ch6.htm

JAVA Developing Professional JavaApplets

is also a good place to begin threads since their existence is
also not confined to the page where they began.
<LI><TT><B>stop()</FONT></B></TT> This method
is called whenever the user leaves a page-it is the converse of
the <TT>start()</TT> method. Therefore,
it's a good place to hide frames and terminate threads.
<LI><TT><B>destroy()</FONT></B></TT><B> </B>The
<TT>destroy()</TT> method is called
whenever the applet is being shut down. Typically, this will occur
when the browser is being closed, although there could be other
circumstances that could lead to <TT>destroy()</TT>
being invoked. This method is a good place to do some cleanup.
However, since it's unpredictable when <TT>destroy()</TT>
will be called, it should be used with some discretion.
</UL>
<P>
The example that follows shows how these four methods work with
the Frame class, providing an interesting insight into their behavior.
Listing 6.1 shows code used to create a simple frame from an applet.
The <TT>init()</TT> and <TT>destroy()</TT>
methods print out their invocations to standard output so their
behavior can be tracked.
<HR>
<BLOCKQUOTE>
<B>Listing 6.1. An applet that creates Frames at initialization.
<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.awt.*;<BR>
import java.lang.*;<BR>
import java.applet.Applet;<BR>
<BR>
// This applet illustrates how Frames work <BR>
// with basic Applet methods<BR>
public class AppletBasics extends Applet&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;Frame f;<BR>
&nbsp;&nbsp;&nbsp;// Called once and only once upon<BR>
&nbsp;&nbsp;&nbsp;// applet initialization<BR>
&nbsp;&nbsp;&nbsp;public void init() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;In
Applet init. Create frame!&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Create a frame and make
it visible...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;f = new Frame(&quot;Applet
Basics test!&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;f.resize(300,200);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;f.show();<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Applet destroyed.&nbsp;&nbsp;Browser probably
shutting down...<BR>
&nbsp;&nbsp;&nbsp;public void destroy() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// Destroy the frame<BR>
&nbsp;&nbsp;&nbsp;&nbsp;f.dispose();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;In applet destroy!&quot;);
<BR>
&nbsp;&nbsp;&nbsp;}<BR>
</TT>}
</BLOCKQUOTE>
<HR>
<P>
Frame's behavior is curious. If you leave the page where the Frame
was created, the Frame will still be active and visible. This
may or may not be the behavior you want. Furthermore, if you remove
the <TT>dispose()</TT> method from
the destroy call, you can get some downright undesirable behavior.
In some browsers, you could create multiple instances of the frame
by reopening its location reference. In Figure 6.1, three instances
of the frame were created by going to its location several times.
<P>
<A HREF="f6-1.gif" ><B>Figure 6.1 : </B><I>Creating multiple frames by reloading its location </I></A>
<P>
Listing 6.2 gives the applet its desirable behavior of hiding
the frame every time you leave the page and redisplaying it when
you come back. It simply calls the <TT>show()</TT>
method of the Frame class in an overridden Applet <TT>start()</TT>
method and hides the Frame in an overridden <TT>stop()</TT>
method.
<HR>
<BLOCKQUOTE>
<B>Listing 6.2. An applet that displays the frame only when you
are on the applet's page.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.awt.*;<BR>
import java.lang.*;<BR>
import java.applet.Applet;<BR>
<BR>
// This applet illustrates how Frames work <BR>
// with basic Applet methods<BR>
public class AppletBasics extends Applet&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;Frame f;<BR>
&nbsp;&nbsp;&nbsp;// Called once and only once upon<BR>
&nbsp;&nbsp;&nbsp;// applet initialization<BR>
&nbsp;&nbsp;&nbsp;public void init() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;In
Applet init. Create frame!&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Create a frame and make
it visible...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;f = new Frame(&quot;Applet
Basics test!&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;f.resize(300,200);<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Move the show to the start method so the
Frame<BR>
&nbsp;&nbsp;&nbsp;// disappears when you leave the page...<BR>
&nbsp;&nbsp;&nbsp;public void start() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;In
applet start!&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;f.show();<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Hide the frame when you leave the page...
