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 These slides were created by S.P.N.Durga Prasad for CSE470, Spring 2000 and updated by Chirantana Sriram for CSE470, Fall 2000. CSE470 Software Engineering Fall 2000
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 CSE470 Software Engineering Fall 2000
Sample .html file for Appletviewer <body> <html> <applet code = “programfile.class” width=200 height=200> </applet> </html> </body> CSE470 Software Engineering Fall 2000
CSE470 Software Engineering Fall 2000 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 CSE470 Software Engineering Fall 2000
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 CSE470 Software Engineering Fall 2000
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 CSE470 Software Engineering Fall 2000
Delegation Event Model … Key Components of the delegation event model: Event Source (usually GUI object) Event Listener object Event object Event registration CSE470 Software Engineering Fall 2000
CSE470 Software Engineering Fall 2000 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 ……*/ } } CSE470 Software Engineering Fall 2000
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) { …..} } CSE470 Software Engineering Fall 2000
CSE470 Software Engineering Fall 2000 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) CSE470 Software Engineering Fall 2000
CSE470 Software Engineering Fall 2000 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 CSE470 Software Engineering Fall 2000
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"); CSE470 Software Engineering Fall 2000
CSE470 Software Engineering Fall 2000 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. CSE470 Software Engineering Fall 2000