ch6.htm

JAVA Developing Professional JavaApplets

<P><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD WIDTH=185><TT><B>getCodeBase()</FONT></B></TT>
</TD><TD WIDTH=406>Returns the URL of this Applet class (the one used to start the applet). This could be the value specified in the &lt;APPLET&gt; tag CODEBASE attribute or the directory of the HTML file in which this applet is embedded.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=185><TT><B>GetDocumentBase()</FONT></B></TT>
</TD><TD WIDTH=406>Returns the URL of the HTML containing this applet.
</TD></TR>
</TABLE></CENTER>
<P>
<P>
Perhaps the <TT>getImage()</TT> method
you will find the most useful is the one that takes two arguments:
a base URL and a String representing the path or filename in relation
to the base. In the example that follows, the image is loaded
from a subdirectory off where the applet HTML is located:
<BLOCKQUOTE>
<TT>Image img = getImage(getDocumentBase(),&quot;images/mail.gif&quot;);</TT>
</BLOCKQUOTE>
<P>
The other <TT>getImage()</TT> method
takes a URL object as its only parameter. This will have the full
path and filename of the image. The multiple constructors of URL
objects will be discussed shortly, but the construction of a URL
object with a String is illustrated as follows:
<BLOCKQUOTE>
<TT>Image img = getImage(new URL(&quot;http://AFakeServer.com/images/mail.gif&quot;));</TT>
</BLOCKQUOTE>
<P>
Once you get the image, it is ready to be drawn. When the <TT>paint()</TT>
method is invoked, an image is drawn by calling the <TT>drawImage()</TT>
method of the Graphics object:
<BLOCKQUOTE>
<TT>g.drawImage(img,10,10,this);</TT>
</BLOCKQUOTE>
<P>
This code draws the Image created in the previous example at an
x, y coordinate. The last <BR>
parameter refers to an implementation of the ImageObserver interface.
This class is used for tracking the progress of an image as it
is loaded and decoded. This makes it possible to show partial
images as the full image is being constructed. The ImageObserver
interface will be discussed in more detail later, along with its
related classes and methods. For now, it's enough to say that
the AWT Component class provides the basic methods necessary for
managing this image display behavior. The &quot;this&quot; of
the above <TT>drawImage()</TT> sample
code refers to the component displaying the object.
<P>
Listing 6.5 and Figure 6.3 illustrate an applet that loads an
image and displays it at different scales each time the applet
receives a mouse click. The image is loaded in the applet <TT>init()</TT>
method by using <TT>getImage()</TT>.
The first time the image appears, it is at its normal size using
the version of <TT>drawImage()</TT>
previously discussed. When you click the mouse, it changes the
image's scale and forces the applet to be redrawn. If the image
is not at its normal scale, then it's displayed at its modified
size with the following code:
<P>
<A HREF="f6-3.gif" ><B>Figure 6.3 : </B><I>Drawing a scaled Applet image </I></A>
<BLOCKQUOTE>
<TT>int width = img.getWidth(this);<BR>
int height = img.getHeight(this);<BR>
g.drawImage(img,10,10,scale * width,scale * height,this);</TT>
</BLOCKQUOTE>
<P>
The first two statements use methods of the Image class to get
the width and height of the image. A different version of the
<TT>drawImage()</TT> method is used
to draw the image scaled to fit inside a bounding box specified
by a width and height. The image is automatically scaled to fit
tightly inside the box. In the case of this example, the width
and height are multiplied by values of <TT>2</TT>,
<TT>3</TT>, and <TT>4</TT>.
