Event Handling in Java

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 • We are primarily interested in developing GUIs in Java and in Event Handling mechanisms of Java

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

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 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 …. 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 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 …. • 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 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 • 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

Event Handling in Java