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n Multiple traversals of objects need to be supported in the collection. n A universal interface for traversing different structures needs to be provided
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Java EE Technology Feature and Java SE API Association The Java EE technology feature associated with the Command pattern is ValueListHandler in the J2EE Patterns Catalog. The Java SE APIs associated with the Iterator pattern are
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n java.util.Iterator n java.util.Enumeration
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The following example Java code demonstrates the Iterator
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package javaee.architect.Iterator; public class IteratorPattern { public static void main(String[] args) { System.out.println("Iterator Pattern Demonstration."); System.out.println("-------------------------------"); System.out.println("Building string array of books."); String[] books = new String[8]; books[0] = "PowerBuilder Developers Guide, 1994"; books[1] = "Informix Developers Guide, 1995"; books[2] = "Informix Universal Data Option, 1996"; books[3] = "SQL Server Developers Guide, 1999"; books[4] = "SilverStream Success I, 1999"; books[5] = "SilverStream Success II, 2000"; books[6] = "J2EE Unleashed, 2001"; books[7] = "Enterprise Architect Study Guide, 2002";
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// Turn the string array into a collection. System.out.println("Turning string array into a collection."); CollectionIF collection = new ConcreteCollection(books); // Get an iterator for the collection. System.out.println("Getting an iterator for the collection.."); IteratorIF iterator = collection.iterator(); // Iterate through and print each object in the list. System.out.println("Iterate through the list."); int i = 0; while (iterator.hasNext()) { System.out.println((++i)+" "+iterator.next()); } System.out.println(); } } package javaee.architect.Iterator; import java.util.*; public interface CollectionIF { // Interface for creating a // collection that needs iterating. public IteratorIF iterator(); public Collection elements(); } package javaee.architect.Iterator; import java.util.*; public class ConcreteCollection implements CollectionIF { // Builds an iterable list of elements private List list = new ArrayList(); public ConcreteCollection(Object[] objectList) { for (int i=0; i < objectList.length; i++) { list.add(objectList[i]); } } public IteratorIF iterator() { return new ConcreteIterator(this); } public Collection elements() { return Collections.unmodifiableList(list); } } package javaee.architect.Iterator;
5: Design Patterns
import java.util.*; public class ConcreteIterator implements IteratorIF { private List list; private int index; public ConcreteIterator(CollectionIF parm) { list = (List) parm.elements(); index = 0; } public Object next() throws RuntimeException { try { return list.get(index++); } catch (IndexOutOfBoundsException ioobe) { throw new RuntimeException("No Such Element"); } } public boolean hasNext() { return (index < list.size()) true : false; } } package javaee.architect.Iterator; public interface IteratorIF { // Interface for Iterators. public boolean hasNext(); public Object next(); }
Mediator
The Mediator pattern s intent is to define an object that encapsulates how a set of objects interacts. It helps to promote a looser coupling by keeping objects from referring to each other explicitly, therefore allowing any interaction to vary independently. The UML is shown in Figure 5-17.
Benefits Following is a list of benefits of using the Mediator pattern:
n It decouples colleagues. n It simplifies object protocols. n It centralizes control. n The individual components become simpler and much easier to deal with
because they do not need to pass messages to one another.
n The components do not need to contain logic to deal with their
intercommunication and are therefore more generic.
Identify the Most Appropriate Design Pattern
FIGURE 5-17
UML for the Mediator pattern
Applicable Scenarios The following scenarios are most appropriate for the Mediator pattern:
n A set of objects communicates in complex but well-defined ways. n Custom behavior distributed between several objects is required without
subclassing. It is commonly used structurally in message-based systems. The messages themselves are the means by which related objects are decoupled.
Example Code pattern:
The following example Java code demonstrates the Mediator
package javaee.architect.Mediator; public class MediatorPattern { public static void main(String[] args) { System.out.println("Mediator Pattern Demonstration."); System.out.println("-------------------------------"); // Construct mediator and colleagues System.out.println("Constructing mediator and colleagues."); MediatorIF mediator = new ConcreteMediator(); ColleagueIF colleague1 = new ConcreteColleague1(mediator); ColleagueIF colleague2 = new ConcreteColleague2(mediator); // Display colleague values. System.out.println("Displaying colleague states."); System.out.println("colleague1.toString()="+colleague1); System.out.println("colleague2.toString()="+colleague2); // Change state on colleague1 and the mediator
5: Design Patterns
// will coordinate the change with colleague2. System.out.println("Calling colleague1.changeState()"); ((ConcreteColleague1) colleague1).changeState(); // Display colleague values now. System.out.println("Displaying colleague states now."); System.out.println("colleague1.toString()="+colleague1); System.out.println("colleague2.toString()="+colleague2); // Change state on colleague2 and see what happens. System.out.println("Calling colleague2.changeState()"); ((ConcreteColleague2) colleague2).changeState(); // Display colleague values now. System.out.println("Displaying colleague states again."); System.out.println("colleague1.toString()="+colleague1); System.out.println("colleague2.toString()="+colleague2); System.out.println(); } } package javaee.architect.Mediator; public interface ColleagueIF { } package javaee.architect.Mediator; public class ConcreteColleague1 implements ColleagueIF { private MediatorIF mediator; // This colleague uses a boolean for it's state. private boolean state; public ConcreteColleague1(MediatorIF parm) { this.mediator = parm; this.mediator.registerColleague1(this); } public void setState(boolean parm) { this.state = parm; } public void changeState() { state = state false : true; mediator.state1Changed(); } public String toString() { return new Boolean(state).toString(); } } package javaee.architect.Mediator; public class ConcreteColleague2 implements ColleagueIF { private MediatorIF mediator; // This colleague uses a string for its state.
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