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5: Design Patterns
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clone2.action(); System.out.println(); } } package javaee.architect.Prototype; public class ConcretePrototype1 implements PrototypeIF { public ConcretePrototype1() { System.out.println("ConcretePrototype1 constructed."); } public PrototypeIF getClone() { // if required, put deep copy code here return new ConcretePrototype1(); } public void action() { System.out.println("ConcretePrototype1.action() called"); } } package javaee.architect.Prototype; public class ConcretePrototype2 implements PrototypeIF { public ConcretePrototype2() { System.out.println("ConcretePrototype2 constructed."); } public PrototypeIF getClone() { // if required, put deep copy code here return new ConcretePrototype1(); } public void action() { System.out.println("ConcretePrototype2.action() called."); } } package javaee.architect.Prototype; public interface PrototypeIF { public PrototypeIF getClone(); // as opposed to Object.clone() public void action(); }
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Singleton
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The Singleton pattern s intent is to ensure that a class has only one instance and provides a global point of access to it. It ensures that all objects that use an instance of this class are using the same instance. Figure 5-5 shows the UML.
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Identify the Most Appropriate Design Pattern
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FIGURE 5-5
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UML for the Singleton pattern
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Benefits Following are the benefits of using the Singleton pattern:
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n It controls access to a single instance of the class. n It reduces name space usage. n It permits refinement of operations and representation. n It can also permit a variable number of instances. n It is more flexible than class methods (operations).
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Applicable Scenario The scenario most appropriate for the Singleton pattern is when a single instance of a class is needed and must be accessible to clients from a well-known access point. Java SE API Association The Java SE API associated with the Singleton pattern is java.lang.Runtime. Example Code The following example Java code demonstrates the Singleton pattern:
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package javaee.architect.Singleton; public class SingletonPattern { public static void main(String[] args) { System.out.println("Singleton Pattern Demonstration."); System.out.println("--------------------------------"); System.out.println("Getting Singleton instance (s1)"); Singleton s1 = Singleton.getInstance(); System.out.println("s1.getInfo()="+s1.getInfo()); System.out.println("Getting Singleton instance (s2)"); Singleton s2 = Singleton.getInstance(); System.out.println("s2.getInfo()="+s2.getInfo()); System.out.println("s1.setValue(42)"); s1.setValue(42);
5: Design Patterns
System.out.println("s1.getValue()="+s1.getValue()); System.out.println("s2.getValue()="+s2.getValue()); System.out.println("s1.equals(s2)="+s1.equals(s2) + ", s2.equals(s1)="+s2.equals(s1)); // The following will not compile // Singleton s3 = (Singleton) s1.clone(); System.out.println(); } } package javaee.architect.Singleton; /* * Singletons really are "per classloader" and * in a Java EE application, many developers make * the mistake of assuming that a singleton really * is a singleton in a cluster of application servers. * This is not true! */ public final class Singleton { private static Singleton instance; private int value; private Singleton() {System.out.println("Singleton constructed.");} public static synchronized Singleton getInstance() { // if it has not been instantiated yet if (instance == null) // instantiate it here instance = new Singleton(); return instance; } // remaining methods are for demo purposes // your singleton would have it's business // methods here... public String getInfo() { return getClass().getName() + // Uncomment line below to also see the loader //+", loaded by " + getClass().getClassLoader(); ", id#" + System.identityHashCode(this); } public int getValue() {return value;} public void setValue(int parm) {value = parm;} public boolean equals(Singleton parm) { return (System.identityHashCode(this) == System.identityHashCode(parm)); } }
Identify the Most Appropriate Design Pattern
GoF Structural Design Patterns
Structural patterns are concerned with composition or the organization of classes and objects, how classes inherit from each other, and how they are composed from other classes. Common Structural patterns include Adapter, Proxy, and Decorator patterns. These patterns are similar in that they introduce a level of indirection between a client class and a class it wants to use. Their intents are different, however. Adapter uses indirection to modify the interface of a class to make it easier for a client class to use it. Decorator uses indirection to add behavior to a class, without unduly affecting the client class. Proxy uses indirection transparently to provide a stand-in for another class. The following Structural patterns are described by GoF:
n Adapter n Bridge n Composite n Decorator n Facade n Flyweight n Proxy
Adapter
The Adapter pattern converts the interface of a class into an interface that a client requires. It acts as an intermediary and lets classes work together that couldn t otherwise because of an incompatible interface. The Adapter pattern is also known as Wrapper. The UML is shown in Figure 5-6.
FIGURE 5-6
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