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Applets – Lecture 1, 2 & 3
Prepared by: Ahmad Ramin Rahimee
Assistant Professor
ICTI
1
What is an applet?

Applet: a Java program that can be inserted into a
web page and run by loading that page in a browser
 brings web pages to life with interactive content,
multimedia, games, and more


the feature of Java that is primarily responsible for
its initial popularity
users can run applets simply by visiting a web page
that contains an applet program (if they have the
Java runtime environment installed on their
computer)
2
Applet
3
APPLET…
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Loaded into a java-enabled program
(Netscape, Applet Viewer)
Applet is not a stand-alone program (no main
method), structured to run inside another
program (browser)
JVM runs applications, but not applets. An
applet can run as an application if it defines a
main().
Applets are usually event-driven.
4
APPLET…

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Does I/O through a GUI, only displays error
messages with stdout.
Applets subclass java.applet
Security - applets barred from reading/writing
files on the client, can’t load libraries, can’t
make network connections (except to its host),
can’t execute programs, etc...
5
Applet life cycle

browser visits page containing an applet
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browser goes away from that page
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browser calls init on that applet, once
browser calls start on that applet
browser calls start again on that applet
...
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browser shuts down
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start()
browser calls stop on that applet
browser comes back to that page

init()
browser calls destroy on the applet, once
do some work
stop()
destroy()
6
APPLET

Applet lifecycle: (methods overriden from Applet
class):
init(), start(), paint(g),stop(), destroy()

Loading an applet

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Leaving and returning to applet page
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when page is left, applet stops
upon page return, applet starts
Applet reloading/unloading
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an instance of the applet’s controlling class (Applet subclass)
created
applet initializes itself
applet starts running
applet performs final cleanup, before reloading
Browser quitting - (applet stops, performs final
cleanup)
7
Applet Skeleton
import java.awt.*;
import java.applet.*;
/*
<applet code="AppletSkel" width=300 height=100>
</applet>
*/
public class AppletSkel extends Applet {
// Called first.
public void init() {
// initialization
}
/* Called second, after init(). Also called whenever
the applet is restarted. */
public void start() {
// start or resume execution
}
// Called when the applet is stopped.
public void stop() {
// suspends execution
}
/* Called when applet is terminated. This is the last
method executed. */
public void destroy() {
// perform shutdown activities
}
// Called when an applet's window must be restored.
public void paint(Graphics g) {
// redisplay contents of window
}
}
8
Simple Applet Display Methods

To output a string to an applet, use drawString( ),
which is a member of the Graphics class. Typically, it
is called from within paint( ). It has the following
general form:
void drawString(String message, int x, int y)

Here, message is the string to be output beginning at
x,y.
In a Java window, the upper-left corner is location 0,0.
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Simple Applet Display Methods…
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To set the background color of an applet’s window, use
setBackground( ).
To set the foreground color (the color in which text is
shown, for example), use setForeground( ).
These methods are defined by Component, and they
have the following general forms:
void setBackground(Color newColor)
void setForeground(Color newColor)
A good place to set the foreground and background
colors is in the init( ) method.
Of-course, you can change these colors as often as
necessary during the execution of your applet.
10
Simple Applet Program

Here is a very simple applet that sets the background
color to cyan, the foreground color to red, and displays
a message that illustrates the order in which the init(
), start( ), and paint( ) methods are called when an
applet starts up.
import java.awt.*;
import java.applet.*;
public class Sample extends Applet{
String msg;
public void init() {
setBackground(Color.cyan);
setForeground(Color.red);
msg = "Inside init( ) --";
}
public void start() {
msg += " Inside start( ) --";
}
public void paint(Graphics g) {
msg += " Inside paint( ).";
g.drawString(msg, 10, 30);
}
}
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Simple Applet Program…

The applet generates the window shown here:
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HTML APPLET TAG (Element)
<applet code="..." width=xxx height=xxx ...>
...
</applet>
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Required Attributes
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CODE
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Designates the filename of the Java class file to load
Filename interpreted with respect to directory of current HTML page
(default) unless CODEBASE is supplied
WIDTH and HEIGHT
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Specifies area the applet will occupy
Values can be given in pixels or as a percentage of the browser window
width
13
Web page with an applet
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Put a page like this in your project's folder:
<HTML>
<HEAD>
<TITLE>My Applet Page</TITLE>
</HEAD>
<BODY>
<APPLET code="mypackage/MyApplet.class"
width=400 height=300> </APPLET>
</BODY>
</HTML>
14
Useful Applet Methods

getCodeBase, getDocumentBase

The URL of the:
Applet file - getCodeBase
HTML file - getDocumentBase
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getParameter
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getSize
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Retrieves the value from the associated HTML PARAM element
Returns the Dimension (width, height) of the applet
getGraphics
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Retrieves the current Graphics object for the applet
The Graphics object does not persist across paint invocations
15
Useful Applet Methods…
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showStatus
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getCursor, setCursor
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Defines the Cursor for the mouse, for example,
CROSSHAIR_CURSOR, HAND_CURSOR, WAIT_CURSOR
getBackground, setBackground
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Displays a string in the status line at the bottom of the browser
Gets/sets the background color of the applet
SystemColor class provides access to desktop colors
getForeground, setForeground
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Gets/sets foreground color of applet (default color of drawing
operations)
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Useful Applet Methods…
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getAudioClip(URL url, String fileName)
play
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Retrieves an audio file from a remote location and plays it
JDK 1.1 supports .au only. Java 2 also supports MIDI, .aiff and
.wav
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Sample Playing Audio Clip Program
import java.applet.Applet;
import java.applet.AudioClip;
public class NewApplet extends Applet {
public void start(){
AudioClip acl;
acl = this.getAudioClip(getDocumentBase(),"M1F1Alaw-AFsp.au");
acl.play();
}
}
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Useful Applet Methods…
Loading images in applet
 public Image getImage(URL url)
Loads and returns the image file at the given URL.
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public Image getImage(URL folder, String file)
Loads the image in the given folder with the given
name. Often used with getCodeBase().
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Requesting Repainting
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repaint() method is defined by AWT.
Causes AWT runtime system to execute paint()
method.
repaint() method has four forms:
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void repaint()
this method causes the entire window to be repainted.
void repaint(int left, int top, int width, int height)
this method causes a specific region in a window to be
repainted.
void repaint(long maxDelay)
Here maxDelay specifies the maximum number of milliseconds
that can elapse before calling repaint().
void repaint(long maxDelay, int x, int y, int width, int height)
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Summary
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Applet operations are restricted
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The init method
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Called only when applet loaded, not each time executed
This is where you use getParameter to read PARAM data
The paint method
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Applet cannot read/write local files, call local programs, or
connect to any host other than the one from which it was
loaded
Called each time applet is displayed
Coordinates in drawing operations are wrt top-left corner
Drawing images
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getImage(getCodeBase(), "imageFile") to “load”
drawImage(image, x, y, this) to draw
21
Exercises
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What is an applet? How does an applet differ from
applications?
Explain with an example how we implement an applet
into a webpage?
Explain how to set background area in applet?
What are the methods that controls an applet’s life
cycle?
22
Assignment
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Write a program to load an image to your applet.
Write a program to draw a home in to your applet
using Graphics methods.
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