<HR>
<BLOCKQUOTE>
<B>Listing 6.5. Code for drawing scaled images.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.awt.*;<BR>
import java.lang.*;<BR>
import java.applet.Applet;<BR>
<BR>
public class TestImage extends Applet {<BR>
&nbsp;&nbsp;&nbsp;// Load the image off the document base...<BR>
&nbsp;&nbsp;&nbsp;Image img;<BR>
&nbsp;&nbsp;&nbsp;int scale = 1;<BR>
&nbsp;&nbsp;&nbsp;public final int MAX_SCALE = 4;<BR>
&nbsp;&nbsp;&nbsp;public void init() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;img = getImage(getDocumentBase(),&quot;images/mail.gif&quot;);
<BR>
// Set toggle state...<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Paint the image at its normal size or at
twice<BR>
&nbsp;&nbsp;&nbsp;// its normal size...<BR>
&nbsp;&nbsp;&nbsp;public void paint(Graphics g) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Show at normal scale<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (scale == 1) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;g.drawImage(img,10,10,this);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Or make bigger...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;int width
= img.getWidth(this);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;int height
= img.getHeight(this);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;g.drawImage(img,10,10,scale
* width,<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;scale
* height,this);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Mouse clicks change the scale of the image
<BR>
&nbsp;&nbsp;&nbsp;public boolean mouseDown(Event ev, int x, int
y) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;++scale;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// If the max size of the
image is reached, then<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// go back to normal size...
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (scale &gt; MAX_SCALE)
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;scale
= 1;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Force a repaint of the
image...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;repaint();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return true;<BR>
&nbsp;};<BR>
}<BR>
</TT>
</BLOCKQUOTE>
<HR>
<P>
An interesting thing to note is that the Toolkit class provides
its own versions of <TT>getImage()</TT>.
One takes a single URL parameter as in the Applet single parameter
<TT>getImage()</TT> method; this Toolkit
method is used in this chapter's project. The other version of
<TT>getImage()</TT> takes a single
String parameter that describes the file containing the image.
<H2><A NAME="AppletsandAudio"><FONT SIZE=5 COLOR=#FF0000>Applets
and Audio</FONT></A></H2>
<P>
The Applet class has two simple methods for loading and playing
an audio clip. These <TT>play()</TT>
methods have two variations, as the <TT>getImage()</TT>
method does: One takes a fully constructed URL of the desired
audio clip; the other method takes a base URL, plus a String specifying
additional directory and filename information. For example, the
following code could be called to play a sound located off the
HTML directory:
<BLOCKQUOTE>
<TT>play(getDocumentBase(),&quot;audio/song.au&quot;);</TT>
</BLOCKQUOTE>
<P>
The only audio format currently supported by Java is the AU format,
but this limitation will probably be relaxed in the near future.
<P>
Another way to play a sound is to create an AudioClip object.
AudioClip is an interface implemented by the native environment
Java is running on. Just like <TT>play()</TT>
and <TT>getImage()</TT>, the Applet
class offers two ways to get a reference to an AudioClip. The
Applet class <TT>getAudioClip()</TT>
method with a single URL parameter is one way of getting a reference
to an AudioClip object:
<BLOCKQUOTE>
<TT>import java.applet.AudioClip;<BR>
// &#133;<BR>
AudioClip sound = getAudioClip(new URL(&quot;http://AFakeServer.com/audio/song.au&quot;));</TT>
</BLOCKQUOTE>
<P>
Note that the AudioClip interface is actually located in the Applet
package. You can also refer to an AudioClip by giving a URL and
a String representing additional directory and filename information.
<P>
Once you have an AudioClip, it can be played with the <TT>play()</TT>
method. This method takes no parameters and plays the clip only
once. The <TT>loop()</TT> method plays
the sound repeatedly:
<BLOCKQUOTE>
<TT>sound.loop();</TT>
</BLOCKQUOTE>
<P>
The <TT>stop()</TT> method is used
to terminate the playing of the audio clip:
<BLOCKQUOTE>
<TT>sound.stop();</TT>
</BLOCKQUOTE>
<P>
It is important to remember to stop an audio clip when you leave
the page that started the clip. The <TT>stop()</TT>
method of the applet should, therefore, call the AudioClip <TT>stop()</TT>
when appropriate.
