Instantiating class variables in parent constructor.

[RB]

Hi guys (and gals!),

I've got 2 classes, "TypesafeConsta nt" and "Color". "Color" inherits
from "TypesafeConsta nt", and adds no new functionality. All "Color" does
is to instantiate some class variables which are public instances of
"Color".

What "TypesafeConsta nt" does is to create a shared list, and add every
declaration of itself to that list. I can then query that list with a
string (e.g. "Red") to return Color.Red.

Now, if I take out the inheritance, and just have "Color" instantiating
itself, everything works fine, but with the inheritance the Parse
function errors with a "NullReferenceE xception", which is caused by
TypesafeConstan t.list being a null reference.

Any ideas how I could re-write this to make it work?

Oh, and just to say, I want to use this technique a lot, hence the
inheritance - I don't want to have to re-write the entire
TypesafeConstan t each time!!

Many Thanks,

RB.
************* CODE *************** ****
Public Class TypesafeConstan t
Private sShortType

Protected Shared list As ArrayList

Protected Sub New(ByVal sShortType As String)
Me.sShortType = sShortType
If list Is Nothing Then
list = New ArrayList()
End If
list.Add(Me)
End Sub

Public ReadOnly Property Value() As String
Get
Return sShortType
End Get
End Property

Public Shared Function Parse(ByVal sValue As String) As
TypesafeConstan t
Dim o As TypesafeConstan t
For Each o In list.ToArray(Ge tType(TypesafeC onstant))
If o.Value.ToUpper = sValue.ToUpper Then
Return o
End If
Next
End Function
End Class
Public Class Color
Inherits TypesafeConstan t

Public Shared Red As Color = New Color("Red")
Public Shared GreenAs Color = New Color("Green")
Public Shared BlueAs Color = New Color("Blue")

Private Sub New(ByVal sShort As String, ByVal sLong As String)
MyBase.New(sSho rt, sLong)
End Sub
End Class

[=?Utf-8?B?VGVycnk=?=]

You might try calling your constructors with the correct number of arguments.
--
Terry
"RB" wrote:

Hi guys (and gals!),

I've got 2 classes, "TypesafeConsta nt" and "Color". "Color" inherits
from "TypesafeConsta nt", and adds no new functionality. All "Color" does
is to instantiate some class variables which are public instances of
"Color".

What "TypesafeConsta nt" does is to create a shared list, and add every
declaration of itself to that list. I can then query that list with a
string (e.g. "Red") to return Color.Red.

Now, if I take out the inheritance, and just have "Color" instantiating
itself, everything works fine, but with the inheritance the Parse
function errors with a "NullReferenceE xception", which is caused by
TypesafeConstan t.list being a null reference.

Any ideas how I could re-write this to make it work?

Oh, and just to say, I want to use this technique a lot, hence the
inheritance - I don't want to have to re-write the entire
TypesafeConstan t each time!!

Many Thanks,

RB.
************* CODE *************** ****
Public Class TypesafeConstan t
Private sShortType

Protected Shared list As ArrayList

Protected Sub New(ByVal sShortType As String)
Me.sShortType = sShortType
If list Is Nothing Then
list = New ArrayList()
End If
list.Add(Me)
End Sub

Public ReadOnly Property Value() As String
Get
Return sShortType
End Get
End Property

Public Shared Function Parse(ByVal sValue As String) As
TypesafeConstan t
Dim o As TypesafeConstan t
For Each o In list.ToArray(Ge tType(TypesafeC onstant))
If o.Value.ToUpper = sValue.ToUpper Then
Return o
End If
Next
End Function
End Class
Public Class Color
Inherits TypesafeConstan t

Public Shared Red As Color = New Color("Red")
Public Shared GreenAs Color = New Color("Green")
Public Shared BlueAs Color = New Color("Blue")

Private Sub New(ByVal sShort As String, ByVal sLong As String)
MyBase.New(sSho rt, sLong)
End Sub
End Class

[RB]

Terry wrote:

You might try calling your constructors with the correct number of arguments.

Sorry, my bad. I was trying to simplify the code for the newsgroup.

The code for Color should, of course, read

Private Sub New(ByVal sShort As String)
MyBase.New(sSho rt)
End Sub

Cheers,

RB.

