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Java applets are java programs that can be embedded in webpages
and run. So there are three steps in creating
applet applications:
(1) You first write and compile a java program.
(2) You create an html file that calls this
java program.
(3) You place this html file in a web-accessible location.
REFERENCES:
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A Simple Java Applet Program
The following Applet reads an image file called ``gifIcon.gif''
and print a label with the text "Hello Icon!".
To compile the above problem, you must first
write the program a file named HelloIcon.java.
Note that the file named must agree with the
by the name of the class in the above program.
Use your favorite editor to do this. Then compile
it using the command:
The result is the compiled program called HelloIcon.class.
This is the file that will be called by the web browser.
Note that our example requires has an image file ``gifIcon.gif''.
You can download all program files from this directory:
http://cs.nyu.edu/~yap/classes/visual/03s/lect/progs/applet/
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Browsers understand the HTTP protocol. The contents read by a browser
is found in a text file called an HTML file. We now
create such an HTML file which tells the browser to
load the HelloIcon.class that we just created.
In the html file below, we do two things:
Again, use your favorite editor to write this in a file
named HelloIcon.html. In this case, the
actual name is not important, though the file entension
should be .html or .htm.
Finally, you must place this file in a place
which is web-accessible. Assuming you have an account
on a unix-based server named cs.nyu.edu that is web-accessible.
Suppose your account (or user) name is yap. Then
you should create a directory named public\_html
which should be readable and executable by the world.
Place your file HelloApplet.html in this directory,
making it world-readable.
In the same directory you should place HelloIcon.class
and gifIcon.gif, both also world-readable.
Now you are ready. Now any browser can access your
Hello applet through the URL
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In standard applications, we pass command line arguments.
But applets receive parameters from their html files
read by browsers. This requires two steps:
(1) In the Html file, you need to use the PARAM tags:
These tags specify NAME-VALUE pairs to be passed to the
applet. Consider a Hello applet that takes three arguments:
"greeting", "who" and "fontsize":
(2) In your Java code, you use the getParameter() to access
the NAME-VALUE pairs:
import java.applet.*;
import java.awt.*;
public class Hello
extends Applet
String greeting, who;
int fontsize;
public void init()
setBackground(Color.black);
setForeground(Color.white);
greeting = getParameter("greeting");
who = getParameter("who");
fontsize = Integer.parseInt(getParameter("fontsize"));
setFont(new Font("SansSerif", Font.BOLD, fontsize);
public void paint(Graphics g)
Dimension dim = getSize(); // size of Applet
if (greeting != null)
g.drawString(greeting + " " + who + "!",
0, dim.height/2 ); // position to draw
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/***************************************************
* @file HelloIcon.java
* @author Chee Yap (yap@cs.nyu.edu)
* @date Jan 24, 2003
* @purpose A first example of Java Applet
* @notes Visualization Class, Spring 2003
* $Id: l.tex,v 1.3 2003/03/09 14:43:05 yap Exp yap $
***************************************************/
import java.applet.Applet;
import java.awt.*;
public class HelloIcon
extends Applet {
// Declare variables
public int COUNT = 0;
private Image myImage;
// Init method
public void init() {
setBackground(Color.white);
setFont(new Font("Sanserif", Font.BOLD, 18));
myImage = getImage(getDocumentBase(),
"gifIcon.gif");
add(new Label("Hello Icon!"));
}
// Paint method
public void paint(Graphics g) {
g.drawImage(myImage, 0, 50, this);
}
}//class HelloIcon
> javac HelloIcon.java
2 HTML Basics
(1) We announce the applet as "My First Applet".
(2) We load the applet program HelloIcon.class.
Note that single line comments are enclosed inside the matching
pair <! ... >. Multiline comments are
enclosed inside <!-- ... -->.
<html> <! this tells the browser the protocol>
<!--
Tbe main part of the html document is called the "body".
In this simple example, there is nothing before the body,
but in general, there will be a title for this
html page, style files, etc.
-->
<body>
<h1> My First Applet</h1> <! header >
<APPLET CODE="HelloIcon.class"
WIDTH=500 HEIGHT=300>
Error! You need a Java-enabled Browser!
<APPLET>
</body>
</html>
http://cs.nyu.edu/~yap/HelloIcon.html
On other systems, please ask your web administrator.
3 Parameters