Applets CS 3331 Fall 2009 1 Outline Basics of applets First applet HelloWorldApplet java More on applets Animation applet digital clock 2 What Are Applets An applet is a special Java program that can be embedded in HTML documents It is automatically executed by appletenabled web browsers In Java non applet programs are called applications 3 Application vs Applet Application Trusted i e has full access to system resources Invoked by Java Virtual Machine JVM java e g java HelloWorld Should contain a main method i e public static void main String Applet Not trusted i e has limited access to system resource to prevent security breaches Invoked automatically by the web browser Should be a subclass of class java applet Applet 4 Examples HelloWorld java public class HelloWord public static void main String args System out println Hello World HelloWorldApplet java 5 First Java Applet Java source in HelloWorldApplet java import java awt import java applet Applet public class HelloWorldApplet extends Applet public void paint Graphics g Dimension d getSize g setColor Color BLACK g fillRect 0 0 d width d height paint background g setFont new Font San serif Font BOLD 24 g setColor new Color 255 215 0 g drawString Hello world 60 40 g drawImage getImage getCodeBase Rabbit jpg 20 60 this 6 Graphics Coordinate 0 0 x height y width 7 Embedding Applet into HTML HTML source in HelloWorld html HelloWorld html html head title HelloWord title head body center applet code HelloWorldApplet class width 300 height 350 applet center hr a href HelloWorldApplet java The source a body html 8 Compiling and Running To compile javac HelloWorldApplet java Produces HelloWorldApplet class To run Open page HelloWorld htmlfrom web browser or Use appletviewer of JDK appletviewer HelloWorld html 9 Elements of Applets Superclass java applet Applet No main method paint method to paint the picture Applet tag applet applet code width and height 10 Outline Basics of applets First applet HelloWorldApplet java More on applets Animation applet digital clock 11 Framework Based Programming OO Frameworks Semi complete applications Provide the structure backbone and utilities for applications Often domain specific Inversion of control Examples applets GUI frameworks etc 12 The Class Applet java awt Panel java applet Applet destroy init start stop 13 The Life Cycle of Applet init Called exactly once in an applet s life Called when applet is first loaded which is after object creation e g when the browser visits the web page for the first time Used to read applet parameters start downloading any other images or media files etc 14 Applet Life Cycle Cont start Called at least once Called when an applet is started or restarted i e whenever the browser visits the web page stop Called at least once Called when the browser leaves the web page 15 Applet Life Cycle Cont destroy Called exactly once Called when the browser unloads the applet Used to perform any final clean up init start stop destroy start 16 Outline Basics of applets First applet HelloWorldApplet java More on applets Animation applet digital clock 17 Animation Applet Digital Clock import import import import java awt java awt event javax swing java util Calendar public class DigitalClock extends java applet Applet Fields Methods An applet must be a subclass of java applet Applet 18 Program Structure java applet Applet DigitalClock 1 start void stop void paint g Graphics void use java awt use javax swing Timer java util Calendar use java awt event 19 Methods for Digital Clock public void start invoked when entering the web page that contains the applet public void stop invoked when leaving the web page that contains the applet public void paint Graphics g paint the picture 20 Field Declarations protected Timer timer protected Font font new Font Monospaced Font BOLD 48 protected Color color Color GREEN 21 Object Initialization public DigitalClock timer new Timer 1000 createTimerTickHandler protected ActionListener createTimerTickHandler return new ActionListener public void actionPerformed ActionEvent event repaint 22 The start and stop Methods public void start timer start public void stop timer stop Start and stop the timer Stopped timer will not consume CPU time 23 The paint Method public void paint Graphics g Calendar calendar Calendar getInstance int hour calendar get Calendar HOUR OF DAY int minute calendar get Calendar MINUTE int second calendar get Calendar SECOND g setFont font g setColor color g drawString hour 10 hour 10 minute 10 minute 10 second 10 second 10 10 60 24 Who Calls the paint method Timer ticks and calls ActionListener actionPerformed ActionListener actionPerformed calls DigitalClock repaint DigitalClock repaint calls DigitalClock paint The paint method is usually not called directly 25 Drawing Strings g drawString A sample string x y 26 HTML Source DigitalClock html html head title Digital Clock Applet title head body bgcolor black h1 The Digital Clock Applet h1 p applet code DigitalClock class width 250 height 80 applet p hr a href DigitalClock java The source a body html 27 The java awt Color Class Instances of the Color class represent colors new Color r g b where r g b are the values of the red green and blue components respectively They are in the in the range of 0 to 255 Predefined constants BLACK ORANGE YELLOW BLUE GREEN PINK CYAN LIGHTGRAY RED ARKGRAY MAGENTA WHITE 28 The java awt Font Class Fonts are specified with three attributes font name Serif Sans serif Monospaced Dialog DialogInput TimesRoman Helvetica Courier font style PLAIN BOLD ITALIC Styles can be combined Font BOLD Font ITALIC font size a positive integer A font can be created as follows new Font name style size 29 Exercise Write an applet class named CuckooDigitalClock that behaves like DigitalClock except that it cuckoos at every hour by playing an audio file cuckoo au Hints To play an audio file use play getCodeBase cuckoo au To get the current time use the class java util Calendar e g Calendar calendar Calendar getInstance int hour calendar get Calendar HOUR OF DAY int minute calendar get Calendar MINUTE int second calendar get Calendar SECOND 30