[=?Utf-8?B?VGVycnk=?=]

It may be better if you actually show how you hoped to use the color class.
Since the constructor is private, how do you create an instance? Maybe Color
should have a shared sub Initialize with code something like:
If list is nothing then
Red = New Color("Red")
.....etc
end if
--
Terry
"RB" wrote:

Terry wrote:

You might try calling your constructors with the correct number of arguments.

Sorry, my bad. I was trying to simplify the code for the newsgroup.

The code for Color should, of course, read

Private Sub New(ByVal sShort As String)
MyBase.New(sSho rt)
End Sub

Cheers,

RB.

[Patrice]

The list is initialized in an instance constructor and used in a shared
method. How do you use those classes ? If you try to use the shared method
without creating an instance first it could fail...

--
Patrice

"RB" <ow************ @mailinator.com a écrit dans le message de news:
YK************* *************** **@eclipse.net. uk...

Hi guys (and gals!),

I've got 2 classes, "TypesafeConsta nt" and "Color". "Color" inherits from
"TypesafeConsta nt", and adds no new functionality. All "Color" does is to
instantiate some class variables which are public instances of "Color".

What "TypesafeConsta nt" does is to create a shared list, and add every
declaration of itself to that list. I can then query that list with a
string (e.g. "Red") to return Color.Red.

Now, if I take out the inheritance, and just have "Color" instantiating
itself, everything works fine, but with the inheritance the Parse function
errors with a "NullReferenceE xception", which is caused by
TypesafeConstan t.list being a null reference.

Any ideas how I could re-write this to make it work?

Oh, and just to say, I want to use this technique a lot, hence the
inheritance - I don't want to have to re-write the entire TypesafeConstan t
each time!!

Many Thanks,

RB.
************* CODE *************** ****
Public Class TypesafeConstan t
Private sShortType

Protected Shared list As ArrayList

Protected Sub New(ByVal sShortType As String)
Me.sShortType = sShortType
If list Is Nothing Then
list = New ArrayList()
End If
list.Add(Me)
End Sub

Public ReadOnly Property Value() As String
Get
Return sShortType
End Get
End Property

Public Shared Function Parse(ByVal sValue As String) As
TypesafeConstan t
Dim o As TypesafeConstan t
For Each o In list.ToArray(Ge tType(TypesafeC onstant))
If o.Value.ToUpper = sValue.ToUpper Then
Return o
End If
Next
End Function
End Class
Public Class Color
Inherits TypesafeConstan t

Public Shared Red As Color = New Color("Red")
Public Shared GreenAs Color = New Color("Green")
Public Shared BlueAs Color = New Color("Blue")

Private Sub New(ByVal sShort As String, ByVal sLong As String)
MyBase.New(sSho rt, sLong)
End Sub
End Class

[=?Utf-8?B?VGVycnk=?=]

.....and since the constructor is Private, you can't create an instance!
--
Terry
"Patrice" wrote:

The list is initialized in an instance constructor and used in a shared
method. How do you use those classes ? If you try to use the shared method
without creating an instance first it could fail...

--
Patrice

"RB" <ow************ @mailinator.com a écrit dans le message de news:
YK************* *************** **@eclipse.net. uk...

Hi guys (and gals!),

I've got 2 classes, "TypesafeConsta nt" and "Color". "Color" inherits from
"TypesafeConsta nt", and adds no new functionality. All "Color" does is to
instantiate some class variables which are public instances of "Color".

What "TypesafeConsta nt" does is to create a shared list, and add every
declaration of itself to that list. I can then query that list with a
string (e.g. "Red") to return Color.Red.

Now, if I take out the inheritance, and just have "Color" instantiating
itself, everything works fine, but with the inheritance the Parse function
errors with a "NullReferenceE xception", which is caused by
TypesafeConstan t.list being a null reference.

Any ideas how I could re-write this to make it work?

Oh, and just to say, I want to use this technique a lot, hence the
inheritance - I don't want to have to re-write the entire TypesafeConstan t
each time!!