<BR>
&nbsp;&nbsp;&nbsp;public void stop() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;In
applet stop!&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;f.hide();<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Applet destroyed.&nbsp;&nbsp;Browser probably
shutting down...<BR>
&nbsp;&nbsp;&nbsp;public void destroy() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;f.dispose();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;In
applet destroy!&quot;);<BR>
&nbsp;&nbsp;&nbsp;}<BR>
}</TT>
</BLOCKQUOTE>
<HR>
<P>
It is worth spending a few minutes playing around with this applet
to get a full understanding of how these methods work. Try discarding
or moving the code to see what will happen.
<H2><A NAME="AppletsandHTML"><FONT SIZE=5 COLOR=#FF0000>Applets
and HTML</FONT></A></H2>
<P>
A special HTML tag is used for including applets within a Web
page. The &lt;APPLET&gt; tag is used to indicate that an applet
is to be run at the current location in the HTML. The typical
syntax for the tag would be like the following:
<BLOCKQUOTE>
<TT>&lt;APPLET CODE=&quot;AppletTags&quot;
WIDTH=200 HEIGHT=60&gt;<BR>
&lt;/APPLET&gt;</TT>
</BLOCKQUOTE>
<P>
The &lt;APPLET&gt; tag has required attributes, as this example
shows. The CODE attribute states the name of the class file that
runs the applet. The &quot;AppletTags&quot; class of this example
will be a subclass of Applet. Java will look in the path specified
in the <TT>CLASSPATH</TT> variable
to find the class. The <TT>WIDTH</TT>
and <TT>HEIGHT</TT> parameters describe
the bounding region of the applet. These are also required and
need to be set to the proper values if the applet is to look the
way you want. Instances of the Window class, like frames, can
appear outside this region. The &lt;/APPLET&gt; tag is used to
indicate the end of the HTML applet block.
<P>
An interesting characteristic of this &lt;APPLET&gt; tag pair
is that any non-tagged text appearing within this block will appear
in a browser that supports Java, but will not for non-Java-capable
browsers. This feature can be used to indicate that a page is
missing some functionality because the browser does not support
Java. For example, the modifications of the previous lines
<BLOCKQUOTE>
<TT>&lt;APPLET CODE=&quot;AppletTags&quot;
WIDTH=200 HEIGHT=60&gt;<BR>
You need to get a Java capable browser!<BR>
&lt;/APPLET&gt;</TT>
</BLOCKQUOTE>
<P>
will show the appropriate message for a browser that does not
support Java.
<P>
The &lt;APPLET&gt; tag supports a couple of other optional attributes.
The ALIGN attribute is used to control the alignment of the applet
on the page. This attribute has about half a dozen possible values,
including <TT>LEFT</TT>, <TT>RIGHT</TT>,
<TT>TOP</TT>, <TT>MIDDLE</TT>,
and so forth. The ALIGN attribute can be used to wrap text around
an applet. The accompanying CD-ROM has a variation of the following
example that uses the ALIGN attribute to right-align the text.
(The file, which is in the <A HREF="ch6.htm" >Chapter 6</A> directory,
is launched by <TT>appletrighttags.html</TT>.)
The HSPACE and VSPACE attributes use the pixel values to control
spacing between the applet and the text around it.
<P>
The CODEBASE attribute can be used to complement the CODE attribute.
It specifies an alternative path where the class specified by
CODE can be found.
<P>
Programmers will find the &lt;PARAM&gt; tag to be of the most
interest. It is a separate tag from &lt;APPLET&gt;, although it
appears within its block. The &lt;PARAM&gt; tag consists of NAME-VALUE
attribute pairs that describe the name of a parameter variable
and its corresponding value. It can appear multiple times with
the &lt;APPLET&gt; block. This is illustrated with an example.
<P>
Figure 6.2 shows a page displaying a couple of strings of text.
