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Java Coffee Break Newsletter Volume 2 Issue 12
Java Coffee Break Newsletter Volume 2, Issue 12
http://www.javacoffeebreak.com/
ISSN 1442-3790
=================================================================
In this issue
* Java in the news
* Merry Christmas, and a Happy New Year
* Book Review - JBuilder 3 Unleashed
* Book Review - Developing JavaBeans Using VisualAge for Java
* Article : Using MediaTracker to help load images
* Q&A : Why did Java skip from Version 1.1 to Version 2?
* Q&A : How do I create a drop-down list of items in my GUI?
=================================================================
In the News
Here are a selection of recent news items that may be of
interest to Java developers.
/*/ Judge releases "Findings of Fact" in DOJ Microsoft trial
The eagerly anticipated findings of fact in the DOJ trial
into Microsoft have been publically released. No surprise
that Microsoft has been declared a monopoly, but has this
really damaged consumers? Will this impact the long term
future of Microsoft? Reaction to the findings have been
mixed, but the general consensus is that Microsoft isn't
down for the count just yet!
See an ongoing special report on CNN into the trial, at
http://cnnfn.com/specials/antitrust/
/*/ Findings of fact reveal attempts to undermine Java
The findings of fact in United States of America v Microsoft
reveal attempts by Microsoft to undermine the portability of
Java. An except of relevant paragraphs from the findings of
fact pertaining to Java is included in our special report,
at http://www.javacoffeebreak.com/articles/microsoftjava/
/*/ Java Coffee Break launches Java directory
If you're looking for information on any topic related
to Java, you'll find it in the new Java Coffee Break Directory.
It's packed full of hundreds of great sites, from general
tutorials, to the exotic and specialized. We proudly use
data supplied by the Open Directory Project, the world's
largest volunteer directory effort.
Visit the directory at
http://www.javacoffeebreak.com/directory
=================================================================
Merry Christmas, and a Happy New Year
T'is the season to be jolly. Whether you celebrate Christmas,
Chanukah,or Kwanzaz, may your holiday season be bright and
festive! And if you find yourself leaving things to the last
minute like me, you'll probably want to avoid the hustle and
bustle of holiday shopping. Why not consider shopping online
this season? Amazon.com, one of the world's largest and most
respected book and music retailers makes it easy to buy gifts
for friends, collegues and loved ones. You'll also be
supporting a great Java newsletter!
http://www.davidreilly.com/goto.cgi?id=amazon
=================================================================
Book Review - JBuilder 3 Unleashed
Author : Various
Publisher : Sams
ISBN : 0672315483
Experience: Beginner - Intermediate
Review Courtesy of Amazon.com
JBuilder 3 Unleashed does an excellent job of explaining how to
write Java programs in Borland's newest Integrated Development
Environment (IDE) for the language. With plenty of code (all of
which ships on the accompanying CD-ROM) and lots of documentation
on Java classes, this book enables the intermediate-level Java
programmer to become more capable with JBuilder and the language
as a whole. You'll find good information if you want to write
servlets, interface with databases, or do any of dozens of other
software development tasks.
JBuilder 3 Unleashed naturally covers the JBuilder user
interface, but the authors don't explain interface elements for
their own sake. Rather, they explain the JBuilder interface in
the context of doing certain Java programming jobs. Subjects
include some fairly exotic aspects of the Java language,
including elaborate user interface design, multithreading,
internationalization, and client/server architecture.
Coverage of programming JavaBeans (and especially serialization
as it applies to Beans) with JBuilder is exemplary, as are the
sections on Remote Method Invocation (RMI) and CORBA. In addition
to their coverage of the mechanics of JBuilder and Java, the
authors include quite a bit of management and engineering
information on using Borland's development environment as the
centerpiece of a development team, including versioning, testing,
and object-oriented analysis and design. --David Wall
For more information about this title, or to order it, visit
http://www.davidreilly.com/goto.cgi?isbn=0672315483
=================================================================
Book Review - Developing JavaBeans Using VisualAge for Java v2
Author : Nilsson et al.
