1. Component-Based Software Engineering
The Java Event Model
Paul Krause

2. Callbacks and Events
Contents
Callbacks
Events
Events in Java

3. Problems Pauls Pictures Pauls Documents Pauls Sums Pauls Pictures
Pauls Homework
Pauls Pictures
Pauls Documents
Pauls Sums
Pauls Homework
Pauls ToDo Lists

4. Window Library Client File System But I need to tell you I’m in charge
something!
I’m in charge
here, guys!
My Documents
Reports
Papers
Presentations
Client
File System
Slide Shows
Calls

5. Callbacks
Callbacks are used in procedural libraries when they need to handle asynchronous events
The alternative is for the client to continuously poll the library for events
This is inefficient, especially if a large number of library components are registered with a client
But the use of callback means a client may observe “intermediate” states of a library
“Classically” a library’s operations would run to completion before returning control

6. Call Sequence Client aUser Library Client installs callback
Third party calls library
“I need to tell you something!”
Library invokes callback
“What’s happened?”
Callback queries library
“He’s changed a name!”
“That’s cool”
Callback returns
Library returns

7. Directory Service public class Directory {
public void addEntry(String name, File file) {
// pre name != “” and file != null
// post File file = map.get(name) }
public void removeEntry(String name) {
// pre name != “”
// post map.get(name) = nil
public void registerNotifier(Notifier n) {
// pre n != nil
// post n registered, will be called on addEntry and removeEntry

8. Callbacks and Events
Contents
Callbacks
Events
Events in Java

9. Events
An abstraction of Callback that is applicable to “federations” of interacting components
The firing of an event is a way of one object telling one or more other recipients that something interesting has happened
The sender fires an event
A recipient is called a listener and handles the event

10. Register Event Listener
Java Event Model
Event Source
Register Event Listener
Fire Event
Event
Object
Event Listener

11. Event Objects
Encapsulates information specific to an instance of an event
E.g. a “mouse click” event may contain:
The position of the mouse pointer
Which mouse button was clicked (and how many times)
The event object is passed as a parameter to the event notification method

12. Event Listeners
These are objects that need to be notified when a certain event occurs
Event notifications are made through method invocations in the listening object
The event object is passed as a parameter
The event source must know which listener object(s) to call
This information is contained in an event-listener interface

13. Event Sources Objects that fire events
Implement methods that allow listeners to:
Register their interest in the events they generate;
Unregister their interest in the events they generate.
Multicast event delivery enables an event to be fired to a number of event-listeners

14. invokes notifications in
Summary
EventObject
source
getSource()
toString()
passed to
fires
EventSource
addListener()
removeListener()
EventListener
notification(evt)
registers
0..*
invokes notifications in
0..*

15. Callbacks and Events
Contents
Callbacks
Events
Events in Java

16. Java Events
An “Event” encapsulates information about some state change in its source
Pressing a button
Entering text in a text field
Moving a slider on a scroll bar, …
Events need not be generated by users:
Expiration of a timer
Arrival of incoming data, …

17. Event Sources An event source must:
Provide methods to enable “listeners” to register and unregister interest in its events
Fire the events, of course!
“Send” each event to all listeners who are interested in that type of event
For an event of Type, and listener e1:
public void
addTypeListener(TypeListener e1);
removeTypeListener(TypeListener e1);

18. Listeners Have three responsibilities:
Register for events of a certain Type
Implement an interface to “receive” events of a certain Type
Unregister if they no longer wish to receive events of Type

19. Events EventObject: AWTEvent: EventObject(Object source)
Object getSource( )
String toString( )
AWTEvent:
AWTEvent(Object source, int id)
int getID( )
Object
EventObject
AWTEvent

20. Example: Temperature Converter
// Event Source
convertTemp = new JButton("Convert..."); …
// Listen to events from Convert button.
convertTemp.addActionListener(this);
// Implementation of ActionListener interface.
public void actionPerformed(ActionEvent event) {
// Parse degrees Celsius as a double and convert to Fahrenheit.
int tempFahr = (int)((Double.parseDouble(tempCelsius.getText()))
* );
fahrenheitLabel.setText(tempFahr + " Fahrenheit"); }
//v 1.3
// Example taken from the Sun website: //
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
// This example demonstrates the use of JButton, JTextField
// and JLabel.
public class CelsiusConverter implements ActionListener {
JFrame converterFrame;
JPanel converterPanel;
JTextField tempCelsius;
JLabel celsiusLabel, fahrenheitLabel;
JButton convertTemp;
// Constructor
public CelsiusConverter() {
// Create the frame and container.
converterFrame = new JFrame("Convert Celsius to Fahrenheit");
converterFrame.setSize(40, 40);
converterPanel = new JPanel();
converterPanel.setLayout(new GridLayout(2, 2));
// Add the widgets.
addWidgets();
// Add the panel to the frame.
converterFrame.getContentPane().add(converterPanel, BorderLayout.CENTER);
// Exit when the window is closed.
converterFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// Show the converter.
converterFrame.pack();
converterFrame.setVisible(true); }
// Create and add the widgets for converter.
private void addWidgets() {
// Create widgets.
tempCelsius = new JTextField(2);
celsiusLabel = new JLabel("Celsius", SwingConstants.LEFT);
convertTemp = new JButton("Convert...");
fahrenheitLabel = new JLabel("Fahrenheit", SwingConstants.LEFT);
// Listen to events from Convert button.
convertTemp.addActionListener(this);
// Add widgets to container.
converterPanel.add(tempCelsius);
converterPanel.add(celsiusLabel);
converterPanel.add(convertTemp);
converterPanel.add(fahrenheitLabel);
celsiusLabel.setBorder(BorderFactory.createEmptyBorder(5,5,5,5));
fahrenheitLabel.setBorder(BorderFactory.createEmptyBorder(5,5,5,5));
// Implementation of ActionListener interface.
public void actionPerformed(ActionEvent event) {
// Parse degrees Celsius as a double and convert to Fahrenheit.
int tempFahr = (int)((Double.parseDouble(tempCelsius.getText()))
* );
fahrenheitLabel.setText(tempFahr + " Fahrenheit");
// main method
public static void main(String[] args) {
// Set the look and feel.
try {
UIManager.setLookAndFeel(
UIManager.getCrossPlatformLookAndFeelClassName());
} catch(Exception e) {}
CelsiusConverter converter = new CelsiusConverter();

21. Summary
The components of a software product must interact to achieve a goal
In general, it is hard to identify a single component that has overall control
So a general model is for components to interact by triggering “Events” that other components register an interest in