The first line is produced by the normal HTML text mechanism,
but the second line is written out by an applet. Within the &lt;APPLET&gt;
and &lt;/APPLET&gt; tag pair are two parameters. The first one,
with the NAME of &quot;text,&quot; is used by the applet to display
the second line of text, which is specified by the VALUE attribute.
The second parameter, called &quot;unused,&quot; is ignored but
shows how multiple parameters can be included in the &lt;APPLET&gt;
block.
<P>
<A HREF="f6-2.gif" ><B>Figure 6.2 : </B><I>An applet that uses the PARAMETER attribute to specify display text </I></A>
<HR>
<BLOCKQUOTE>
<B>Listing 6.3. An HTML listing of Figure 6.2 illustrating the
use of &lt;APPLET&gt; and &lt;PARAMETER&gt; tags.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>&lt;TITLE&gt;Applet Tags First Test&lt;/TITLE&gt;
<BR>
&lt;HR&gt;<BR>
&lt;P&gt; This is HTML text.<BR>
&lt;P&gt;<BR>
&lt;P&gt;<BR>
&lt;APPLET CODE=&quot;AppletTags&quot; WIDTH=200 HEIGHT=60&gt;
<BR>
&lt;PARAM NAME=text VALUE=&quot;This is the AppletTags applet&quot;&gt;
<BR>
&lt;PARAM NAME=unused VALUE=&quot;This doesn't matter&quot;&gt;
<BR>
You need to get a Java capable browser!<BR>
&lt;/APPLET&gt;<BR>
&lt;P&gt;<BR>
&lt;HR&gt;</TT>
</BLOCKQUOTE>
<HR>
<P>
The Applet class uses the &lt;PARAMETER&gt; tags through the <TT>getParameter()</TT>
method. This method takes a String indicating the NAME attribute
and returns a String specifying the VALUE attribute. It will return
null if the NAME attribute is not found. Listing 6.4 shows the
code that uses the <TT>getParameter()</TT>
method to display the second line of text in Figure 6.2.
<HR>
<BLOCKQUOTE>
<B>Listing 6.4. Applet code of Figure 6.2 illustrating the use
of parameters.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.awt.*;<BR>
import java.lang.*;<BR>
import java.applet.Applet;<BR>
<BR>
// This applet takes an applet parameter<BR>
// and displays it...<BR>
public class AppletTags extends Applet&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;String text;<BR>
&nbsp;&nbsp;&nbsp;public void init() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Get the text to be displayed
from<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// the applet tag parameters...
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if ((text = getParameter(&quot;text&quot;))
== null)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;text
= &quot;Parameter not found!&quot;;<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Display the parameter text...<BR>
&nbsp;&nbsp;&nbsp;public void paint(Graphics g) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;g.drawString(text,10,10);
<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
}</TT>
</BLOCKQUOTE>
<HR>
<P>
The project at the end of this chapter makes extensive use of
parameters and gives you a more involved illustration of how parameters
can be constructed in the HTML and read in by the applet.
<H2><A NAME="AppletsandImages"><FONT SIZE=5 COLOR=#FF0000>Applets
and Images</FONT></A></H2>
<P>
The Applet class provides some basic methods for loading an image
into memory. Both of these methods return an instance of an implementation
of the Image class. The Image class is an abstract class providing
methods that define the kind of behaviors an image should have.
The underlying implementation of images in the JDK is somewhat
complex. However, it isn't necessary to understand how this works
to display images. Consequently, the internals of the Image and
related classes will be postponed to later chapters. In this chapter,
the focus will be on the basic mechanics of loading an image and
displaying it.
<P>
The two forms of the <TT>getImage()</TT>
method of the Applet class give you a simple way to load an image.
Both these methods require an instance of a URL object. Although
the specifics of the URL class will be discussed in more detail
shortly, it's enough to say here that the URL class is used to
encapsulate a URL. Typically, the URL object used in the <TT>getImage()</TT>
method will be, in part, generated by one of two Applet class
methods:<BR>