Consider the following problem (which I state with a simple example,
but it corresponds to a real problem that I'm facing): Junior loves
pets, and that's fine with Mom and Dad, as long as he only has no more
than one of any given species.
Now, how can I use a Singleton to restrict Junior from having too many
pets?
+----------------+
+ ----->| << abstract >> |<-----+
| | Animal | |
| +----------------+ |
| | | |
| +----------------+ |
| | | |
| | Speak() | |
| | \ | |
| +-----\----------+ |
| \ |
| <<abstract>> |
+-----+ +-----+
| Dog | | Cat |
+-----+ +-----+
So, we could implement our Speak() methods like this:
For Dog:
Overrides Sub Speak()
Console.WriteLine("Arf!")
End Sub
For Cat:
Overrides Sub Speak()
Console.WriteLine("Meow!")
End Sub
The problem I'm facing is this: how do I restrict the creation of Dog and Cat
classes while avoiding redundant code. I mean, it's easy simple to implement
each class as separate singletons, but that's going to get old after a while
if I also have to create a Hampster, a Parrot, a Mouse, a Rat, a Guinea Pig, etc.
I started working with an Abstract Factory--or something like it:
+----------------------+
| << abstract >> |
| AnimalCreator |
+----------------------+
| m_objAnimal: Animal |
+--->+----------------------+
| | Create(): Animal |
| +----------------------+
| /|\
| |
+------------------+ |
| DogCreator | |
+------------------+ |
| Create(): Animal | |
+------------------+ |
|
+------------------+ |
| CatCreator | |
+------------------+------+
| Create(): Animal |
+------------------+
(Animal, Dog, and Cat as above)
Problem is: I wanted to use a static variable (AnimalCreator.m_objAnimal) to store a reference
to a single instance of each animal. Little did I know (until the code behaved incorrectly)
that a single instance of m_objAnimal is shared by all classes derived from AnimalCreator.
So, what's a good way around this problem?
The code follows at the end of this message.
Pierre the Perplexed Programmer
Imports Microsoft.VisualBasic
Imports System
Imports System.Collections
Public MustInherit Class Animal
Public MustOverride Sub Speak()
End Class
Public Class Dog
Inherits Animal
Public Overrides Sub Speak()
Console.WriteLine("Arf!")
End Sub
End Class
Public Class Cat
Inherits Animal
Public Overrides Sub Speak()
Console.WriteLine("Meow")
End Sub
End Class
Public MustInherit Class AnimalCreator
Protected Shared m_objAnimal As Animal
Public MustOverride Function Create() As Animal
End Class
Public Class DogCreator
Inherits AnimalCreator
Public Overrides Function Create() As Animal
If m_objAnimal Is Nothing Then
m_objAnimal = New Dog
End If
Return m_objAnimal
End Function
End Class
Public Class CatCreator
Inherits AnimalCreator
Public Overrides Function Create() As Animal
If m_objAnimal Is Nothing Then
m_objAnimal = New Cat
End If
Return m_objAnimal
End Function
End Class
Public Class Test
Public Shared Sub Main()
Dim objDogCreator As AnimalCreator = New DogCreator()
Dim objCatCreator As AnimalCreator = New CatCreator()
Dim objDog As Animal = objDogCreator.Create()
Dim objCat As Animal = objCatCreator.Create()
objDog.Speak()
objCat.Speak()
Console.ReadLine()
End Sub