Publisher : John Wiley & Sons
ISBN : 0471345342
Experience: Beginner-Intermediate
Review courtesy of Amazon.com
IBM VisualAge for Java is a powerful Java development tool, but
it doesn't always get a lot of press. The authors of Developing
JavaBeans Using VisualAge for Java, Version 2, meet a real need
with their well-organized tutorial using this capable Java tool.
This book can also serve as an introduction to Java programming.
There is good coverage of JavaBeans with both visual and
non-visual components. Later chapters whet the user's appetite
with newer Java features (like Swing/JFC, enterprise beans, and
servlets). You'll want to go elsewhere for more thorough coverage
of these technologies, but the book lets you try out these
features with small working examples that are scaled just right
for readers new to VisualAge development. (A working version of
the VisualAge tool is included on the accompanying CD-ROM.) The
authors show the strengths (and quirks) of VisualAge, with good
coverage of its visual programming style (where beans are "wired"
together). They even provide a list of "anti-patterns" -- things
to watch out for in your VisualAge designs.
There aren't many tools-based guides to Java. (Most concentrate
on the language itself.) Developing JavaBeans shows that a good
way to learn the Java language is with the VisualAge tool.
--Richard Dragan
For more information about this title, or to order it, visit
http://www.davidreilly.com/goto.cgi?isbn=0471345342
=================================================================
Using MediaTracker to help load images
When loading images over a network, a common problem
which Java applets face is images not loading properly.
On a fast network connection the image will load
quickly, but over a slow modem or network connection,
images take longer, or may not arrive at all. Users
may be confused by blank images - particularly when a
sequence of images are being displayed. One simple way
to let your users know you're still loading is to use
MediaTracker, which allows applets to detect when
images are loaded. By David Reilly.
Ask any user who is connected via a modem dial-up connection,
what they think about applets, and the answer is frequently that
they are slow to load. As one user puts it, "I hate the ugly gray
applet box that sits there on the page". Quite often, an applet
is loading images for some form of animation - but the user
doesn't know what is going on. Other times, applets that fail to
load properly just sit there with a blank gray box. This adds to
the confusion, because a slow loading applet is often equated
with a broken one.
When writing applets, we as programmers have a responsibility to
users to let them know what's going on. If your applet loads
images over a network connection, it's important to track their
progress, and notify the user.
That's where MediaTracker comes in. MediaTracker allows applets
to check to see whether an image has loaded or not. Applets can
register images with a MediaTracker object, and then wait until
one or all images have loaded. While the images are loading, a
message can be displayed to the user. When all the images are
loaded, they can then be processed by the applet - for example,
displayed as part of an animation. Note though MediaTracker can
benefit applications, as well as applets - even when loading from
a local file-system, there can be a noticeable delay when many
large images are loaded.
MediaTracker is a class in the AWT package. If you're already
working with images, the AWT package will be imported. If not,
you'll need to add the following lines to the beginning of your
code :-
// Import the AWT graphics package
import java.awt.*;
Next, you need to create an instance of java.awt.MediaTracker.
You must pass an instance of java.awt.Component to MediaTracker's
constructor. An applet is a subclass of Component, so when
writing an applet for a web-page, the applet can pass itself by
using the this keyword.
// Create a MediaTracker instance,
// to montior loading of images
tracker = new MediaTracker(this);
Once you have a MediaTracker, simply load each image and register
it, using the MediaTracker.addImage (Image, int) method. It takes
as its first parameter an image, and the idcode of the image as
its second parameter. The idcode can be used to inquire about the
status of a particular image, rather than a group of images.
// Load the image
Image img = getImage( getDocumentBase(),
"picture.jpg" );
// Register it with media tracker
tracker.addImage(img, 1);
Once you've registered the image, you can display a text message,
or a simple animation, to let your users know that data is
loading. Of course, you'll also have to check to see if the
images are ready for use, by querying MediaTracker.
There are two ways to check on the progress of images. The first
is to continually poll MediaTracker, by using the
MediaTracker.checkID(int) method.
// Check to see if it has loaded
if (tracker.checkID(1) == false)
{
// Not yet, display message .....
}
else
{
// Display the animation
}
Another way, and often the simpler, is to just wait indefinitely,
until all images are loaded. Note that it's best to do this in a
separate thread of execution, so that your applet can continue to
respond to user events and display a message. This is a blocking
operation, whereas the MediaTracker.checkID(int) method is
non-blocking and returns immediately.
