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Transcript 
Internet Software
Development
Applets
Paul J Krause
Contents
 Applet
Basics
 Passing parameters to applets
 Simple graphics operations
 Animating applets
Hello.java
// Filename: Hello.java
public class Hello {
public static void main(String args) {
System.out.println(“Hello fom Venus”);
}
}
Applets
 Java
applications are executed from the
command line
 Applets are small applications that can be
embedded in html pages
 They can then be downloaded via the
world wide web and invoked in a browser
Template for an Applet
 Subclass
one of the Java foundation
classes:


java.applet.Applet;
javax.swing.JApplet;
 It’s
behaviour is invoked in a “paint”
method, not a “main” method
// File: MyApplet.java
import java.awt.*;
import java.applet.Applet;
public class MyApplet extends Applet {
public void paint(Graphics g) {
// Do something interesting
}
}
Viewing Area of Applet
(0,0)
x
These dimensions are set in the HTML file
height
y
width
Contract for the paint method
 Dimensions
of the region on the Web page
assigned to the applet are set within an
<applet> tag in an HTML file
 The paint( ) method defines the
appearance of the applet within this
rectangular region
 The paint( ) method is invoked when
the applet is initially loaded in the browser
(or applet viewer)
HelloFromVenus.java
import java.awt.*;
import java.applet.Applet;
public class HelloFromVenus extends Applet {
public void paint(Graphics g) {
Dimension d = getSize( );
// Gets the size of the rectangle allocated to an
// instance of this applet
}
}
painting the background
import java.awt.*;
import java.applet.Applet;
public class HelloFromVenus extends Applet {
public void paint(Graphics g) {
Dimension d = getSize( );
g.setColor(Color.black);
g.fillRect(0, 0, d.width, d.height);
}
}
painting the text
public void paint(Graphics g) {
Dimension d = getSize( );
g.setColor(Color.black);
g.fillRect(0, 0, d.width, d.height);
// set font style and size:g.setFont(new Font(“Sans-serif”, Font.BOLD, 24);
// change colour on paint brush:g.setColor(255, 215, 0); // note different way!
g.drawString(“Hello from Venus!”, 40, 25);
}
finally add the image
public void paint(Graphics g) {
Dimension d = getSize( );
g.setColor(Color.black);
g.fillRect(0, 0, d.width, d.height);
g.setFont(new Font(“Sans-serif”, Font.BOLD, 24);
g.setColor(255, 215, 0);
g.drawString(“Hello from Venus!”, 40, 25);
g.drawImage(getImage(getCodeBase( ),
“
Venus.gif”, 20, 60, this);
}
Embedding Applets in HTML
< applet code = bytecode-filename
width = pixels
height = pixels >
</applet>
<HTML>
<HEAD>
<TITLE>
HelloFromVenus Applet
</TITLE>
</HEAD>
<BODY BGCOLOR=BLACK TEXT=white>
<CENTER>
<H2>Here is the <EM>Hello From Venus</EM> Applet</H2>
<APPLET CODE="HelloFromVenus.class" WIDTH=300
HEIGHT=350>
</APPLET>
</CENTER>
&nbsp; &nbsp; &nbsp; &nbsp; Venus photo courtesy of NASA.
</BODY>
</HTML>
Parameter passing
 To
make applets reusable in different
contexts, we need to be able to pass
parameters from the html files
 Can do this by using:


<param name> tag in html, and
getParameter(String) method in the
applet
in the html file
< applet code = bytecode-filename
width = pixels
height = pixels >
<param name=String value=String>
…
</applet>
in the applet
String input = getParameter(String);
Example
 In

 In

the html file:
<param name=“text” value=“Web Technology”>
the applet
textV = getParameter(“text”);
Initialisation of an applet





Initialisation of an applet is not done in the
Constructor method, but in an init( ) method
Why?
An applet context interprets the <applet> tag in
an html file
The applet context first constructs an instance of
the applet, then interprets the remaining
parameters in the <applet> tag
The init( ) method is invoked after the
information in the <applet> tag has been
processed
import java.awt.*;
import java.applet.*;
public class Font02 extends Applet {
String tmpSt, textV;
int sizeV;
public void init( ) {
textV = getParameter("text");
tmpSt = getParameter("size");
sizeV = Integer.parseInt(tmpSt)
}
public void paint(Graphics gg) {
…}
}
<html>
<head>
<title>Font Rotation and Translation</title>
</head>
<body style="font-size:18pt; text-align:center;font-weight:bold">
Font Rotation and Translation<br /><br />
<applet code="Font02.class" width=450 height=400>
<param name="text" value="....Web Technology">
<param name="size" value="18">
</applet>
</body>
</html>
shifting graphical objects
 To

translate(xpos, ypos);
 To

translate a graphical object, use:
rotate a graphical object, use:
rotate(angle_in_radians);
public void paint(Graphics gg) {
int ii;
Graphics2D g = (Graphics2D) gg;
g.setFont( new Font("Arial", Font.BOLD, sizeV) );
g.setColor(Color.blue)
g.translate(200, 200);
for(ii = 1; ii <= 16; ii++)
{
g.rotate(Math.PI/8.0);
g.drawString( textV, 20, 0);
}
public void paint(Graphics gg) {
int ii;
Graphics2D g = (Graphics2D) gg;
g.setFont( new Font("Arial", Font.BOLD, sizeV) );
g.translate(200, 200);
for(ii = 1; ii <= 16; ii++)
{
g.rotate(Math.PI/8.0);
g.setColor( new Color( (int) (Math.random( )*256),
(int) (Math.random( )*256),
(int) (Math.random( )*256) ));
g.drawString( textV, 20, 0);
}
Animated applets
 Create
a thread for an instance of the
applet
 paint( ) the applet into the applet
context
 Each time the relevant graphical
properties of the applet are changed,
repaint( ) the applet
Applet Lifecycle
 init()
is invoked when the applet is
loaded
 start() is invoked when the page
containing the applet has been entered
 stop() is invoked when the page is left
 destroy() is invoked when the page is
discarded
Font03.java
import java.awt.*;
import java.applet.*;
public class Font03 extends Applet implements Runnable {
protected Thread paintThread = null;
protected String tmpSt, textV;
protected int sizeV;
protected double rotation=0.0;
public void init( ) { // as before
}
}
starting and stopping
public void start( ) {
if (paintThread == null) {
paintThread = new Thread(this);
paintThread.start( );
}
}
public void stop( ) {
paintThread = null;
}
running the applet
public void run( ) {
while (Thread.currentThread( ) == paintThread) {
repaint( );
try {
Thread.currentThread( ).sleep(100);
rotation += Math.PI/8.0;
}
catch(InterruptedException e) { }
}
}
public void paint(Graphics gg) {
int ii;
Graphics2D g = (Graphics2D) gg;
g.setFont( new Font("Arial", Font.BOLD, sizeV) );
g.translate(200, 200);
g.rotate(rotation);
g.setColor( new Color( (int) (Math.random( )*256),
(int) (Math.random( )*256),
(int) (Math.random( )*256) ));
g.drawString( textV, 20, 0);
}
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