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Identify the Most Appropriate Design Pattern
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private String state = "false"; public ConcreteColleague2(MediatorIF parm) { this.mediator = parm; this.mediator.registerColleague2(this); } public void setState(String parm) { this.state = parm; } public void changeState() { state = state.equals("false") "true" : "false"; mediator.state2Changed(); } public String toString() { return state; } } package javaee.architect.Mediator; public class ConcreteMediator implements MediatorIF { ColleagueIF colleague1; ColleagueIF colleague2; public void registerColleague1(ColleagueIF parm) { this.colleague1 = (ConcreteColleague1) parm; } public void registerColleague2(ColleagueIF parm) { this.colleague2 = (ConcreteColleague2) parm; } public void state1Changed() { String s = (colleague2.toString().equals("true")) "false" : "true"; ((ConcreteColleague2) colleague2).setState(s); } public void state2Changed() { boolean b = (colleague1.toString().equals("true")) false : true; ((ConcreteColleague1) colleague1).setState(b); } } package javaee.architect.Mediator; public interface MediatorIF { //Interface for communicating with colleagues public void registerColleague1(ColleagueIF parm); public void registerColleague2(ColleagueIF parm); public void state1Changed(); public void state2Changed(); }
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5: Design Patterns
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UML for the Memento pattern
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The Memento pattern s intent is to capture and internalize an object s internal state so that objects can be restored to this state later. It must do this without violating encapsulation. The Memento pattern is also known as Token. The UML is shown in Figure 5-18.
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Benefits Following is a list of benefits of using the Memento pattern:
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n It preserves encapsulation boundaries. n It simplifies the originator.
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Applicable Scenarios The following scenarios are most appropriate for the Memento pattern:
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n A snapshot containing enough information regarding the state of an object
can be saved so that it can be restored to the complete state using the snapshot information later.
n Using a direct interface to obtain the state would impose implementation
details that would break the rules of encapsulation for the object.
Java EE Technology Feature The Java EE technology feature associated with the Memento pattern is EntityBeans using Bean-Managed Persistence (BMP). Example Code The following example Java code demonstrates the Memento pattern:
package javaee.architect.Memento; public class MementoPattern { public static void main(String[] args) { System.out.println("Memento Pattern Demonstration."); System.out.println("-------------------------------"); // Run the caretaker
Identify the Most Appropriate Design Pattern
Caretaker.run(); System.out.println(); } } package javaee.architect.Memento; public class Caretaker { public static void run() { // Create originator and set initial values. System.out.println("Creating originator and setting initial values."); Originator originator = new Originator(); originator.setState(true); originator.setName("The Originator"); // Create memento. System.out.println("Creating memento."); Memento memento = originator.createMemento(); System.out.println(originator); // Change originator values. System.out.println("Changing originator values."); originator.setState(false); originator.setName("To be undone."); System.out.println(originator); // Recover state from memento. System.out.println("Recovering originator values from memento."); originator.recoverFromMemento(memento); System.out.println(originator); } } package javaee.architect.Memento; public class Memento { private boolean state; private String name; Memento(boolean parm1, String parm2) { this.state = parm1; this.name = parm2; } boolean getState() {return this.state;} String getName() {return this.name;} } package javaee.architect.Memento; public class Originator { private boolean state; private String name; private String other; // Create memento, save critical data in it.
5: Design Patterns
public Memento createMemento() { return new Memento(state, name); } // Recover critical data from memento. public void recoverFromMemento(Memento memento) { this.state = memento.getState(); this.name = memento.getName(); } public void setState(boolean parm) { this.state = parm; } public void setName(String parm) { this.name = parm; } public String toString() { return "Originator.toString() state="+state+", name="+name; } }
Observer
The Observer pattern s intent is to define a one-to-many dependency so that when one object changes state, all its dependents are notified and updated automatically. Java provides support for implementing the Observer pattern via the java.util. Observer interface and the java.util.Observable class. The Observer pattern is also known as Dependents or Publish-Subscribe. The UML is shown in Figure 5-19.
Benefits Following is a list of benefits of using the Observer pattern:
n It abstracts the coupling between the subject and the observer. n It provides support for broadcast-type communication.
Applicable Scenarios The following scenarios are most appropriate for the Observer pattern:
n A change to an object requires changing other objects, and the number of
objects that need to be changed is unknown.
n An object needs to notify other objects without making any assumptions
about the identity of those objects.
Identify the Most Appropriate Design Pattern
FIGURE 5-19
UML for the Observer pattern
Java EE Technology Feature and Java SE API Association The Java EE technology feature associated with the Observer pattern is the JMS (Java Message Server) Publish/Subscribe Model. The Java SE APIs associated with the Observer pattern are
n java.lang.Observable n java.lang.Observer
Example Code The following example Java code demonstrates the Observer pattern:
package javaee.architect.Observer; public class ObserverPattern { public static void main(String[] args) { System.out.println("Observer Pattern Demonstration."); System.out.println("-------------------------------"); // Constructing observers. System.out.println("Constructing observer1 and observer2."); ObserverIF observer1 = new ConcreteObserver(); ObserverIF observer2 = new ConcreteObserver(); // Constructing observable (subject). System.out.println("Constructing observerable (subject)."); ConcreteSubject subject = new ConcreteSubject(); // Add observer object references to the subject. System.out.println("Registering observers with subject."); subject.addObserver(observer1);
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