// Wait until all images have loaded
tracker.waitForAll();
An example applet using MediaTracker
The following example demonstrates the use of MediaTracker. No
images will be loaded until you click on the applet. Depending on
the speed of your connection, it may take a few seconds, or even
a minute to load the images. Note that the applet displays a
message, indicating a load is occurring, and that the message
disappears once the load is finished. Without MediaTracker,
there'd be no way to tell if the image was completely loaded -
you might end up seeing only part of the image until it was
completely finished.
(To see the applet, you must be viewing the HTML version
of this article at
http://www.javacoffeebreak.com/articles/mediatracker/)
Listing One - MediaTrackerDemo.java
import java.applet.Applet;
import java.awt.*;
//
//
// MediaTrackerDemo
//
//
public class MediaTrackerDemo extends Applet implements Runnable
{
Image[] imgArray = null;
MediaTracker tracker = null;
int current = 0;
Thread animThread=null;
// Check for a mouse click, to start the images downloading
public boolean mouseDown(Event evt, int x, int y)
{
if (tracker == null)
{
// Create a new media tracker, to track loading
images
tracker = new MediaTracker(this);
// Create an array of three images
imgArray = new Image[3];
// Start downloading the images
for (int index=0; index < 3; index++)
{
// Load the image
imgArray[index] = getImage( getDocumentBase(),
"anim" + (index+1) + ".gif");
// Register it with media tracker
tracker.addImage(imgArray[index], index);
}
// Start animation thread
animThread = new Thread(this);
animThread.start();
}
return true;
}
public void update(Graphics g) {
// Don't repaint gray background
paint(g);
}
public void paint (Graphics g)
{
g.setColor(Color.white);
g.fillRect(0,0, 200, 200);
g.setColor(Color.black);
// Check to see if images have started loading
if (tracker == null)
{
g.drawString ("Click to start loading",20,20);
}
else
// Check to see if images have loaded
if (tracker.checkAll())
{
g.drawImage(imgArray[current++], 0, 0, this);
if (current >= imgArray.length) current=0;
}
else
// Still loading
{
g.drawString ("Images are loading...", 20,20);
}
}
public void run()
{
try
{
tracker.waitForAll();
for (;;)
{
// Repaint the images
repaint();
Thread.sleep(2000);
}
}
catch (InterruptedException ie) {};
}
}
Running the MediaTrackerDemo
The demonstration applet loads three images, called anim1.gif,
anim2.gif, anim3.gif, from the base directory of the page from
which the applet is loaded. If you'd like to modify the applet,
you could increase the number of images, change the name or
location, or change the speed of animation.
For the full text of this article, including the demonstration
applet, read it at the Java Coffee Break
http://www.javacoffeebreak.com/articles/mediatracker/
=================================================================
Q&A: Why did Java skip from Version 1.1 to Version 2?
The jump in version numbers between Java 1.1 and Java 2 has
caused confusion for many programmers, who were expecting Java
1.2. Indeed, many publishers released titles bearing the title of
Java 1.2 or JDK1.2, only to find that the version number changed!
The official name for the new version of Java is the Java 2
Platform, version 1.2. This corresponds to JDK1.2, and the terms
can be used interchangeably.
=================================================================
Q&A: How do I create a drop-down list of items in my GUI?
There are several types of GUI components that provide a
selectable list. The java.awt.List component, for example,
provides a scrollable list of items. However, the most efficient
use of GUI space is a drop-down list, which is provided by the
java.awt.Choice component.
Take the following snippet, for example, which provides a list of
days of the week.
import java.awt.*;
public class ChoiceDemo
{
public static void main(String args[])
{
Frame f = new Frame();
f.setLayout(new FlowLayout());
// List contains 7 elements
Choice list = new Choice();
list.add ("Monday");
list.add ("Tuesday");
list.add ("Wednesday");
list.add ("Thursday");
list.add ("Friday");
list.add ("Saturday");
list.add ("Sunday");
f.add (list);
f.add ( new Button ("Ok") );
f.pack();
f.setVisible(true);
}
}
Choice boxes allow you to get the index value of the list
(getSelectedIndex), or the currently selected String
(getSelectedItem).
=================================================================
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