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battle is in milliseconds； the light remains on）。 A smarter approach would be to allow a timing
of the light。 But how much time—a quarter of an hour； a half hour； an hour？
Another approach is not to use a time interval； but to enhance the interface and allow the
controller to figure out the state。 This enhanced interface； called ISensorRoom； is defined as
follows （in LibLightingSystem）：
Public Interface ISensorRoom
Inherits IRemoteControlRoom
ReadOnly Property IsPersonInRoom（） As Boolean
End Interface
The ISensorRoom interface has a single property IsPersonInRoom； which is a Boolean prop
erty。 If the property has a value of True； then a person is in the room； otherwise； no person is in
the room。 How the implementation determines whether or not a person is in the room is not
the problem of the kernel。 The kernel assumes the implementation knows how to figure this
out。
■Note As a general rule of thumb； the kernel can municate with the implementation only via the inter
face。 The kernel should never assume a certain implementation of an interface。 The kernel should take the
approach that what it sees is what it gets。 Thus； if the kernel needs additional information； the interface
should be extended during design； or another interface should be implemented。 Of course； this does not mean
for every piece of state the interface should be extended。 Sometimes； you will need to define a specific inter
face； such as in the tax application example in the previous chapter （ ICanadianTaxEngine）。
Now that we’ve created the interfaces； we’re ready to implement the kernel。
Implementing the Kernel
In this example； the kernel will be a single class that contains all of the functionality of the
controller。 This definition means that the individual implementations； testing； and applica
tions will interact with a single class。
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204 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
Here is an example of implementing the light…dimming method using the
LightingController class （in LibLightingSystem）：
Public Class LightingController
Public Sub DimLights（ByVal grouping As Object； ByVal level As Double）
End Sub
End Class
The user of LightingController would dim a light using this code：
Dim controller As LightingController = New LightingController（）
Dim grouping As Object = Nothing
controller。DimLights（grouping； 0。50）
The user code instantiates the type LightingController directly and uses the method
DimLights（） directly。 Using classes directly has the cost that the controller code cannot change
without affecting the users； as there is a tight coupling between the user code and the kernel。
And this is why I have been writing at length about using interfaces； ideas； and implementa
tions。 Yet； the controller appears to throw all of that out of the window。
The reason for using a class goes back to the previous chapter’s example and the interfaces
ITaxDeduction and ITaxIne。 That example had only a single implementation of each inter
face； and those implementations were not going to change。 As I explained in the previous chapter；
the interfaces could have been represented as classes。 The same logic applies with respect to
the controller。 The controller is not going to change much from a method and property signature
perspective； and there is going to be only a single implementation of the controller。 Therefore； an
interface is not necessary。 Using a class is pletely acceptable； and it’s the approach we’re
using in this chapter。 However； I’ll talk about when you might want to implement the kernel as
an interface a little later in the chapter； in the “Defining the Kernel As an Interface Instead of a
Class” section。
The controller represents a building that has the ability to organize rooms into groupings。
Based on the groupings； the controller can perform operations such as turning the lights on
or off； or setting lights to a specific dimness。 When each of these operations is executed； the
controller must respect the intentions of the individual rooms； by querying for a particular
interface as defined by the previous section。
The controller has two main responsibilities： calling the appropriate interface methods
and organizing the interface instances。 The organization of the instances involves using collec
tions； arrays； or a linked list。 We’ll use a linked list in this example。
Storing a Collection Using a Linked List
In the examples in previous chapters； we created a collection of objects using an array； like this：
Dim array As MyType（） = New MyType（9） ｛ ｝
array（0） = New MyType（）
array（2） = New MyType（）
This creates an array that can contain 10 elements at most （MyType（9））。 If you needed to store
20 elements； you would need to use the ReDim keyword with Preserve （as explained in Chapter 4）。
Another feature of an array is that you don’t need to assign the elements in a sequential manner。
The example assigns the first and third positions； with the second position being Nothing。
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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 205
Thus； some code that iterates the array will need to verify