Event%20Handling%20in%20Java

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Event Handling in Java

• CSE 470 - Software Engineering
• Spring 2000
• By Prasad

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Introduction to Java ..

• Java, platform independent language
• Two types of programs in Java
• Applets (embedded in web pages)
• Command line programs
• We are primarily interested in developing GUIs in
  Java and in Event Handling mechanisms of Java

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To Compile and run Java Applets

• Include path
• setenv PATH PATH/usr/java1.2/bin in your
  .personal file
• To compile Java programs
• javac programfile.java
• To run Java applets
• appletviewer programfile.html
• A sample programfile.html is given in next slide

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Running Java applets continued ..

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

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Event Model of Java

• Java uses delegation event Model

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

• Event Listener Object has to implement a event
  listener interface.
• Examples of Event Listener interfaces
• Action Listener Interface
• Mouse Listener Interface
• Mouse Motion Listener Interface
• Examples
• Class mylistener implements ActionListener
• / The following procedure is called when a
  Button is
• pressed. /
• public void actionPerformed ( ActionEvent e )
  

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Event Listeners contd .

• Class mylistener implements MouseListener
• / 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)
  ..

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Event Object

• Event Objects carry information about the event.
• Event Listeners get information from Event
  sources thru Event Objects.
• Examples
• 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

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Event Registration .

• A Listener Object needs to register itself with
  an Event Source to be able to catch events on it.
• Example
• Button1.addActionListener( new mylistener1() )
• Button1.addMouseListener( new mylistener2() )
• Multiple Listeners can listen to a single event
  source
• A single Listener can listen to multiple event
  sources

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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")

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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