do this, in the body of the interface, you can create the necessary members that you want to make available to new classes. The members can be the same types of methods or properties as those we have used in classes so far. Here are examples: File: RegularTriangle.vb Public Interface ITriangle ReadOnly Property Name() As String Property Base() As Double Property Height() As Double Function CalculatePerimeter() As Double Function CalculateArea() As Double End Interface The primary rule you must observe when deriving a class from an interface is that you must implement each member of the interface in the derived class. If you omit implementing a member of the parent interface, you would receive an error. When implementing a member of the interface, it must be followed by the Implements keyword, the name of the interface, the period operator, and the name of the member that it is implementing. Based on this, our Regular class can implement the ITriangle interface as follows: File: RegularTriangle.vb Public Interface ITriangle ReadOnly Property Name() As String Property Base() As Double Property Height() As Double Function CalculatePerimeter() As Double Function CalculateArea() As Double End Interface Public Class RegularTriangle Implements ITriangle Public bas As Double Public hgt As Double Public sd1 As Double Public sd2 As Double ' Default constructor: the user will specify the dimensions Public Sub New() bas = 0 hgt = 0 sd1 = 0 sd2 = 0 End Sub ' A triangle based on known base and height Public Sub New(ByVal b As Double, ByVal h As Double) bas = b hgt = h End Sub ' A triangle based on the measurements of the sides Public Sub New(ByVal b As Double, ByVal side1 As Double, ByVal side2 As Double) bas = b sd1 = side1 sd2 = side2 End Sub ' A triangle whose all sides and the height are known Public Sub New(ByVal b As Double, ByVal h As Double, _ ByVal side1 As Double, ByVal side2 As Double) bas = b hgt = h sd1 = side1 sd2 = side2 End Sub Public Property Base() As Double Implements ITriangle.Base Get Return bas End Get Set(ByVal Value As Double) If bas < 0 Then bas = 0 Else bas = Value End If End Set End Property Public Function CalculateArea() As Double Implements ITriangle.CalculateArea Return bas * hgt / 2 End Function Public Function CalculatePerimeter() As Double Implements _ ITriangle.CalculatePerimeter Return bas + sd1 + sd2 End Function Public Property Height() As Double Implements ITriangle.Height Get Return hgt End Get Set(ByVal Value As Double) If hgt < 0 Then hgt = 0 Else hgt = Value End If End Set End Property Public ReadOnly Property Name() As String Implements ITriangle.Name Get Return "Regular Triangle" End Get End Property End Class Once the class has been defined like this, you can then instantiate and use it. Here is an example: File: Exercise.vb Module Exercise Public Function Main() As Integer Dim reg As RegularTriangle = New RegularTriangle(35.28, 26.44) Response.Write("Triangle Type: " & reg.Name) Response.Write("=-= Characteristics =-=") Response.Write("Base: " & reg.Base) Response.Write("Height: " & reg.Height) Response.Write("Area: " & reg.CalculateArea) Return 0 End Function End Module This would produce: Triangle Type: Regular Triangle =-= Characteristics =-= Base: 35.28 Height: 26.44 Area: 466.4016 In the same way, you can derive other classes from an interface. For example, from our ITriangle class, you can derive an isosceles, a right, or an equilateral triangle. Always remember that when you derive a class, you must implement all of the members of the interface. You can also add new members as you see fit.

