Transcript Intro_to_Java_6

Object-Oriented
Programming with Java
Lecture 6
Using the Java Event Model
Using the Java Event Model
Contents
 Quick Review of the Java Event model
 Using this model to notify Customers that
Pizzas are ready from a Bakery
 This example is taken from JavaBeans by
Example, by Henri Jubin, Prentice Hall
Java Event Model
Event Source
Register Event Listener
Event
Object
Fire Event
Event Listener
Chili PizzaExpress
EventObject
«interface»
OrderListener
source
getSource()
toString()
Bakery
fires
PizzaEvent
registers with
addOrderListener()
removeOrderListener()
sendMessage(PizzaEvent)
invokes notifications in
pizzaStatus(evt)
passed to
0..*
0..*
Customer
iNumber
iSliceNumber
run( )
PizzaEvent.java
import java.util.EventObject;
EventObject
source
getSource()
toString()
PizzaEvent
public class PizzaEvent extends EventObject
{
public PizzaEvent(Object aSource) {
super(aSource);
}
}
OrderListener.java
import java.util.EventListener;
EventListener
OrderListener
pizzaStatus
public interface OrderListener extends
EventListener {
public void pizzaStatus(PizzaEvent anEvent);
}
Bakery.java
Bakery
addOrderListener
removeOrderListener
sendMessage(PizzaEvent)
public class Bakery
public Bakery( ) {
// constructor of a Bakery instance
}
public addOrderListener( eL ) {
// inserts OrderListeners in some
// structure
}
public removeOrderListener( eL ) {
// deletes OrderListeners from that
// structure
}
private void sendMessage( evt ) {
// broadcast evt somehow
}
Properties of Bakery.java
import java.lang.Thread;
import java.util.*;
public class Bakery implements Runnable {
private Vector iCustomers = new Vector( );
private Thread iThread;
// methods go here…
}
Constructor for Bakery
public Bakery ( ) {
iThread = new Thread(this);
iThread.start( );
}
// When a new instance of Bakery is created,
// a flow of control owned by it is started.
The main flow of control
public void run( ) {
while(true) {
iThread.sleep(4000);
PizzaEvent event = new PizzaEvent(this);
sendMessage(event);
}
}
// a Bakery broadcasts a message that Pizza is ready
// every 4 seconds
Adding and removing
Listeners
public void addOrderListener(OrderListener aListener) {
iCustomers.addElement(aListener);
}
// Remember iCustomers is a Vector field in Bakery
public void removeOrderListener(OrderListener aListener) {
iCustomers.removeElement(aListener);
}
Broadcasting the message
private void sendMessage(PizzaEvent anEvent) {
Vector v;
v = iCustomers.clone( );
for (int i = 0; i<v.size( ); i++) {
OrderListener ol = v.elementAt(i);
ol.pizzaStatus(anEvent); // implement in Customer
}
System.out.println(“Pizza ready …”);
}
Summary for Sources
 Record
all the references to Listener
Objects in a “Vector”
 Register Listeners by adding their name to
the Vector
 Unregister Listeners by removing their
name from the Vector
 Step through the elements of the Vector to
notify all the Listeners
The Story so Far
EventObject
«interface»
OrderListener
source
getSource()
toString()
Bakery
fires
PizzaEvent
registers with
addOrderListener()
removeOrderListener()
sendMessage(PizzaEvent)
invokes notifications in
pizzaStatus(evt)
passed to
0..*
0..*
Customer
iNumber
iSliceNumber
run( )
The Customer Class
 A Customer
also has its own flow of
control
public class Customer implements OrderListener,
Runnable {
private int iNumber;
// identify customer
private boolean iHaveSlice;
// something to eat?
private Thread iThread;
// identifiy flow of control
private Randon iRandom;
// gaps between bites
private int iSliceNumber;
// Slices eaten
…}
Construct a Customer
public Customer(int aNumber) {
iNumber = aNumber;
iRandom = new Random(aNumber);
iThread = new Thread(this);
iThread.start( );
}
// Construct a Customer with a specified identifier, and
// start its own flow of control
Making your Customer Run
public void run( ) {
while(true) {
if (iHaveSlice) {
for (int bites=0; bites<4; bites++) {
System.out.println(“customer: “ + iNumber +
bites + “ slice:” + iSliceNumber);
iThread.sleep(iRandom.nextFloat( ) * 3000);
}
iHaveSlice = false;
iThread.suspend( );
}
}
// Takes 4 bites, with a rest between each, then
}
// waits for some more Pizza.
Response to PizzaEvents
 Remember,
we invoked a method called
“pizzaStatus” when we broadcast
messages from the Bakery. Customer
must implement this:
public void pizzaStatus(PizzaEvent anEvent) {
if ( ! iHaveSlice) {
iHaveSlice = true;
iSliceNumber++;
iThread.resume( );
}
Warning

These slides have simplified the implementation
a little bit
 We have missed out:



Explicit type conversions;
“Synchronisation” of critical sections in threads
The full implementation can be found on the
CSM-15 Web site
 This is taken from JavaBeans by Examples,
Henri Jubin, Prentice Hall
Running the Bakery
public class TestApp {
public static void main(String args[ ]) {
TestApp t = new TestApp( );
}
public TestApp( ) {
Bakery b = new Bakery( );
Customer c1 = new Customer( 1 );
Customer c2 = new Customer( 2 );
b.addOrderListener( c1 );
b.addOrderListener( c2 );
}
}
Summary
 We
have explored a simple example of a
general Notifier-Observer design pattern
 Everything in this example is available in
the Java 2 Software Development Kit
 The trick has been to use a design pattern
that allows as many Observers
(Customers, in our case) to be added as
required