Introduction to Java Programming

1
Introduction to Java Programming

• An object-oriented, platform independent language
• Two types of programs in Java
• Applets (run from web browsers)
• Command line programs
• Primary focus of this lab
• Event Handling mechanisms of Java
• Developing GUI prototypes in Java

2
To Compile and run Java Applets

• Include path
• setenv PATH PATH/usr/java1.2/bin in your
  .personal file
• Create java program file programfile.java using
  any editor
• Compile Java program
• javac programfile.java
• This creates a file with .class extension,
  programfile.class
• Create .html file for appletviewer, run
  appletviewer
• appletviewer programfile.html
• A sample programfile.html is given on the next
  slide

3
Sample .html file for Appletviewer

• ltbodygt
• lthtmlgt
• ltapplet code programfile.class width200
  height200gt
• lt/appletgt
• lt/htmlgt
• lt/bodygt

4
Event Handling in Java

• Graphical User Interfaces are event-driven
• To process a GUI event, a programmer should
• register an event listener
• implement an event handler
• Number of different event classes and
  event-listener interfaces defined in the package
  java.awt.event

5
Abstract Interfaces, Delegation

• Event-listener interfaces define only abstract
  methods
• Programmer must define all methods of an
  interface in a class that implements the
  interface
• Event-listener objects for an event are created
  from the class that implements the required
  interface
• Event handling model of Java is known as
  delegation event model because event processing
  is delegated to particular objects

6
Delegation Event Model
Register with source
Event Source
Event Listener
Event Object e
Ex Button, Canvas, Text Box
An object of a class that will implement methods
which will respond to events
7
Delegation Event Model

• Key Components of the delegation event model
• Event Source (usually GUI object)
• Event Listener object
• Event object
• Event registration

8
Event Listeners

• Some Event Listener interfaces of java.awt.event
• ActionListener
• MouseListener
• MouseMotionListener
• Examples of classes to implement interfaces
• Class mylistener implements ActionListener
• / The following method is called when a Button
  is
• pressed. /
• public void actionPerformed ( ActionEvent e )
  /event handler code goes here /

9
Event Listeners (contd.)

• Example of a listener for multiple events
• Class mylistener implements MouseListener
• / Following method is called when the mouse
  button is clicked /
• public void mouseClicked(MouseEvent evt)
  .
• / Called when the mouse button is
  depressed /
• public void mousePressed(MouseEvent evt)
  ..
• / Called when the mouse button is released /
• public void mouseReleased(MouseEvent evt)
  ..

10
Event Objects

• Event Objects carry information about the event
• Event Listeners obtain information from Event
  Sources through Event Objects
• Example MouseEvent Evt
• Evt.getX() --- gets the X-coordinate of mouse
  click
• Evt.getY() --- gets the Y-coordinate of mouse
  click
• Evt.getSource() --- gets the source object of the
  mouse click event (like a button)

11
Event Registration

• A Listener object has to register itself with an
  Event Source to catch events on the source
• Example
• Button1.addActionListener( new mylistener1() )
• Button1.addMouseListener( new mylistener2() )
• Multiple Listeners can listen to a single event
  source (for example, a KeyListener and a
  MouseListener could both listen to the same
  source)
• A single Listener can listen to multiple event
  sources

12
Example -- Simple Action Event

• public class SimpleEvent extends Applet
• Button button1 new Button("Press me")
• public void init()
• this.add(button1)
• button1.addActionListener(new mylistener())
• Class mylistener implements ActionListener
• public void actionPerformed(ActionEvent e)
• if(e.getSource() button1)
• button1.setLabel("Again")

13
References

• G. Cornell and C. Horstmann, Core Java, The
  Sunsoft Press Java Series, Second Edition, 1997
• D. Joshi, L. Lemay, and C. Perkins, Teach
  yourself Java in Café in 21 days, Sams.net
  publishing, 1996
• The Java Tutorial
• http//java.sun.com/docs/books/tutorial/index.htm
• D. Geary and A. McClellan, Graphic Java, The
  Sunsoft Press Java Series, 1997
• Deitel Deitel, Java How To Program Third
  Edition, Prentice Hall Inc.