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5: Design Patterns
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subject.addObserver(observer2); System.out.println("Doing something in the subject over time..."); System.out.println(); System.out.println(" Observable Observer1 Observer2"); System.out.println("Iteration changed notified notified "); // Use loop to simulate time. for(int i=0;i < 10;i++) { System.out.print(i+": "); subject.doSomething(); System.out.println(); } System.out.println(); System.out.println("Removing observer1 from the subject...repeating..."); System.out.println(); subject.removeObserver(observer1); // Another loop to simulate time. for(int i=0;i < 10;i++) { System.out.print(i+": "); subject.doSomething(); System.out.println(); } } } package javaee.architect.Observer; public class ConcreteObserver implements ObserverIF { private ConcreteSubject subject; // Reference to subject public void update() { if (subject == null) { subject = new ConcreteSubject(); } System.out.print(" Yes!"); } } package javaee.architect.Observer; import java.util.*; public class ConcreteSubject implements SubjectIF { List observers = new ArrayList(); public void addObserver(ObserverIF parm) {observers.add(parm);} public void removeObserver(ObserverIF parm) {observers.remove(observers.indexOf(parm));} public void notifyObservers() { for (Iterator i = observers.iterator(); i.hasNext();) { ((ObserverIF) i.next()).update(); } } public void doSomething() { double d = Math.random();
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Identify the Most Appropriate Design Pattern
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if (d<0.25 || d>0.75) { System.out.print("Yes"); notifyObservers(); } else { System.out.print("No"); } } } package javaee.architect.Observer; public interface ObserverIF { public void update(); } package javaee.architect.Observer; public interface SubjectIF { public void addObserver(ObserverIF parm); public void removeObserver(ObserverIF parm); public void notifyObservers(); }
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State
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The State pattern s intent is to allow an object to alter its behavior when its internal state changes, appearing as though the object itself has changed its class. Another view of the intent of the State pattern is to encapsulate the states of an object as discrete objects, with each object belonging to a separate subclass of an abstract state class. The State pattern is also known as Objects for States and acts in a similar way to the Receiver in the Command pattern. The UML is shown in Figure 5-20.
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FIGURE 5-20
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UML for the State pattern
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Benefits Following is a list of benefits of using the State pattern:
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n It keeps state-specific behavior local and partitions behavior for different
states.
n It makes any state transitions explicit.
Applicable Scenarios pattern:
The following scenarios are most appropriate for the State
n The behavior of an object depends on its state and it must be able to change
its behavior at runtime according on the new state.
n Operations have large, multipart conditional statements that depend on the
state of the object.
Example Code pattern:
The following example Java code demonstrates the State
package javaee.architect.State; public class StatePattern { public static void main(String[] args) { System.out.println("State Pattern Demonstration."); System.out.println("----------------------------"); // Construct context. System.out.println("Constructing context."); Context context = new Context(); // Call request, make state handle the request. System.out.println("Calling context.request()."); context.request(); // Flip state. System.out.println("Calling context.changeState()."); context.changeState(); // call request. System.out.println("Calling context.request()."); context.request(); System.out.println(); } } package javaee.architect.State; public class ConcreteState1 implements StateIF { public void handle() { System.out.println("ConcreteState1.handle() called."); } }
Identify the Most Appropriate Design Pattern
package javaee.architect.State; public class ConcreteState2 implements StateIF { public void handle() { System.out.println("ConcreteState2.handle() called."); } } package javaee.architect.State; public class Context { // Initial state. private StateIF state = new ConcreteState1(); // Request operation. public void request() { state.handle(); } // Switch states public void changeState() { if (state instanceof ConcreteState1) state = new ConcreteState2(); else state = new ConcreteState1(); } } package javaee.architect.State; public interface StateIF { public void handle(); }
Strategy
The Strategy pattern s intent is to define a family of functionality, encapsulate each one, and make them interchangeable. The Strategy pattern lets the functionality vary independently from the clients that use it. The Strategy pattern is also known as Policy. The UML is shown in Figure 5-21.
Benefits Following is a list of benefits of using the Strategy pattern:
n It provides a substitute to subclassing. n It defines each behavior within its own class, eliminating the need for
conditional statements.
n It makes it easier to extend and incorporate new behavior without changing
the application.
5: Design Patterns
FIGURE 5-21
UML for the Strategy pattern
Applicable Scenarios The following scenarios are most appropriate for the Strategy pattern:
n Multiple classes differ only in their behaviors. The servlet API is a classic
example of this.
n You need different variations of an algorithm. n An algorithm uses data that is unknown to the client.
Example Code The following example Java code demonstrates the Strategy pattern:
package javaee.architect.Strategy; public class StrategyPattern { public static void main(String[] args) { System.out.println("Strategy Pattern Demonstration."); System.out.println("-------------------------------"); // Construct strategies. System.out.println("Constructing strategies."); StrategyIF strategy1 = new ConcreteStrategy1(); StrategyIF strategy2 = new ConcreteStrategy2(); // Construct contexts. System.out.println("Constructing contexts."); Context context1 = new Context(strategy1); Context context2 = new Context(strategy2); // Execute contextInterface. System.out.println("Constructing context interfaces."); context1.contextInterface("J2EE Unleashed"); context2.contextInterface("J2EE Unleashed");
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