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5: Design Patterns
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package javaee.architect.ChainOfResponsibility; public class ConcreteHandler2 implements HandlerIF { public void processRequest(Request parm) { // You could add on to the processing chain here... handleIt(parm); } private void handleIt(Request parm) { System.out.println("ConcreteHandler2 has handled the processing."); } } package javaee.architect.ChainOfResponsibility; public interface HandlerIF { public void processRequest(Request request); } package javaee.architect.ChainOfResponsibility; public class Request { // The universe of known requests that can be handled. public final static int EQUITY_ORDER = 100; public final static int BOND_ORDER = 200; // This objects type of request. private int type; public Request(int parm) throws Exception { // Validate the request type with the known universe. if ((parm == EQUITY_ORDER) || (parm == BOND_ORDER)) // Store this request type. this.type = parm; else throw new Exception("Unknown Request type "+parm+"."); } public int getType() { return type; } }
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The Command pattern s intent is to encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support rollback types of operations. The Command pattern is also known as Action or Transaction. The UML is shown in Figure 5-14.
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Identify the Most Appropriate Design Pattern
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FIGURE 5-14
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UML for the Command pattern
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Benefits Following is a list of benefits of using the Command pattern:
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n It separates the object that invokes the operation from the object that
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actually performs the operation.
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n It simplifies adding new commands, because existing classes remain
unchanged.
Applicable Scenarios The following scenarios are most appropriate for the Command pattern:
n You need to parameterize objects according to an action to perform. n You create, queue, and execute requests at different times. n You need to support rollback, logging, or transaction functionality.
Java EE Technology Feature These are the Java EE technology features associated with the Command pattern:
n MessageBeans invoke business logic based on content of messages dispatched
to them.
n Servlets/JSPs are invoked corresponding to the type of HTTP request that is
received by the web container.
Example Code pattern:
The following example Java code demonstrates the Command
5: Design Patterns
package javaee.architect.Command; public class CommandPattern { public static void main(String[] args) { System.out.println("Command Pattern Demonstration."); System.out.println("------------------------------"); // Create receiver objects. System.out.println("Creating receivers."); ReceiverIF order = new Order(); ReceiverIF trade = new Trade(); // Create commands passing in receiver objects. System.out.println("Creating commands."); CommandAbstract cmdOrder = new ConcreteCommand(order); CommandAbstract cmdTrade = new ConcreteCommand(trade); // Create invokers. System.out.println("Creating invokers."); Invoker invOrder = new Invoker(); Invoker invTrade = new Invoker(); // Storing commands in invokers respectively. System.out.println("Storing commands in invokers."); invOrder.storeCommand(cmdOrder); invTrade.storeCommand(cmdTrade); // Call invoke on the invoker to execute the command. System.out.println("Invoking the invokers."); invOrder.invoke(); invTrade.invoke(); System.out.println(); } } package javaee.architect.Command; abstract class CommandAbstract { public abstract void execute(); } package javaee.architect.Command; public class ConcreteCommand extends CommandAbstract { // The binding between action and receiver private ReceiverIF receiver; public ConcreteCommand(ReceiverIF receive) { this.receiver = receive; } public void execute() { receiver.action(); } }
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package javaee.architect.Command; public class Invoker { private CommandAbstract command; public void storeCommand(CommandAbstract cmd) { this.command = cmd; } public void invoke() { command.execute(); } } package javaee.architect.Command; public class Order implements ReceiverIF { public void action() { System.out.println("Order.action() called."); } } package javaee.architect.Command; public interface ReceiverIF { public void action(); } package javaee.architect.Command; public class Trade implements ReceiverIF { public void action() { System.out.println("Trade.action() called."); } }
Interpreter
The Interpreter pattern s intent is to define a representation of the grammar of a given language, along with an interpreter that uses this representation to interpret sentences in the language. The UML is shown in Figure 5-15.
Benefits Following is a list of benefits of using the Interpreter pattern:
n It is easier to change and extend the grammar. n Implementing the grammar is straightforward.
5: Design Patterns
FIGURE 5-15
UML for the Interpreter pattern
Applicable Scenarios The following scenarios are most appropriate for the Interpreter pattern:
n The grammar of the language is not complicated. n Efficiency is not a priority.
Example Code The following example Java code demonstrates the Interpreter pattern:
package javaee.architect.Interpreter; import java.util.ArrayList; import java.util.ListIterator; import java.util.StringTokenizer; public class InterpreterPattern { public static void main(String[] args) { System.out.println("Interpreter Pattern Demonstration."); System.out.println("----------------------------------"); BookInterpreterContext bookInterpreterContext = new BookInterpreterContext(); bookInterpreterContext.addTitle("Pickwick Papers"); bookInterpreterContext.addTitle("Great Expectations"); bookInterpreterContext.addTitle("Wuthering Heights"); bookInterpreterContext.addTitle("Crossfile"); bookInterpreterContext.addAuthor("William Shakespeare"); bookInterpreterContext.addAuthor("Emily Bronte"); bookInterpreterContext.addAuthor("James Marathon"); bookInterpreterContext.addTitleAndAuthor( new TitleAndAuthor("Pickwick Papers", "William Shakespeare")); bookInterpreterContext.addTitleAndAuthor( new TitleAndAuthor("Great Expectations", "William Shakespeare")); bookInterpreterContext.addTitleAndAuthor(
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new TitleAndAuthor("Wuthering Heights", "Emily Bronte")); bookInterpreterContext.addTitleAndAuthor( new TitleAndAuthor("Crossfire", "James Marathon")); BookInterpreterClient bookInterpreterClient = new BookInterpreterClient(bookInterpreterContext); System.out.println("show author ->" + bookInterpreterClient.interpret("show author")); System.out.println("show title ->" + bookInterpreterClient.interpret("show title")); System.out.println("show author for title <Crossfire> ->" + bookInterpreterClient.interpret("show author for title <Crossfire>")); System.out.println("show title for author <William Shakespeare> ->" + bookInterpreterClient.interpret( "show title for author <William Shakespeare>")); System.out.println(); } } class BookInterpreterClient { BookInterpreterContext bookInterpreterContext; public BookInterpreterClient(BookInterpreterContext parm) { bookInterpreterContext = parm; } // language syntax: // show title // show author // show title for author <author-name> // show author for title <title-name> public String interpret(String expression) { StringTokenizer expressionTokens = new StringTokenizer(expression); String currentToken; char mainQuery = ' '; char subQuery = ' '; String searchString = null; boolean forUsed = false; boolean searchStarted = false; boolean searchEnded = false; StringBuffer result = new StringBuffer(); while (expressionTokens.hasMoreTokens()) { currentToken = expressionTokens.nextToken(); if (currentToken.equals("show")) { continue;//show in all queries, not really used } else if (currentToken.equals("title")) { if (mainQuery == ' ') { mainQuery = 'T'; } else { if ((subQuery == ' ') && (forUsed)) { subQuery = 'T'; } } } else if (currentToken.equals("author")) {
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