to labor to find the bug。 A debugger has its uses； but when writing good tests in many scenarios； you will rarely need to use it。 And speaking of writing tests； as I noted in Chapter 6； in this book； I could introduce a testing framework like NUnit （http：//nunit。org） or Microsoft Visual Studio Team System （http：// msdn2。microsoft。/en…us/vstudio/default。aspx）。 When you are writing production code； you will probably use such a testing framework。 Testing frameworks do not help you to write your tests； but rather help you by providing support code to generate errors； log problems； and indicate progress of the tests。 Do not get misled by tools that say they can write the tests for you。 No tool can write your tests； because that would imply the tool understands the context of your code。 And since such a tool does not yet exist； you will need to write your own tests。 Implementing Room Groupings Room groupings are collections of rooms that fall into a specific organization。 The idea behind a grouping is to perform group operations without having to explicitly examine a room before performing an operation。 For example； in the case of the museum； we don’t need to figure out whether a room is public or private each time a global operation is being performed。 The organization of the collection is that there can be multiple room groupings that are linked together； and within a grouping； there are multiple rooms that can be grouped together。 The linked list structure has two levels and is coded as follows （in LibLightingSystem）： Class RoomGrouping Inherits BaseLinkedListItem Public Rooms As Room Public Description As String End Class Class Room Inherits BaseLinkedListItem Public ObjRoom As IRoom End Class …………………………………………………………Page 235…………………………………………………………… 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 213 The class declaration Room represents an individual room。 But notice how it derives from BaseLinkedListItem； which seems to imply that Room is actually many rooms。 This is part of the linked list implementation； it is like a chain； where the chain is created by individual links。 The RoomGrouping class has two data members： Rooms； which represents the list of rooms in the grouping； and Description； which represents an easy…to…understand description of the grouping。 The single data member for Room is a reference to an IRoom interface instance。 The room groupings are managed by the LightingController class。 An initial implemen tation of LightingController is as follows： Public Class LightingController Private _roomGroupings As BaseLinkedListItem = New RoomGrouping（） End Class When dealing with linked lists； you have a problem： which is the first element of a list？ When you use arrays； an empty list of arrays is an array with no references。 But there is an explicit array object。 Using the linked list； an empty linked list is a list that does not exist。 Thus； when you want to create a list； you need a room。 In LightingController； the first element is an instance of RoomGrouping； which is not a room grouping； but serves as a placeholder。 To insert a room grouping； you could simply use this code： _roomGroupings。Insert（NewRoomGroup（）） Without the placeholder； you would need to write the following code whenever you 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 …………………………………………………………Page 236…………………………………………………………… 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 o