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wanted to add an element into the list。
If _roomGroupings Is Nothing Then
_roomGroupings = NewRoomGroup（）
Else
_roomGroupings。Insert（NewRoomGroup（））
End If
The code that uses the placeholder is shorter and simpler； however； it also requires a dangling
instance of RoomGrouping that has no real value。 I chose the dangling approach because I am
making the decision that a room grouping with no identifier is the default room grouping。
Adding a Room Grouping
The following code adds a room grouping （added to the class LightingController）。
Public Function AddRoomGrouping（ByVal description As String） As Object
Dim grouping As RoomGrouping = New RoomGrouping（） _
With ｛ _
。Description = description； _
。Rooms = Nothing
｝
_roomGroupings。Insert（grouping）
Return grouping
End Function
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214 CH AP T E R 8 ■ L E A R N IN G AB OU T CO M P O N E N TO R IE N T E D AR C HI TE CT U R E
To add a new room grouping； you instantiate RoomGrouping； assign the data members； and
then call the method _roomGroupings。Insert（） to insert the new room grouping into the linked list。
Let’s look at the technique for assigning data members； called object initialization。 In
previous examples； when an object was instantiated and we wanted to assign default values；
we would create a constructor with the appropriate parameters。 However； another way is to instan
tiate the object and define a block that assigns the appropriate data members or properties。 In the
case of RoomGrouping； there are two publicly defined data members： Description and Rooms：
。Description = description； _
。Rooms = Nothing
The Description and Rooms data members have assign access； which is important as this
technique only works with properties that are not read…only。 To assign a data member or prop
erty； after the object instantiation； add the With keyword； and then within curly brackets； assign
each individual data member using a key/value pair； in this form：
With ｛ 。Key1 = value1； 。Key2 = value2 ｝
The key represents the data member property to assign； and the value is the data that is
assigned to the data member or property。 In the form example； the properties Key1 and Key2
are set。
Another technique of interest in the code to add a room grouping is the definition of a data
handle when passing information：
Return grouping
In the implementation of AddRoomGrouping（）； the variable grouping is assigned an
instance of RoomGrouping。 The declaration of the RoomGrouping class limits its scope to the
LibLightingSystem assembly only； while the declaration of LightingController is public。 If the
method AddRoomGrouping（） had attempted to return an instance of RoomGrouping； the piler
would have marked this as an error； because the scope is inconsistent。 Assuming for the moment
that you did want to return an instance of RoomGrouping； your only solution would be to declare
RoomGrouping as public。 The declaration change is the wrong solution； because RoomGrouping is
a class without declared methods （other than the base class methods） and has public data
members。 It is a class for a specific purpose and should not be shared。
Declaring RoomGrouping as public is the wrong approach； so another solution is needed。
You could add a counter data member to the RoomGrouping declaration and return an Integer
value indicating the RoomGrouping instance you are referring to in the list。 However； that would
mean having access to the list somewhere； and then needing to iterate to find the appropriate
RoomGrouping instance。
The solution is to declare the method as returning a type Object。 When you use Object；
you are defining that your method is giving you an object instance。 The caller may or may not
know what the instance type is； and in the case of AddRoomGrouping（）； it doesn’t。 But that is fine；
because you； as the user； will consider the instance as a key that is managed by the class
LightingController。 In technical jargon； the object instance is a handle that you hold and pass
to some other ponent that knows what to do with it。 In the example； it means giving the
handle to LightingController because it knows that the handle is an instance of RoomGrouping。
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C H AP TE R 8 ■ L E AR N IN G AB O U T CO M P O N E N T O R IE N TE D A R CH I TE C TU R E 215
■Note Handles were very popular in the C programming days and were consider pointers to memory。 The
caller did not know what the pointer pointed to； but kept using it when interacting with an API。 These days；
handles have lost significance as we have objects； generics； and other programming constructs。 However； at
times； handles are very useful。 They can help you to avoid the problem of having to expose the internal state
of your API； while not having to maintain an object hierarchy to watch which objects are being referenced。
Finding a Room Grouping
When a number of room groupings have been added； you will want to find a room grouping
with a particular description。 As room groupings are a doubly linked list； it means needing to
iterate the list； as follows （added to LightingController）：
Public Function FindRoomGrouping（ByVal description As String） As Object
Dim curr As RoomGrouping = _roomGroupings。NextItem
Do While curr IsNot Nothing
If curr。Description。pareTo（description） = 0 Then
Return curr
End If
curr = TryCast（curr。NextItem； RoomGrouping）
Loop
Return Nothing
End Function
In the iteration code； the iteration is similar to the code illustrated earlier in the “Storing a
Collection Using a Linked List” section。 The one difference is that the curr variable is of type
RoomGrouping； and because NextItem is of type BaseLinkedListItem； a type cast is necessary。
Then an iteration using a While loop is carried out； during each iteration； a test paring
curr。Description to the parameter description is made。 If an object is found； the handle to
the RoomGrouping is returned； and if nothing is found； Nothing is returned； indicating that the
RoomGrouping could not be found。
This method would be used as follows：
Dim foundHandle As Object = controller。FindRoomGrouping（〃description〃）
Visual Basic has constructs that make it possible to convert the LightingController class
into a class that has array functionality。 The following method in LightingController declares
array…like functionality； which is called a default property
Default Public ReadOnly Property Item（ByVal description As String） As Object
Get
Return FindRoomGrouping（description）
End Get
End Property
A Visual Basic default property is defined like a property； except that the property is prefixed
with a Default keyword。 The default property is named Item by convention； and this name is
used by all collection classes for consistency。 The implementation of a property is just like
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