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Event Handling in Java. CSE 470 - Software Engineering Spring 2000 By Prasad. Introduction to Java. Java, platform independent language Two types of programs in Java Applets (embedded in web pages) Command line programs

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event handling in java

Event Handling in Java

CSE 470 - Software Engineering

Spring 2000

By Prasad

introduction to java
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
to compile and run java applets
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
running java applets continued
Running Java applets continued ..

<body>

<html>

<applet code = “programfile.class” width=200 height=200>

</applet>

</html>

</body>

event model of java
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

event listeners
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 ) { ……}

}

event listeners contd
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) { …..}

}

event object
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

event registration
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
example simple action event
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");

}

}

}

}

references
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
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