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5: Design Patterns
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package javaee.architect.Decorator; public class ConcreteComponent implements ComponentIF { public void action() { System.out.println("ConcreteComponent.action() called."); } } package javaee.architect.Decorator; public class ConcreteDecoratorA extends Decorator { String addedVariable; public void action() { super.action(); System.out.println("ConcreteDecoratorA.action() called."); addedVariable = "extra"; System.out.println("ConcreteDecoratorA.addedVariable="+addedVariable); } } package javaee.architect.Decorator; public class ConcreteDecoratorB extends Decorator { public void action() { super.action(); System.out.println("ConcreteDecoratorB.action() called."); addedMethod(); } private void addedMethod() { System.out.println("ConcreteDecoratorB.addedMethod() called."); } } package javaee.architect.Decorator; public class Decorator implements ComponentIF { ComponentIF component = new ConcreteComponent(); public void action() { component.action(); } }
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The Facade pattern s intent is to provide a unified and simplified interface to a set of interfaces in a subsystem. The Facade pattern describes a higher-level interface that makes the subsystem(s) easier to use. Practically, every Abstract Factory is a type of Facade. Figure 5-10 shows the UML.
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Identify the Most Appropriate Design Pattern
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UML for the Facade pattern
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Benefits Following is a list of benefits of using the Facade pattern:
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n It provides a simpler interface to a complex subsystem without reducing the
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n It shields clients from the complexity of the subsystem components. n It promotes looser coupling between the subsystem and its clients. n It reduces the coupling between subsystems provided that every subsystem
uses its own Facade pattern and other parts of the system use the Facade pattern to communicate with the subsystem.
Applicable Scenarios The following scenarios are most appropriate for the Facade pattern:
n You need to provide a simple interface to a complex subsystem. n Several dependencies exist between clients and the implementation classes of
an abstraction.
n Layering the subsystems is necessary or desired.
Java SE API Association java.net.URL.
The Java SE API associated with the Facade pattern is
Example Code The following example Java code demonstrates the Facade pattern:
package javaee.architect.Facade; public class FacadePattern { public static void main(String[] args) { System.out.println("Facade Pattern Demonstration."); System.out.println("-----------------------------");
5: Design Patterns
// Construct and call Facade System.out.println("Constructing facade."); Facade facade = new Facade(); System.out.println("Calling facade.processOrder()."); facade.processOrder(); System.out.println(); } } package javaee.architect.Facade; public class Facade { public void processOrder() { // Call methods on sub-systems to complete the process SubSystem1 subsys1 = new SubSystem1(); subsys1.getCustomer(); subsys1.getSecurity(); subsys1.priceTransaction(); SubSystemN subsysN = new SubSystemN(); subsysN.checkBalances(); subsysN.completeOrder(); } } package javaee.architect.Facade; public class SubSystem1 { public void getCustomer() { // Place functionality here... System.out.println("SubSystem1.getCustomer() called.");} public void getSecurity() { // Place functionality here... System.out.println("SubSystem1.getSecurity() called.");} public void priceTransaction() { // Place functionality here... System.out.println("SubSystem1.priceTransaction() called.");} } package javaee.architect.Facade; public class SubSystemN { public void checkBalances() { // Place functionality here... System.out.println("SubSystemN.checkBalances() called.");} public void completeOrder() { // Place functionality here... System.out.println("SubSystemN.completeOrder() called.");} }
Identify the Most Appropriate Design Pattern
FIGURE 5-11
UML for the Flyweight pattern
Flyweight
The Flyweight pattern s intent is to utilize sharing to support large numbers of finegrained objects in an efficient manner. Figure 5-11 shows the UML.
Benefits Following are benefits of using the Flyweight pattern:
n It reduces the number of objects to deal with. n It reduces the amount of memory and storage devices required if the objects
are persisted.
Applicable Scenarios The following scenarios are most appropriate for the Flyweight pattern:
n An application uses a considerable number of objects. n The storage costs are high because of the quantity of objects. n The application does not depend on object identity.
Java SE API Association tern is java.lang.String.
The Java SE API associated with the Flyweight pat-
Example Code The following example Java code demonstrates the Flyweight pattern:
package javaee.architect.Flyweight; public class FlyweightPattern { public static void main(String[] args) { System.out.println("Flyweight Pattern Demonstration.");
5: Design Patterns
System.out.println("--------------------------------"); // Create states State stateF = new State(false); State stateT = new State(true); // Get reference to (and in doing so create) flyweight FlyweightIF myfwkey1 = FlyweightFactory.getFlyweight("myfwkey"); // Get new reference to the same flyweight FlyweightIF myfwkey2 = FlyweightFactory.getFlyweight("myfwkey"); // Call action on both references System.out.println("Call flyweight action with state=false"); myfwkey1.action(stateF); System.out.println("Call flyweight action with state=true"); myfwkey2.action(stateT); System.out.println(); } } package javaee.architect.Flyweight; public class ConcreteFlyweight implements FlyweightIF { // Add state to the concrete flyweight. private boolean state; public ConcreteFlyweight(State parm) { this.state = parm.getState(); } public void action(State parm) { // Display internal state and state passed by client. System.out.println("ConcreteFlyweight.action(" +parm.getState()+") called."); this.state = parm.getState(); System.out.println("ConcreteFlyweight.state = " + this.state); } } package javaee.architect.Flyweight; import java.util.*; public class FlyweightFactory { private static Map map = new HashMap(); public static FlyweightIF getFlyweight(String parm) { // Return the Flyweight if it exists, // or create it if it doesn t. FlyweightIF flyweight = null; try { if (map.containsKey(parm)) { // Return existing flyweight
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