<H2><A NAME="UndertheAppletHood"><FONT SIZE=5 COLOR=#FF0000>Under
the Applet Hood</FONT></A></H2>
<P>
It is worth spending a few moments to look at the underpinnings
of the Applet class. Closely related to the Applet class is the
AppletContext interface, which represents the underlying applet
environment. This will typically be a browser, such as Netscape
Navigator or HotJava. Therefore, the AppletContext provides a
link to the resources of the browser. The Applet method <TT>getAppletContext()</TT>
is used to return a reference to this underlying context.
<P>
Once you get access to the AppletContext object, it's possible
to do all kinds of interesting things. One of the simplest and
most useful things to do is to display a message on the browser's
status bar:
<BLOCKQUOTE>
<TT>getAppletContext().showStatus(&quot;This
is a message&quot;);</TT>
</BLOCKQUOTE>
<P>
The various projects throughout this book use this technique to
display problems to the user. An interesting thing to do is to
use this to show the results of an exception:
<BLOCKQUOTE>
<TT>try {<BR>
&nbsp;// &#133; do something<BR>
}<BR>
// If exception, then show detail message to status bar&#133;
<BR>
catch (Exception e) {<BR>
&nbsp;&nbsp;&nbsp;getAppletContext().showStatus(e.getMessage());
<BR>
}</TT>
</BLOCKQUOTE>
<P>
Since <TT>getAppletContext()</TT>
is a method of the Applet context, this code needs to be called
from within an Applet subclass. However, now that this has been
explained, it needs to be pointed out that the Applet class has
its own <TT>showStatus()</TT> method
with the same function. Therefore, the first code fragment could
just as easily have been the following:
<BLOCKQUOTE>
<TT>showStatus(&quot;This is a message&quot;);</TT>
</BLOCKQUOTE>
<P>
Actually, this code does little more than call the underlying
AppletContext <TT>showStatus()</TT>
method.
<P>
Another important thing that the AppletContext can do is return
references of Applets running on the current HTML page. The <TT>getApplet()</TT>
method takes a String name of an Applet and returns a reference
to it. The <TT>getApplet()</TT> method
enumerates the applets on the current page. Both of these are
useful for inter-applet communication.
<P>
The AppletContext also ties in to one of the basic functions of
a browser-dynamically linking to another HTML page. This can be
done in one simple method call! The basic form of <TT>showDocument()</TT>
takes a URL and makes it the current HTML of the browser. This
implies that the <TT>stop()</TT> method
of the current applet will be called, because its container page
will no longer be current. This chapter's project will use <TT>showDocument()</TT>
to link from one page to another. Here is a code fragment from
the project:
<BLOCKQUOTE>
<TT>try {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;URL
u = new URL(URLBase,link);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;Show
new doc: &quot; + u);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a.getAppletContext().showDocument(u);
<BR>
}<BR>
catch (MalformedURLException e) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a.showStatus(&quot;Malformed
URL: &quot; + link);<BR>
}<BR>
catch (Exception e) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a.showStatus(&quot;Unable
to go to link: &quot; + e);<BR>
}</TT>
</BLOCKQUOTE>
<P>
It creates a URL of the new page to be loaded and calls <TT>showDocument()</TT>
to go to it. Note how the browser status bar is used to display
any errors.
<P>
An alternative form of <TT>showDocument()</TT>
takes a second String parameter that specifies the target frame
or window to place the loaded page. This is useful for browsers
that support framed pages. Note, however, that this method (like
the other AppletContext methods) might not do anything if the
native browser does not support the action.
<P>
Finally, a word should be said about the AppletStub interface.
This interface is used to create a program to view applets. Consequently,
any browser that supports Java will need to use the AppletStub
interface to make the Applet class functional.
<H2><A NAME="CreatingandReadingaURL"><FONT SIZE=5 COLOR=#FF0000>Creating
and Reading a URL</FONT></A></H2>
<P>
Since the Uniform Resource Locator, or URL, is at the heart of
the World Wide Web, it is only appropriate that an Internet language