==> Gossip.java <==
/**
 * Observer Design Pattern -- "Gossip" example for COM1204 Summer 2003.
 * This is the driver/tester class.
 * <br>Spaces and TalkerListeners are created and the latter register to
 * listen to certain spaces. Then the TalkerListeners talk and what
 * they hear is reported as a printout.
 *
 * @author Bob Futrelle
 * @version 0.1, 28 June 2003
 *
 */
 public class Gossip {
 	
 	public static void main(String[] args) {
 	
		// First, set up the spaces, talkers and register them
		Space outdoorTable = new Space("Outdoor table");
		Space insideTable = new Space("Inside table");
		Space inbetweenTable = new Space("Inbetween table");
		
		// Jing is the first talker, at the outdoor table.
 		TalkerListener jing = new TalkerListener("Jing", outdoorTable);
		// Jing registers to listen to two Spaces.
		outdoorTable.addTalkListener(jing);
		inbetweenTable.addTalkListener(jing);
		
		// Bob is the second talker, at the same table
 		TalkerListener bob = new TalkerListener("Bob", outdoorTable);
		// Bob also registers to listen to the same two Spaces.
		outdoorTable.addTalkListener(bob); 
		inbetweenTable.addTalkListener(bob);
		
 		TalkerListener elliott =
			new TalkerListener("Elliot, from the Matrix", inbetweenTable);
		// Elliott can't hear much, only himself.
		inbetweenTable.addTalkListener(elliott);	
		
		// Vorzdun monitors all; all herhisherherhis talk is telepathic
 		TalkerListener vorzdun = 
			new TalkerListener("Vorzdun from Planet 9",insideTable);
		// Of course, Vorzdun  is registered to listen to all Spaces.
		outdoorTable.addTalkListener(vorzdun); 
		inbetweenTable.addTalkListener(vorzdun);		
		insideTable.addTalkListener(vorzdun);	
		
		// Let the gossipping begin
		bob.says("I heard that there's a Matrix agent nearby.");
		jing.says("So what, we're all so Neo.");
		bob.says("Yeah, I guess you're right.");
		elliott.says("Which one of these guys is which?");
		bob.says("Did you hear that? I think we have an agent.");
		jing.says("Let's go up and away.");
		vorzdun.transmits("Leader: It is clear that these creatures are still quite primitive.");
 	} // main()
 	
 } // class Gossip
 
 // Output from the run above:
 /*
 Jing at the Outdoor table just heard: 
"I heard that there's a Matrix agent nearby."
from the place, the Outdoor table

Vorzdun from Planet 9 at the Inside table just heard: 
"I heard that there's a Matrix agent nearby."
from the place, the Outdoor table

Bob at the Outdoor table just heard: 
"So what, we're all so Neo."
from the place, the Outdoor table

Vorzdun from Planet 9 at the Inside table just heard: 
"So what, we're all so Neo."
from the place, the Outdoor table

Jing at the Outdoor table just heard: 
"Yeah, I guess you're right."
from the place, the Outdoor table

Vorzdun from Planet 9 at the Inside table just heard: 
"Yeah, I guess you're right."
from the place, the Outdoor table

Jing at the Outdoor table just heard: 
"Which one of these guys is which?"
from the place, the Inbetween table

Bob at the Outdoor table just heard: 
"Which one of these guys is which?"
from the place, the Inbetween table

Vorzdun from Planet 9 at the Inside table just heard: 
"Which one of these guys is which?"
from the place, the Inbetween table

Jing at the Outdoor table just heard: 
"Did you hear that? I think we have an agent."
from the place, the Outdoor table

Vorzdun from Planet 9 at the Inside table just heard: 
"Did you hear that? I think we have an agent."
from the place, the Outdoor table

Bob at the Outdoor table just heard: 
"Let's go up and away."
from the place, the Outdoor table

Vorzdun from Planet 9 at the Inside table just heard: 
"Let's go up and away."
from the place, the Outdoor table

Telepathic message:
"Leader: It is clear that these creatures are still quite primitive."
*/

==> Message.java <==
/**
 * The Message classes bundles the TalkerListener speaking and what they say.
 * This allows the recipient to find the place at which the message was generated.
 * <br>Observer Design Pattern -- "Gossip" example for COM1204 Summer 2003.
 *
 * @author Bob Futrelle
 * @version 0.1, 28 June 2003
 *
 */
 public class Message {
 	
	public TalkerListener talkerListener;
	public String whatTalkerSaid;
	
	public Message(TalkerListener talkerListener, String whatTalkerSaid){
		this.talkerListener = talkerListener;
		this.whatTalkerSaid = whatTalkerSaid;
	}
 	
 } // class Messagew

==> Space.java <==
import java.util.*;

/**
 * The Space class where messages are sent and which notifies registered listeners.
 * <br>Observer Design Pattern -- "Gossip" example for COM1204 Summer 2003.
 *
 * @author Bob Futrelle
 * @version 0.1, 28 June 2003
 *
 */
 public class Space {
 	
	public String name;
	public ArrayList registeredListeners;
	
	public Space(String name){ 
			this.name = name;
			registeredListeners = new ArrayList();			
	}
	
	/** Allows a TalkerListener to be placed on the registered listeners list.
	*/
	public void addTalkListener(TalkerListener listener) {
			registeredListeners.add(listener);
	}
	/** When a TalkerListener says something, its local space is informed.
	*/
	public void notifySpace(Message msg) {
		Iterator listeners = registeredListeners.iterator();
		while(listeners.hasNext()){
			((TalkerListener)listeners.next()).notifyMe(msg);
			}
	}

 } // class Space

==> TalkerListener.java <==
/**
 * The TalkerListener "talks" in a space and registers to "listen" to other spaces.
 * <br>Observer Design Pattern -- "Gossip" example for COM1204 Summer 2003.
 *
 * @author Bob Futrelle
 * @version 0.1, 28 June 2003
 *
 */
 
 public class TalkerListener {
 	
	public String name;
	public Space whereIAmAt;
	
	/** The TalkerListener has a name and a location
	*/
	public TalkerListener(String name, Space whereIAmAt){
			this.name = name;
			this.whereIAmAt = whereIAmAt;	
	}
	
	/** When the TalkerListener speaks, the local space is "notified".
	*/
	public void says(String said) {
		whereIAmAt.notifySpace(
			new Message(this,said));
	}
	
	/** When a space that the TalkerListener has registered with receives
	 * a message, it notifies the TalkerListener using this method.
	 */
	public void notifyMe(Message msg) {
		// Don't report what you yourself just said.
		if(msg.talkerListener != this) 
		System.out.println(name + " at the " + whereIAmAt.name + " just heard: \n"
		+ "\"" + msg.whatTalkerSaid + "\"\nfrom the place, the " +
		msg.talkerListener.whereIAmAt.name + "\n");
	}
	
	/** Tells us the contents of any telepathic message being sent.
	*/
	public void transmits(String telepathic) {
		System.out.println("Telepathic message:\n\"" +
		telepathic + "\"");
	}
 } // class TalkerListener