I2PUJ4 - Chapter 6 - Applets, HTML, and GUI’s
The Basic Java Applet and JApplet
Rob Dempster
School of Computer Science University of KwaZulu-Natal
Pietermaritzburg Campus
Abstract
This is not a paper. It is the lecture presentation slides I used to provide a framework for the lectures I presented dealing with the material covered in Section 1 of Chapter 6 of David J.
Eck’s book entitled Introduction to Programming Using Java [1]. The slides were prepared using SuSE Linux, Emacs, LATEXand Prosper.
2005, Robert Dempster. These are free slides. There are no restrictions on using orc
redistributing or posting on the web a complete, unmodified copy of this material. There are some restrictions on modified copies. To be precise: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License,
Version 1.1 or any later version published by the Free Software Foundation; with no invariant sections, front cover text, or back cover text.
The most recent version of these slides are always available, at no charge, for downloading and for on-line use at the Web address http://saturn.cs.ukzn.ac.za/ robd/javaslides/. There you will find the LATEXsource code together with the slides in formats suitable for slide
presentations (.sp.) and hand-outs (.ho.).
Interfaces Introduction
Java is a programming language designed for networked computers and the World Wide Web i.e., the design of distributed computing applications
It contains many neat features to make this easy to do in a reliable and secure manner.
Part of learning Java is learning to program applets and other Graphical User Interface (GUI) programs.
GUI programs are event-driven.
That is, user actions such as clicking on a button or pressing a key on the keyboard, generate events, and the program must respond to
these events as they occur.
Related to GUI programming is Human Computer Interaction (HCI).
The Basic Java Applet and JApplet
Java applets are small programs that are meant to run on a
page in a Web browser.
However:
An applet is not a complete program.
It doesn’t have to be small.
There are other ways to use them.
An applet is inherently part of a graphical user interface.
It is a type of graphical component that can be displayed in a
window which we will hereafter assume, belongs to a Web
browser.
The Basic Java Applet and JApplet (cont’d)
The Applet class, defined in the package java.applet, is really only useful as a basis for making subclasses.
An object of type Applet has certain basic behaviours, but doesn’t actually do anything useful.
There are several methods in the Applet class that are defined to do nothing.
The programmer must override at least some of these methods and give them something to do.
An applet program does not contain a main() routine since it is not a stand-alone program.
However, many of the methods in an applet are similar to main(), in that they are meant to be called by the system.
It is the job of the programmer is to say what happens in response to these system calls.
The Basic Java Applet and JApplet (cont’d)
One of the methods that is defined in the Applet class to
do nothing is the paint() method.
The paint() method is called by the system when the
applet needs to be drawn.
In a subclass of Applet, the paint() method can be
redefined to draw various graphical elements such as
rectangles, lines, and text on the applet.
As a first example of an applet, let’s go the traditional
route and look at an applet that displays the string "Hello
World!".
The “Hello World Applet
import java.awt.*;
import java.applet.*;
public class HelloWorldApplet extends Applet {
// Applet displays the string ’Hello World!’
public void paint(Graphics g) {
g.drawString("Hello World!", 10, 30);
}
} // end of class HelloWorldApplet
The “Hello World Applet - The Theory!
The drawString() method, defined in the Graphics class, actually does the drawing, using the Graphics (context) object referenced by g.
Back to the applet:
It is an object, but we have not created an object here.
This of course begs the question: “Then where does it come from”?
It is the web browser’s responsibility to create the Applet object and add it to its browser window.
Browsers execute html (a markup language) code that describes how a web page should be displayed.
The instructions to the browser regarding the applet are contained in and by (surprise!!!) the <applet> markup tag.
The “Hello World Applet HTML code
<center>
<applet
code="HelloWorldApplet.class"
width=200
height=50>
<p>
If you do not see this applet,
speak to your sysadm asap!!
</p>
</applet>
</center>
The “Hello World Applet - The Theory (cont’d)
The Applet class defines another method that is essential for programming applets, the init() method.
This method is called just after the applet object has been created and before it appears on the screen.
Its purpose is to give the applet a chance to do any necessary initialisation.
Again, this method is called by the system, not by your program.
You might be wondering why initialisation is done in the init() method rather than in a constructor.
It is possible to define a constructor for your applet class, as the system calls the constructor that has no parameters.
Unfortunately when the constructor is called, the size of the applet is not available.
It is when init() is called.
It is customary to do applet initialisation in the init() method.
