Applets
26-Jul-16
Applets
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An applet is a program that is typically embedded in a
Web page and can be run from a browser
You need special HTML in the Web page to tell the
browser about the applet
You don’t need to supply a main method; the browser
does that
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When you write an applet, you are writing only part of a
program
You supply certain methods that the browser calls
For security reasons, applets run in a sandbox: they
have no access to the client’s file system
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Applet Support
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Most modern browsers support applets if they have the
appropriate plugin
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The best support isn't a browser, but the standalone
program appletviewer
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You need a version of the plugin at least as new as the
version of Java you are trying to run
The plugin is usually installed automatically when you
install a new version of Java
appletviewer uses only the applets in a Web page, and
ignores everything else
If you want to reach the widest audience, you should
use Java 1.1, and AWT instead of Swing
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This is because Microsoft no longer supports Java
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What an applet is
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You write an applet by extending the class JApplet
JApplet is just a class like any other; you can even use
it in applications if you want
When you write an applet, you are only writing part of
a program; the browser supplies the main method
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Once you understand how applets work, you can write a
program that function either as an applet or as an
application—just write a main method that calls the right
methods at the right time
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Such programs have the ugly name “appletcations”
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The genealogy of JApplet
java.lang.Object
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+----java.awt.Component
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+----java.awt.Container
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+----java.awt.Panel
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+----java.applet.Applet
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+----javax.swing.JApplet
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The simplest possible applet
TrivialApplet.java
import javax.swing.JApplet;
public class TrivialApplet extends JApplet { }
TrivialApplet.html
<applet
code="TrivialApplet.class"
width="150"
height="100">
</applet>
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The simplest reasonable applet
import java.awt.*;
import javax.swing.JApplet;
public class HelloWorld extends JApplet {
public void paint(Graphics g) {
g.drawString("Hello World!", 30, 30);
}
}
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Applet methods
public void init ()
public void start ()
public void stop ()
public void destroy ()
public void paint (Graphics)
Also:
public void repaint()
public void update (Graphics)
public void showStatus(String)
public String getParameter(String)
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Why an applet works
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You write an applet by extending the class JApplet
Applet defines methods init( ), start( ), stop( ),
paint(Graphics), destroy( )
These methods do nothing--they are stubs
You make the applet do something by overriding
these methods
When you create an applet in BlueJ, it automatically
creates sample versions of these methods for you
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public void init ( )
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init() is the first method to execute
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init() is an ideal place to initialize variables
init() is the best place to define the GUI
Components (buttons, text fields, checkboxes,
etc.), lay them out, and add listeners to them
Almost every applet you ever write will have an
init( ) method
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start( ), stop( ) and destroy( )
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start() and stop( ) are used when the Applet is doing timeconsuming calculations that you don’t want to continue when the
page is not in front
public void start() is called:
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public void stop( ) is called:
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Right after init( )
Each time the page is loaded and restarted
When the browser leaves the page
Just before destroy( )
public void destroy( ) is called after stop( )
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Use destroy() to explicitly release system resources (like threads)
System resources are usually released automatically
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Methods are called in this order
init()
start()
do some work
stop()
destroy()
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init and destroy are only called
once each
start and stop are called
whenever the browser enters and
leaves the page
do some work is code called by
your listeners
paint is called when the applet
needs to be repainted
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public void paint(Graphics g)
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Needed if you do any drawing or painting other than
just using standard GUI Components
Any painting you want to do should be done here, or
in a method you call from here
Painting that you do in other methods may or may not
happen
Never call paint(Graphics), call repaint( )
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repaint( )
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Call repaint( ) when you have changed something
and want your changes to show up on the screen
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You do not need to call repaint() when something in
Java’s own components (Buttons, TextFields, etc.)
You do need to call repaint() after drawing commands
(drawRect(...), fillRect(...), drawString(...), etc.)
repaint( ) is a request--it might not happen
When you call repaint( ), Java schedules a call to
update(Graphics g)
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update( )
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When you call repaint( ), Java schedules a call to
update(Graphics g)
Here's what update does:
public void update(Graphics g) {
// Fills applet with background color, then
paint(g);
}
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Sample Graphics methods
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A Graphics is something you can paint on
g.drawString(“Hello”, 20, 20);
Hello
g.drawRect(x, y, width, height);
g.fillRect(x, y, width, height);
g.drawOval(x, y, width, height);
g.fillOval(x, y, width, height);
g.setColor(Color.red);
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Painting at the right time is hard
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When you modify common components (JButtons, JLabels,
JTextFields, etc.), Java keeps the screen display up to date
When you paint on a Graphics object, you have to make your
changes appear on the screen
To help ensure your changes appear on screen, follow these
rules:
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Rule #1: Never call paint(Graphics g), call repaint( )
Rule #2: Do all your painting in paint, or in a method that is called
from paint
Rule #3: If you paint on any Graphics other than the Applet’s, call its
update method from the Applet’s paint method
Rule #4. Do your painting in a separate Thread
These rules aren't perfect, but they should help
If you follow these rules and the screen still doesn’t change, I
probably won’t be able to find the problem, either :-(
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Other useful JApplet methods
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System.out.println(String s)
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Works from appletviewer, not from browsers
Automatically opens an output window.
showStatus(String s) displays the String in the applet’s
status line.
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Each call overwrites the previous call.
You have to allow time to read the line!
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Applets are not magic!
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Anything you can do in an applet, you can do in an
application.
You can do some things in an application that you can’t
do in an applet.
If you want to access files from an applet, it must be a
“trusted” applet.
Trusted applets are beyond the scope of this course.
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Structure of an HTML page
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HTML
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HEAD
BODY
TITLE
(content)
Most HTML tags
are containers.
A container is
<tag> to </tag>
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HTML
<html>
<head>
<title> Hi World Applet </title>
</head>
<body>
<applet code="HiWorld.class"
width=300 height=200>
<param name="arraysize" value="10">
</applet>
</body>
</html>
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The <applet> tag
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The <applet ...> ... </applet> tag is what tells your browser
to put an applet here
<applet> has three required attributes:
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code="HiWorld.class" tells the browser where to find the code
 Alternatively, if your code is in a .jar file, you can say
archive="HiWorld.jar“
width="300" tells the browser how many pixels wide to make the applet
window
 A typical monitor resolution is 72 pixels/inch
height="200" tells the browser how many pixels high to make the applet
window
In a browser, you cannot resize the applet; but in appletviewer,
you can resize the applet
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<param name="arraysize" value="10">
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You can put parameters in the HTML page, to be read
by the applet
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Parameters go between <applet> and </applet>
All parameters are read by the applet as Strings
To read a parameter, use the applet method
public String getParameter(String name)
Example:
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String s = getParameter("arraysize");
try { size = Integer.parseInt (s) }
catch (NumberFormatException e) {…}
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The End
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