Many Thanks,

RB.
************* CODE *************** ****
Public Class TypesafeConstan t
Private sShortType

Protected Shared list As ArrayList

Protected Sub New(ByVal sShortType As String)
Me.sShortType = sShortType
If list Is Nothing Then
list = New ArrayList()
End If
list.Add(Me)
End Sub

Public ReadOnly Property Value() As String
Get
Return sShortType
End Get
End Property

Public Shared Function Parse(ByVal sValue As String) As
TypesafeConstan t
Dim o As TypesafeConstan t
For Each o In list.ToArray(Ge tType(TypesafeC onstant))
If o.Value.ToUpper = sValue.ToUpper Then
Return o
End If
Next
End Function
End Class
Public Class Color
Inherits TypesafeConstan t

Public Shared Red As Color = New Color("Red")
Public Shared GreenAs Color = New Color("Green")
Public Shared BlueAs Color = New Color("Blue")

Private Sub New(ByVal sShort As String, ByVal sLong As String)
MyBase.New(sSho rt, sLong)
End Sub
End Class

[RB]

I thought that because the instances were themselves shared, they would
be created before the Parse shared method could be called. It certainly
works that way if I don't use inheritance, and have all the code in a
single class.

I should say that I'm a Java programmer by nature, where this technique
is a recognised pattern, so I might be missing something fundemental
about VB...!

Having said that, I never tried it with inheritance in Java either, so
maybe it's just a stupid thing to do :-)

RB.

Patrice wrote:

The list is initialized in an instance constructor and used in a shared
method. How do you use those classes ? If you try to use the shared method
without creating an instance first it could fail...

--
Patrice

[RB]

Terry wrote:

It may be better if you actually show how you hoped to use the color class.
Since the constructor is private, how do you create an instance? Maybe Color
should have a shared sub Initialize with code something like:
If list is nothing then
Red = New Color("Red")
.....etc
end if

The instances are created inside the class itself - see the following
lines in class Color:

Public Shared Red As Color = New Color("Red")
Public Shared GreenAs Color = New Color("Green")
Public Shared BlueAs Color = New Color("Blue")
An example of using it would be something like:

dim oColor as Color
oColor = Color.Parse("Re d")
if oColor Is Color.Red Then
Messagebox.show ("You chose Red!")
End if

Generally speaking I'm parsing values from a database, but it's along
those lines.

The point of the exercise is to have typesafe constants, similar to the
old Java alternative to enumerators.

Cheers,

RB.

[RB]

Terry wrote:

.....and since the constructor is Private, you can't create an instance!

The TypesafeConstan t constructor is protected, which (I think!) means
it's inherited to the child class. The Color constructor is private, but
all instances of it are created inside the class, so that should not be
a problem.

Having said that, I don't really understand why I need a Color
constructor at all. It's just that VB forced me to have one, as it
claimed I needed a no-arg constructor.

The reason it's private by the way, is to limit the set of constants to
those defined in the Color class. Really, Color should be defined as not
inheritable, but that doesn't affect the issue I'm facing.

Cheers,

RB.

[RB]

Charlie Brown wrote:

On Oct 10, 11:18 am, RB <owmdkbqziki... @mailinator.com wrote:

>Terry wrote:

>>It may be better if you actually show how you hoped to use the color class.
Since the constructor is private, how do you create an instance? Maybe Color
should have a shared sub Initialize with code something like:
If list is nothing then
Red = New Color("Red")
.....etc
end if

The instances are created inside the class itself - see the following
lines in class Color:

Public Shared Red As Color = New Color("Red")
Public Shared GreenAs Color = New Color("Green")
Public Shared BlueAs Color = New Color("Blue")

An example of using it would be something like:

dim oColor as Color
oColor = Color.Parse("Re d")
if oColor Is Color.Red Then
Messagebox.show ("You chose Red!")
End if

Generally speaking I'm parsing values from a database, but it's along
those lines.

The point of the exercise is to have typesafe constants, similar to the
old Java alternative to enumerators.

Cheers,

RB.

I may be missing what you are trying to accomplish, but if all you
want is color by name, the System.Drawing. Color class already provides
this function with the Color.FromName method. This would accomplish
your example usage posted above. Then again, you may need something
else I don't see becuase you simplified it for the sake of posting.

Yeah, if it was just colors I would use that method! It's actually
various statuses of various business objects - I just chose colors as an
easier to understand example. Probably Pets would have been a better
choice of an example :-)

Cheers,

RB.