The second “Hello World Applet
import java.awt.*;
import java.applet.*;
public class HelloWorldApplet2 extends Applet {
public void init() {
setBackground(Color.blue);
setForeground(Color.yellow);
}
public void paint(Graphics g) {
g.drawString("Hello World!", 10, 30);
}
JApplets and Swing
The AWT (Abstract Windowing Toolkit) has been part of Java from the beginning, but was not powerful or flexible enough for writing complex, sophisticated applications.
This does not prevent it from being useful – especially for applets, which are generally not as complex as full-scale, independent applications.
The Swing graphical user interface library was created to address the problems with the AWT.
The classes that make up the Swing library can be found in the package javax.swing.
Swing includes the class javax.swing.JApplet as a basis for writing applets.
JApplet is a subclass of Applet, so JApplets are in fact also Applets.
A JApplet Example
import javax.swing.*; // Swing GUI classes are defined here.
import java.awt.event.*; // Event handling class are defined here.
public class HelloSwing extends JApplet implements ActionListener {
public void init() {
JButton bttn = new JButton("Click Me!");
bttn.addActionListener(this);
getContentPane().add(bttn);
} // end init()
public void actionPerformed(ActionEvent evt) {
String title = "Greetings"; // Shown in title bar of dialog box.
String message = "Hello from the Swing User Interface Library.";
JOptionPane.showMessageDialog(null, message, title,
JOptionPane.INFORMATION_MESSAGE);
} // end actionPerformed() } // end class HelloSwing
JApplets and Swing - Some Notes
First we instantiate a Button .
Then we instruct the system to monitor the button for events by
telling it to invoke this objects actionePerformed()
method.
Because our program is a JApplet it already has the content
pane provided by the Browser.
We then add the button to this content pane.
The actionePerformed() method simply pops up a
message dialog window containing an appropriate message.
That is it - heaps and heaps of work done by the GUI API.
JApplets and Swing - Some More Notes
In the previous example the applet itself listened for action
events from the button
The preferred practice is to create a separate object to listen
for, and respond to, events.
This is more object-oriented in the sense that each object has
its own clearly defined area of responsibility.
The most convenient way to make a separate event-handling
object is to use a nested anonymous class.
The next version of HelloSwing uses an anonymous inner
class for event handling.
Another JApplet Example
import javax.swing.*; // Swing GUI classes are defined here.
import java.awt.event.*; // Event handling class are defined here.
public class HelloSwing2 extends JApplet {
public void init() {
JButton bttn = new JButton("Click Me!");
bttn.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
String title = "Greetings"; // Shown in box’s title bar.
String message = "Another hello from Swing.";
JOptionPane.showMessageDialog(null, message, title,
JOptionPane.INFORMATION_MESSAGE);
} // end actionPerformed() });
getContentPane().add(bttn);
} // end init()
} // end class HelloSwing2
The Applet Tag and Modifiers
The <APPLET> tag is used to add a Java applet to a Web page. This tag must have a matching </APPLET>.
A required modifier named CODE gives the name of the compiled class file that contains the applet.
If the class file is not located in the same directory with the HTML
document containing it, then the modifier, CODEBASE must be used to specify the URL of the directory that contains the class file.
If an applet uses a lot of .class files, pacckage all the .class files into a single .zip or .jar file.
You have to specify the name of the archive file in an ARCHIVE modifier in the <APPLET> tag.
Applet Parameters
Applets can use applet parameters to customise their behaviour.
Applet parameters are specified by using <PARAM> tags, which can only occur between an <APPLET> tag and the closing </APPLET>. The PARAM tag takes the form <PARAM NAME="param-name"
VALUE="param-value">
An applet can use the predefined method getParameter() to check for parameters specified in PARAM tags.
If you put anything besides PARAM tags between <APPLET> and
</APPLET>, it will be ignored by any browser that supports Java.
This allows for the inclusion of a message such as "Your browser
doesn’t support Java". This message will only appear in browsers that don’t support Java.
A PARAM Example
<APPLET code="ShowMessage.class" WIDTH=200 HEIGHT=50>
<PARAM NAME="message" VALUE="Goodbye World!">
<p align=center>Sorry, but your browser doesn’t support Java!</p>
</APPLET>
String display; // Instance variable: message to be displayed.
public void init() { String value;
value = getParameter("message"); // Get message PARAM, if any.
if (value == null)
display = "Hello World!"; // default value else
display = value; // Value from PARAM tag.
...
