Section 15.9 Playing One Row Nim Over the Network
In the previous section we developed and tested a generic echo service. It is based on a common root class, ClientServer
, which is a subclass of Thread
. Both EchoServer
and EchoClient
extend the root class, and each implements its own version of run()
. In this section, we will generalize this design so that it can support a wide range of services. To illustrate the effectiveness of the design, we will use it as the basis for a program that plays One Row Nim over the Internet.
In order to generalize our design, we begin by identifying those elements that are common to all servers and clients and what is particular to the echo service and client. Clearly, the general server and client protocols that are defined here in their respective run()
methods, are something that all servers and clients have in common. What differs from one application to another is the particular service provided and requested, as detailed in their respective provideService()
and requestService()
methods. In this example, the service that is provided will be One Row Nim. The clients that use this service will be (human) players of the game.
Therefore, the way to generalize this application is to define the run()
method in the generic Server
and Client
classes. The overall design of the One Row Nim service will now consist of five classes organized into the hierarchy shown in FigureĀ 15.9.1. At the root of the hierarchy is the ClientServer
class, which contains nothing but I/O methods used by both clients and servers. The abstract Server
and Client
classes contain implementations of the Thread.run()
method, which defines the basic protocols for servers and clients, respectively. The details of the particular service are encoded in the provideService()
and requestService()
methods. Because the run()
methods defined in Client
and Server
call provideService()
and requestService()
, respectively, these methods must be declared as abstract
methods in the Server
and Client
classes. Any class that contains an abstract method must itself be declared abstract
.
Note that we have left the readFromSocket()
and writeToSocket()
methods in the ClientServer
class. These methods are written in a general way and can be used, without change, by a wide range of clients and servers. If necessary, they can also be overridden by a client or server. In fact, as we will see, the NimServer
class does override the writeToSocket()
method. Similarly, note that the readFromKeyboard()
method is defined in the Client
superclass. This is a general method that can be used by a large variety of clients, so it is best if they don't have to redefine it themselves.
These design decisions lead to the definitions of Server
and Client
shown in ListingĀ 15.9.3 and ListingĀ 15.9.4, respectively. Note that provideService()
and requestService()
are left unimplemented. Subclasses of Server
, such as NimServer
, and subclasses of Client
, such as NimClient
, can implement provideService()
and requestService()
in a way that is appropriate for the particular service they are providing.
Principle 15.9.2. EFFECTIVE DESIGN: Polymorphism.
Defining a method as abstract
within a superclass, and implementing it in various ways in subclasses, is an example of polymorphism. Polymorphism is a powerful object-oriented design technique.
Subsection 15.9.1 The NimServer
Class
Given the abstract definition of the Server
class, defining a new service is simply a matter of extending Server
and implementing the provideService()
method in the new subclass. We will name the subclass NimServer
.
ListingĀ 15.9.5 provides a definition of the NimServer
subclass. Note how its implementation of provideService()
uses an instance of the OneRowNim
class from ChapterĀ 8. Also, the play()
method, which encapsulates the game-playing algorithm, uses an instance of NimPlayer
, also from ChapterĀ 8. You might recall that OneRowNim
is a TwoPlayerGame
and NimPlayer
defines methods that allow a computer to play an optimal game of One Row Nim. In this example, the server acts as one of the players and use a NimPlayer
to manage its playing strategy. Thus, clients that connect to NimServer
will be faced by a computer that plays an optimal game.
If you compare the details of the NimServer
's play()
method with the play()
method from the implementation of OneRowNim
, you will see that they are very similar. In this implementation, we use public methods of the OneRowNim
object to manage the playing of the game. Thus, addComputerPlayer()
adds an instance of NimPlayer
to the game. The takeSticks()
, changePlayer()
, and gameOver()
methods are used to manage the moves made by both itself and the client. And the getGamePrompt()
and reportGameState()
methods are used to manage the interaction and communication with the client. Note also that whenever it is the server's turn to move, it uses the NimPlayer
's makeAMove()
method to determine the optimal move to make.
Although the programming logic employed in the play()
method looks somewhat complex, it is very similar to the logic employed in the ChapterĀ 8 version of the game. The main difference here is that the server uses the writeToSocket()
and readFromSocket()
methods to manage the communication with the client. In this regard, this instance of provideService()
is no different than the provideService()
method we used in the EchoServer
class.
Finally, note that NimServer
provides an implementation of the writeToSocket()
method. This method is implemented in the ClientServer()
class and is inherited by NimServer
. However, the default implementation assumes that the client will use the a carriage return (\n
) to determine the end of a particular message in the socket. Because OneRowNim
's methods, getGamePrompt()
and reportGameState()
, contain embedded carriage returns, it is necessary to filter these. The new version of writeToSocket()
performs this filtering and calls the default method (super.writeToSocket()
, after it has finished its filtering task.
Subsection 15.9.2 The NimClient
Class
The NimClient
class is even easier to implement. As its task is simply to manage the communication between the human user and the NimServer
, it is very similar to the requestService()
method we used in EchoClient
. The relationship between the abstract Client
class (Fig. ListingĀ 15.9.4) and its extension in NimClient
( ListingĀ 15.9.6) is very similar to the relationship between Server
and NimServer
. TheĀ requestService()
method is called by Client.run()
. It is implemented in NimClient
. In this way, the Client
class can serve as a superclass for any number of clients. New clients for new services can be derived from Client
by simply implementing their own requestService()
method.
Principle 15.9.7. EFFECTIVE DESIGN: Inheritance.
By placing as much functionality as possible into a generic client/server superclass, you can simplify the creation of new services. This is an effective use of Java's inheritance mechanism.
Subsection 15.9.3 Testing the Nim Service
Testing the One Row Nim service will be no different than testing the Echo service. To test the service, you want to run both NimServer
and NimClient
at the same time and preferably on different computers. As they are presently coded, you will have to modify the main()
methods of both NimServer
and NimClient
to provide the correct URL and port for your environment.
Exercises 15.9.4 Self-Study Exercise
1. Scramble Service.
The design of the client/server hierarchy makes it easy to create a new service by extending both the Server
and Client
classes. Describe how you would implement a scramble service with this model. A scramble service can be used by people trying to solve the daily scramble puzzles found in many newspapers. Given a string of letters, the scramble service will return a string containing all possible letter combinations. For example, given ācat,ā the scramble service will return āact atc cat cta tac tca.ā
2. Connection Errors.
Describe what happens when each of the following errors is introduced into the EchoClient
or EchoServer
programs:
Specify the wrong host name when running
EchoClient
.Specify the wrong port number when running
EchoClient
.Remove the reference to
\n
in thewriteToSocket()
call inrequestService()
.