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Java Coffee Break Newsletter Volume 1 Issue 08
Java Coffee Break Newsletter Vol 1, Issue 8
ISSN 1442-3790
One of our most popular sections is our Q&A, where we answer
commonly asked questions about Java programming. We have an even
larger Q&A section this issue, covering a wide range of topics.
If you'd like more Java information, visit our website, at
http://www.davidreilly.com/jcb/
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Java Coffee Break News
* Free Articles & Tutorials
* Merry Christmas, and a Happy New Year
* Java Network Programming FAQ
Question & Answer
* Q&A : How do I playback multimedia files?
* Q&A : How do I execute other applications?
* Q&A : How do I use really large numbers in Java?
* Q&A : How can I change the cursor shape?
* Q&A : How do I make my frames close in response to user requests?
* Q&A : How do I use hashtables?
* Q&A : Is there any way to access C++ objects, and use their methods?
* Q&A : How do I handle timeouts in my networking applications?
* Q&A : How do I use a proxy server for HTTP requests?
1. Free Articles & Tutorials
We've got free articles and tutorials about Java that teach basic
programming concepts, right through to advanced topics like
networking, JavaBeans & CORBA.
For more information, visit the Java Coffee Break at
http://www.davidreilly.com/jcb/
2. 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
3. Java Network Programming FAQ
This issue contains some tips from the Java Network Programming
FAQ, which answers commonly asked networking programming questions. If
you'd like to read more, visit
http://www.davidreilly.com/java/java_network_programming/
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Q&A : How do I playback multimedia files? I'd like to display .avi, .mpg,
and other movie formats.
It'd be a tough task to write routines to playback a wide range of
multimedia formats. Then you'd have to keep an eye out for new formats,
write new code, and make it all so efficient that it can do real-time
playback under a Java Virtual Machine. Fortunately, you don't have to do
any of this!
Sun has been working on the Java Media Framework API, which offers an
easy API to playback multimedia files. The API has been available for
some time now, and implementations are now available from Sun, Intel,
and others. For more information see
http://java.sun.com/products/java-media/jmf/index.html
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Q&A : How do I execute other applications?
Sometimes a pure java implementation doesn't quite cut it. Maybe you need
to call some operating system command, or perhaps another application.
If you need to run other applications from within your app (you won't be
able to do this from an applet for security reasons), then here's what
you need to do.
Step One
Obtain a java.lang.Runtime instance. The java.lang.Runtime class allows
you to execute other applications, but you'll need an object reference
before you can begin.
// Get runtime instance
Runtime r = Runtime.getRuntime();
Step Two
Next, you'll make a call to execute your application. The exec call
returns a Process object, which gives you some control over the new
process. If you just need to start something running, then you might want
to discard the returned process.
// Exec my program
Process p = r.exec ("c:\myprog\inc");
Step Three
Now, if you want to do something with your process (perhaps kill it after
a certain time period, or wait until its finished), you can use the
methods of the java.lang.Process class. Check the Java API documentation
for more information - there's plenty of things you can do. Suppose you
wanted to wait until its terminated, and then continue with your app.
Here's how you'd do it.
// Wait till its finished
p.waitFor();
Easy huh?
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Q&A : How do I use really large numbers in Java?
There are many classes of applications that require support for large
numbers. When primitive data types like long, and double just won't cut
it, Java offers an answer in the form of the java.math package.
Whether its whole numbers or decimals you need, java.math has the
solution. For whole numbers, use the BigInteger class, and for fractions
use the BigDecimal class. Each allows a wide range of mathematical
operations, from simple things like addition, subtraction, multiplication
and division to more complex operations like logical AND, OR, NOT and
powers.
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Q&A : How can I change the cursor shape?
Any graphical component is capable of changing the mouse cursor. As the
user moves the cursor over the component, the cursor will change. Setting
a new cusor type is easy - every subclass of java.awt.Component has a
setCursor method. Individual components, or even applets can specify
their own cursor.
// Create an instance of java.awt.Cursor
Cursor c = new Cursor ( Cursor.WAIT_CURSOR );
// Create a frame to demonstrate use of setCursor method
Frame f = new Frame("Cursor demo");
f.setSize(100,100);
// Set cursor for the frame component
f.setCursor (c);
// Show frame
f.show();
There isn't a "global" cursor type, so this means that you can have
different cursors for different components. But just remember - using
cursors inappropriately will make it difficult for users, so don't put an
hourglass wait component unless you actually want the user to wait! A
full list of cursor types is available from the documentation for
java.awt.Cursor.
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Q&A : How do I make my frames close in response to user requests?
You need to define an event handler for the "WindowClosing" event. Under
JDK1.02, you can add the following code to your Frame subclasses.
