Building a CommPortManager in Java

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Let's consider constructing an enclosing class for these Singleton methods and calling it a PortManager We expect the class to allow us to enumerate the available ports and open any of them Some of the methods in such a class might be as follows:
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public abstract class PortManager { public static Enumeration getAllPorts(); public static Enumeration getAvailablePorts(); public static CommPort openPort(String portName); public static CommPort openPort(); }
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Because of the Java syntax rules, we can't have abstract static classes, so we actually declare each of them in the parent abstract class to return null The getAllPorts method is just like the one shown previously, when we enumerated the ports However, the getAvailablePorts method is interesting because we don't need to do anything at all when we catch the port exceptions We get the port and try to open it If neither operation fails, we add the port name to the list of available ports
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public static Enumeration getAvailablePorts() { Vector portOpenList = new Vector(); CommPortIdentifier portId; Enumeration enum = getAllPorts(); while (enumhasMoreElements()) { String portName = (String) enumnextElement() ; try { //try to get port ownership portId = CommPortIdentifiergetPortIdentifier(portName); //if successful, open the port CommPort cp = portIdopen("SimpleComm",100); //report success portOpenListaddElement(portName); cpclose(); } catch(NoSuchPortException e){} catch(PortInUseException e){} } return portOpenListelements(); }
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The getAvailablePorts method is illustrated in Figure 62
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Figure 62 The available ports assigned and then in use
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Which are the Singletons here We really just wrapped the CommPortIdentifier methods so that we could handle the exceptions internally The static CommPortIdentifier method might be considered a Singleton registry, since it provides access to four or more different ports However, as a static method it does provide the single point of access The ports themselves remain the Singletons, since only one instance of an opened port can exist
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Other Consequences of the Singleton Pattern
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The Singleton Pattern has the following additional consequences: 1 Subclassing a Singleton can be difficult, since this can work only if the base Singleton class has not yet been instantiated 2 You can easily change a Singleton to allow a small number of instances, where this is allowed and meaningful
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1 Consider a system for processing ATM card transactions A thief is using a stolen ATM card number to steal funds, concurrent with the legitimate user's withdrawing funds How could you design a Singleton to reduce this risk
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Program \Singleton\singleSpooler\ Description Shows how to write a print spooler throwing an exception
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SingleSpoolerjava Returns a single instance of a spooler and will not create more \Singleton\finalSpool\ finalspooljava Creates one instance or returns null \Singleton\InstanceSpooler\ InstanceSpoolerjava Simple illustration of Comm class uses \Singleton\Comm SimpleCommjava Uses the CommPortManager class \Singleton\PortManager\ ManageCommJava
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7 The Builder Pattern
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In this chapter we'll consider how to use the Builder pattern to construct objects from components We have already seen that the Factory Pattern returns one of several different subclasses, depending on the data passed in arguments to the creation methods But suppose we don't want just a computing algorithm, but rather a whole different user interface, depending on the data we need to display A typical example might be your e-mail address book You probably have both people and groups of people in your address book, and you expect the display for the address book to change so that the People screen has places for first and last names, company name, e-mail address, and telephone number On the other hand, if you were displaying a group address page, you'd like to see the name of the group, its purpose, and a list of its members and their e-mail addresses You click on a person's name and get one display and click on a group's name and get the other display Let's assume that all e-mail addresses are kept in an Address object and that People and Group classes are derived from this base class, as shown in Figure 71 Figure 71 Both Person and Group are derived from Address
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Depending on which type of Address object we click on, we'd like to see a somewhat different display of that object's properties This is little more than just a Factory pattern because the objects returned are not simple descendents of a base display object, but rather totally different user interfaces made up of different combinations of display objects The Builder pattern assembles a number of objects, such as display widgets, in various ways depending on the data Furthermore, since Java is one of the few languages with which you can cleanly separate the data from the display methods into simple objects, Java is the ideal language to implement the Builder pattern
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