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。 …………………………………………………………Page 226…………………………………………………………… 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。 …………………………………………………………Page 227…………………………………………………………… 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