 

An Interface Derived Like a regular class, an interface can be derived from another interface but an interface cannot derive from a class. To create an interface based on another, use the Inherits keyword as we have used in other classes. Here is an example: File: RegularTriangle.vb Public Interface IGeometricShape End Interface Public Interface ITriangle Inherits IGeometricShape End Interface As mentioned for the interfaces, you can use the parent interface to list the members that the deriving classes would implement. Still remember that since an interface cannot implement a member, the member of the parent interface cannot be defined in a derived interface. This implement would wait for the actual class(es) that would be based on the child (or even the parent) interface. Here is an example: File: RegularTriangle.vb Public Interface IGeometricShape ReadOnly Property Type() As String End Interface Public Interface ITriangle Inherits IGeometricShape ReadOnly Property Name() As String Property Base() As Double Property Height() As Double Function CalculatePerimeter() As Double Function CalculateArea() As Double End Interface After deriving a class from an interface, when defining the class, you must implement the member of the immediate interface and those of the ancestor interface(s). Here is an example: File: RegularTriangle.vb Public Interface IGeometricShape ReadOnly Property Type() As String End Interface Public Interface ITriangle Inherits IGeometricShape ReadOnly Property Name() As String Property Base() As Double Property Height() As Double Function CalculatePerimeter() As Double Function CalculateArea() As Double End Interface Public Class RegularTriangle Implements ITriangle Public bas As Double Public hgt As Double Public sd1 As Double Public sd2 As Double Public ReadOnly Property Type() As String Implements ITriangle.type Get Return "Triangle" End Get End Property ' Default constructor: the user will specify the dimensions Public Sub New() bas = 0 hgt = 0 sd1 = 0 sd2 = 0 End Sub ' A triangle based on known base and height Public Sub New(ByVal b As Double, ByVal h As Double) bas = b hgt = h End Sub ' A triangle based on the measurements of the sides Public Sub New(ByVal b As Double, ByVal side1 As Double, ByVal side2 As Double) bas = b sd1 = side1 sd2 = side2 End Sub ' A triangle whose all sides and the height are known Public Sub New(ByVal b As Double, ByVal h As Double, _ ByVal side1 As Double, ByVal side2 As Double) bas = b hgt = h sd1 = side1 sd2 = side2 End Sub Public Property Base() As Double Implements ITriangle.Base Get Return bas End Get Set(ByVal Value As Double) If bas < 0 Then bas = 0 Else bas = Value End If End Set End Property Public Function CalculateArea() As Double Implements ITriangle.CalculateArea Return bas * hgt / 2 End Function Public Function CalculatePerimeter() As Double Implements _ ITriangle.CalculatePerimeter Return bas + sd1 + sd2 End Function Public Property Height() As Double Implements ITriangle.Height Get Return hgt End Get Set(ByVal Value As Double) If hgt < 0 Then hgt = 0 Else hgt = Value End If End Set End Property Public ReadOnly Property Name() As String Implements ITriangle.Name Get Return "Regular" End Get End Property End Class Here is an example of testing the class: File: Exercise.vb Module Exercise Public Function Main() As Integer Dim reg As RegularTriangle = New RegularTriangle(35.28, 26.44) Response.Write("Shape Type: " & reg.Type) Response.Write("Triangle Type: " & reg.Name) Response.Write("=-= Characteristics =-=") Response.Write("Base: " & reg.Base) Response.Write("Height: " & reg.Height) Response.Write("Area: " & reg.CalculateArea) Return 0 End Function End Module This would produce: Shape Type: Triangle Triangle Type: Regular =-= Characteristics =-= Base: 35.28 Height: 26.44 Area: 466.4016   Multiple Inheritance Multiple inheritance consists of creating a class that is based on more than one parent. In the Microsoft Visual Basic language (in fact in the .NET Framework), you cannot derive a class from more than one class. This functionality is available only with interfaces. To create a class based on more than one interface, after the Implements keyword, enter the name of each interface and separate them with commas. Here is an example: Public Interface IGeometricShape ReadOnly Property Type() As String End Interface Public Interface ICalculation End Interface Public Interface ITriangle Inherits IGeometricShape, ICalculation End Interface The same rules apply for multiple inheritance: you must implements all members of each parent interface. Besides deriving from an interface, you can also create a class that is based on a class and one or more interfaces. To do this, under the line that specifies the name of the class, use the Inherits keyword to specify the name of the parent, press Enter, and use the Implements keyword to specify the name of the class that serves as the parent interface. Here is an example: Public Interface IGeometricShape ReadOnly Property Type() As String End Interface Public Interface ICalculation End Interface Public Class Geometry End Class Public Interface ITriangle Inherits IGeometricShape, ICalculation End Interface Public Class RegularTriangle Inherits Geometry Implements ITriangle End Class In the same way, you can create a class that is based on more than one interface but it can be based on only one class.