197 …………………………………………………………Page 220…………………………………………………………… 198 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 ponents are implemented at the technical level using interfaces and classes that subclass the interfaces。 The interface represents an idea； and the classes represent the implementation of an idea。 A class could implement multiple interfaces； where each interface represents a unique characteristic of the class implementation。 Ideas and interfaces also represent contracts or standards。 The kernel defines a standard； and the ponent has the responsibility of imple menting that standard。 Implementing a kernel is like being a coach。 A coach thinks of where to place players in the field and develops strategies that the players need to implement。 But on game day； the players will do what they think is best； and the coach is powerless。 A coach can teach the players； but whether they apply that knowledge is up to the individual players。 In programming terms； the kernel is the coach； and the external implementations to manipulate are the players。 When designing interfaces that other pieces of functionality will implement； you cannot watch over the shoulders of the programmers and make sure they do things correctly。 You need to trust； but you also need to implement a mode of programming where you assume the programmers are going to do things incorrectly。 This has nothing to do with the personal abilities of the other programmers。 It has to do with the ability to make sure your kernel continues functioning； even when someone makes a mistake。 Remember that when implementing a kernel； you are implementing a controller； and you are devising the strategy of the application。 You are just not doing all of the implementations。 When you write production code； and you have been put in charge of writing a kernel； count yourself lucky。 But remember with the job of building a kernel es great responsibility。 If your kernel is buggy or badly designed； then the external implementations will also be buggy and potentially badly designed。 The kernel is the rock and foundation of the application。 Organizing the Lighting Application For the moment； imagine we are not developing software； but actually building a house。 And in this house； we are going to add a central lighting system。 The make of light bulbs； lamps； and lighting controller are not identical。 This is interesting in that multiple panies make prod ucts that work with each other’s device； because all panies adhere to a particular standard。 The process of standardization is all around us； and is as simple as the electricity that es out of our electrical outlets。 However； what is a standard in one country is not necessarily the standard in another country。 Different countries will have different standard voltages。 In terms of software； the kernel represents a standard that allows the integration of ponents。 The lighting application will include these features： o A room where the light can be controlled using either a simple on/off switch or a gradual light…intensity mechanism。 o The controller represents a building； and the rooms within the building can be grouped； making it simpler to perform group operations。 o Rooms are associated with identifiers that allow each room to be isolated individually。 o Rooms can be associated with a set of attributes that indicate the behavior they do and do not support。 …………………………………………………………Page 221…………………………………………………………… 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 199 The source code will implement the controller as a library project named LibLightingSystem。 This library project will also define the interfaces that the ponents will implement。 To demonstrate building a plete working application； two other projects implement the inter faces and represent ponents： Museum and Home。 The key characteristics of a museum are that some rooms are made dark at night and are pletely managed by the controller； while other rooms are controlled individually in the room itself and also can be managed by the controller。 The key characteristics of a home are that all of the rooms are individually controlled； but they have sensors that allow automatic control。 Some of the lighting associated with the house depends on certain prerequisites being fulfilled。 For example； night…lights might turn on depending on the time of year； or room lights might be turned on and off automatically when no one is home。 As usual； we will use a testing console application； named TestLightingSystem； to test the three assemblies （it is the startup project）。 Figure 8…1 shows the project structure。 Figure 8…1。 Lighting system application project structure Building the Kernel The lighting system is a two…part implementation。 The first part is that we must write code that works properly。 The second part is to integrate code that someone else has written and make sure that if that code does not work properly for some reason； those problems will not affect the code that we wrote。 This application is plicated by the fact that we are dealing with the unknown。 In the previous examples； we ha