Embed Size (px)
Citation preview
CSE470 Software Engineering Fall 2000 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
CSE470 Software Engineering Fall 2000 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.javaThis creates a file with .class extension, programfile.class
• Create .html file for appletviewer, run appletviewer– appletviewer programfile.htmlA sample programfile.html is given on the next slide
CSE470 Software Engineering Fall 2000 3
Sample .html file for Appletviewer
<body><html><applet code = “programfile.class” width=200 height=200>
</applet></html></body>
CSE470 Software Engineering Fall 2000 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
CSE470 Software Engineering Fall 2000 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
CSE470 Software Engineering Fall 2000 6
Delegation Event Model
Event Source Event Listener
Ex: Button, Canvas, Text Box
An object of a class that will implement methods which will respond to events
Register with source
Event Object e
CSE470 Software Engineering Fall 2000 7
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 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 ……*/ }
}
CSE470 Software Engineering Fall 2000 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) { …..}
}
CSE470 Software Engineering Fall 2000 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)
CSE470 Software Engineering Fall 2000 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
CSE470 Software Engineering Fall 2000 12
Example -- Simple Action Eventpublic 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 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.
