A common way to provide polymorphism in VB is by means of Interfaces. An Interface in VB is a class module that we define as a template for the actual classes that will implement this interface. In VB an interface is just like any other class module in your project except that methods and functions have no code at all. In REALbasic an Interface is a special module that you must add to your project. In RB2006 you must select "Class Interface" from the "Add" menu under the "Project" menu.

Just like in VB you must add the empty methods to the Interface Class. A method of an Interface Class doesn't have any code.

Once we define our Interface is time to implement the Interface in some other class. In VB we use the statement "Implements". This statement is followed by the name of the class interface. In REALbasic a class module has a interface property where we type the name of the interfaces separated by comas that our class will implement.

In VB once we enter the line Implements Animal Visual Basic will automatically add the methods defined by the Interface to the entries in the Object drop down in the code window. REALbasic will not populate the interface methods in the class module buth it does have a nice feature. When you enter the name of an interface in the interface property of a class module since REALbasic knows that you are setting an interface it will prompt you if you want REALbasic to add the interface methods to the class that is implementing the interface. Since all methods declared on an Interface must be implemented by the class this feature comes quite handy since it avoids us forgetting to add an interface method to our class.

Now that our class module "dog" implements the interface "animal" we can add the necessary code to each of the methods implemented by "dog".

One big different that you may have noticed already is that in VB the methods of an Interface are implemented more like events on the class module. For example the Bite method of the interface "Animal" would be declared in VB as:

Private Sub Animal_Bite()

End Sub

In REALbasic the method is implemented with the same name as the name it has on the interface. One limitation that REALbasic has is that a class module can not implement two interfaces which have methods with the same name. The VB syntax works as a name space that avoids this problem. Notice that we can easily fix our problem by prefixing the name of the interface to the interface methods.

In VB you can define a property in the Interface. The VB syntax is the same as of a variable:

Public Age As Double

Sub Age(assigns value as integer)

End Sub

Function Age() As integer

End Function

We can do this because REALbasic allow us to overload a method with ease. The assigns keywords will tell REALbasic to treat AGE as a property. Now all we have to do is declare these two methods in the class module implementing our interface.

Lets see an example of how we can use our interface in REALbasic.

dim theAnimal as animal
dim aDog as new dog

theAnimal = aDog
theAnimal.bite

theAnimal.age = 10
MsgBox "my age is " + str(theAnimal.age)

To be continued...