// Response to close events
public void WindowClosing(WindowEvent evt)
{
// Close application
System.exit(0);
}
If you're using JDK1.1 AWT event handlers, you can use the WindowAdapter
class for a similar purpose. Here's a simple application that
demonstrates the use of WindowAdapter, as an anonymous inner class.
import java.awt.*;
import java.awt.event.*;
public class Demo
{
public static void main ( String args[] )
{
// Create a frame with a button
Frame f = new Frame("Demo");
// Add a new button
f.add ( new Button("demo") );
// Resize/pack
f.pack();
// Add a window listener
f.addWindowListener ( new WindowAdapter () {
public void windowClosing ( WindowEvent evt )
{
System.exit(0);
}
});
// Show window
f.setVisible(true);
}
}
It doesn't take much code to respond to close events, and makes
applications look much more professional. Don't forget in your next
application! :)
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Q&A : How do I use hashtables?
Hashtables are an extremely useful mechanism for storing data. Hashtables
work by mapping a key to a value, which is stored in an in-memory data
structure. Rather than searching through all elements of the hashtable for
a matching key, a hashing function analyses a key, and returns an index
number. This index matches a stored value, and the data is then accessed.
This is an extremely efficient data structure, and one all programmers
should remember.
Hashtables are supported by Java, in the form of the java.util.Hashtable
class. Hashtables accept as keys and values any Java object. You can use
a String, for example, as a key, or perhaps a number such as an Integer.
However, you can't use a primitive data type, so you'll need to instead
use Char, Integer, Long, etc.
// Use an Integer as a wrapper for an int
Integer integer = new Integer ( i );
hash.put( integer, data);
Data is placed into a hashtable through the put method, and can be
accessed using the get method. It's important to know the key that maps
to a value, otherwise its difficult to get the data back. If you want to
process all the elements in a hashtable, you can always ask for an
Enumeration of the hashtable's keys. The get method returns an object,
which can then be cast back to the original object type.
// Get all values with an enumeration of the keys
for (Enumeration e = hash.keys(); e.hasMoreElements();)
{
String str = (String) hash.get( e.nextElement() );
System.out.println (str);
}
To demonstrate hashtables, I've written a little demo that adds one
hundred strings to a hashtable. Each string is indexed by an Integer,
which wraps the int primitive data type. Individual elements can be
returned, or the entire list can be displayed. Note that hashtables don't
store keys sequentially, so there is no ordering to the list.
import java.util.*;
public class hash {
public static void main (String args[]) throws Exception {
// Start with ten, expand by ten when limit reached
Hashtable hash = new Hashtable(10,10);
for (int i = 0; i <= 100; i++)
{
Integer integer = new Integer ( i );
// Put entry into hashtable, with an Integer as a key
hash.put( integer, "Number : " + i);
}
// Get specific value matching a key out from hashtable
System.out.println (hash.get(new Integer(5)));
// Get specific value matching a key out from hashtable
System.out.println (hash.get(new Integer(21)));
System.in.read();
// Get all values
for (Enumeration e = hash.keys(); e.hasMoreElements();)
{
System.out.println (hash.get(e.nextElement()));
}
}
}
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Q&A : How do I handle timeouts in my networking applications?
(Reprinted with permission from the Java Network Programming FAQ)
If your application is written for JDK1.1 or higher, you
can use socket options to generate a timeout after a read
operation blocks for a specified length of time. This is
by far the easiest method of handling timeouts. A call to
the java.net.Socket.setSoTimeout() method allows you to
specify the maximum amount of time a Socket I/O operation
will block before throwing an InterruptedIOException. This
allows you to trap timeouts, and handle them correctly.
If your application must support earlier versions of Java,
then another option is the use of threads. Multi-threaded
applications can wait for timeouts, and then perform some
action (such as resetting a connection or notifying the
user). For more information on this topic, see "Dealing
with network timeouts in Java", Java Developers Journal
Volume 3, Issue 5.
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Q&A : How do I use a proxy server for HTTP requests?
(Reprinted with permission from the Java Network Programming FAQ)
When a Java applet under the control of a browser (such
as Netscape or Internet Explorer) fetches content via a
URLConnection, it will automatically and transparently
use the proxy settings of the browser.
If you're writing an application, however, you'll have
to manually specify the proxy server settings. You can
do this when running a Java application, or you can write
code that will specify proxy settings automatically for
the user (providing you allow the users to customize the
settings to suit their proxy servers).
To specify proxy settings when running an application,
use the -D parameter :
jre -DproxySet=true -DproxyHost=myhost
-DproxyPort=myport MyApp
Alternately, your application can maintain a configuration
file, and specify proxy settings before using a
URLConnection :
// Modify system properties
Properties sysProperties = System.getProperties();
// Specify proxy settings
sysProperties.setProperty("proxyHost", "myhost");
sysProperties.setProperty("proxyPort", "myport");
sysProperties.setProperty("proxySet", "true");
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