ch6.htm

JAVA Developing Professional JavaApplets

like Java has a URL class. Several examples of how to create and
use a URL object have already been presented in this chapter.
There are four different constructors of URL objects. Two of them
should already be familiar. The simplest constructor takes a string
and converts it to a URL:
<BLOCKQUOTE>
<TT>URL u = new URL(&quot;http://AFakeServer.com/&quot;);</TT>
</BLOCKQUOTE>
<P>
Another method should also be familiar. It takes a URL and a String
representing a relative path and creates a new URL from it.
<BLOCKQUOTE>
<TT>URL urlNew = new URL(u,&quot;audio/sound.au&quot;);</TT>
</BLOCKQUOTE>
<P>
Recall that a URL typically consists of a protocol, the name of
the host computer, and a path to the location of the resource.
A third URL constructor takes these as protocol, host, and file
Strings, respectively, and returns a URL. The final constructor
adds a String specifying the port as an additional parameter.
However, protocols generally have a fixed port number (HTTP is
80), so this information is usually not needed.
<P>
A couple of methods can be used for deconstructing a URL. The
following code prints the protocol, host, port, file, and finally
the URL itself of the HTML of the applet:
<BLOCKQUOTE>
<TT>System.out.println(&quot;Protocol: &quot;
+ getDocumentBase().getProtocol());<BR>
System.out.println(&quot;Host: &quot; + getDocumentBase().getHost());
<BR>
System.out.println(&quot;Port: &quot; + getDocumentBase().getPort());
<BR>
System.out.println(&quot;File: &quot; + getDocumentBase().getFile());
<BR>
System.out.println(&quot;URL: &quot; + getDocumentBase());</TT>
</BLOCKQUOTE>
<P>
Once constructed, the URL can be used to open up a network connection
to the URL. This is done through the <TT>openStream()</TT>
method, which returns an instance of InputStream. You saw how
InputStream classes worked in <A HREF="ch4.htm" >Chapter 4</A>,
&quot;Enhancing the Spreadsheet Applet.&quot; The FilterInputStream
subclasses can be constructed from this to create high-level interfaces
to the streams. The DataInputStream class can be used to read
in streams according to a specified data type, such as Strings.
Listing 6.6 shows how to combine the URL and stream classes for
a quick and easy printout of the contents of the HTML containing
an applet.
<HR>
<BLOCKQUOTE>
<B>Listing 6.6. Printing the contents of the applet's HTML.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>void printSelf() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// Open up a stream to the document URL
and<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// print its contents...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;try {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;DataInputStream
dis = new DataInputStream(<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;new
BufferedInputStream(<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;getDocumentBase().openStream()
) );<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String s;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;while ( (s = dis.readLine())
!= null)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(s);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;EOF&quot;);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;catch (IOException e) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;URL
read error&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;}</TT>
</BLOCKQUOTE>
<HR>
<P>
The key to this is the first line in the <TT>try</TT>
clause. The URL of the base document is taken from <TT>getDocumentBase()</TT>;
its <TT>OpenStream()</TT> method is
then applied. Once the stream is open, an instance of the easy-to-use
DataInputStream class is created. Each line of the HTML is then
fetched by the <TT>readLine()</TT>
method of DataInputStream and sent to standard output.
<H2><A NAME="ChapterProject"><FONT SIZE=5 COLOR=#FF0000>Chapter
Project</FONT></A></H2>
<P>
This chapter's project begins the development of a kiosk-style
online catalog. It has a couple of interesting characteristics.
First of all, it uses HTML applet parameters to describe how each
applet is constructed and operates. Each page in the catalog has
images describing the current choices. Figure 6.4 shows the main
menu of the catalog. The images are loaded and displayed by the
applet, not the HTML. When a choice is made, the applet jumps
to the next HTML and reloads the applet with the new parameters.
<P>
<A HREF="f6-4.gif" ><B>Figure 6.4 : </B><I>Main menu of the online catalog </I></A>
<P>
Since the applet makes extensive use of images, it poses certain
problems. Images use much network bandwidth and so need to be
used efficiently. The project works around this problem by creating
a MediaLoader class that acts as a cache for images. In the next
chapter, this class is improved by acting as a pre-loader of images
before the next applet is retrieved.
<P>
Another notable feature of the project is its configurability.
The same applet runs on every page of the catalog; its features
are determined by &lt;APPLET&gt; tag parameters in the current
HTML. Furthermore, it uses a URL stream to load in additional
data to be displayed on the button. This data comes from a local
text file, which can be edited outside the actual applet code.
<H3><A NAME="ClassOrganization">Class Organization</A></H3>
<P>
Table 6.1 lists the classes used in this chapter's version of
the catalog applet.<BR>
<P>
<CENTER><B>Table 6.1. Catalog project classes.</B></CENTER>
<P><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD WIDTH=154><I>Class</I></TD><TD WIDTH=436><I>Description</I>
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=154>CacheEntry</TD><TD WIDTH=436>Represents a single entry in the image cache maintained by the MediaLoader class.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=154>Catalog</TD><TD WIDTH=436>The Applet class that takes HTML parameters and constructs the components that represent the current choices.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=154>CatalogButton</TD><TD WIDTH=436>A image-based button that shows text representing a choice and links to another Catalog applet when selected.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=154>MediaLoader</TD><TD WIDTH=436>A class that actually loads the images and uses static methods to employ a cache that exists across Catalog applet invocations.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=154>MediaLoaderException</TD><TD WIDTH=436>An exception thrown by the MediaLoader when there is a problem.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=154>SelectionCanvas</TD><TD WIDTH=436>Displays a large image representing a possible choice of the user. Appears to the right of its companion CatalogButton.
</TD></TR>
</TABLE></CENTER>
<H3><A NAME="CatalogHTML">Catalog HTML</A></H3>
<P>
Listing 6.7 shows the HTML of the catalog page displayed in Figure
6.4. As the listing shows, the HTML does not actually display
anything. The &lt;PARAM&gt; tag fields actually tell the applet
what to display. These are passed to the Catalog applet run for
each page of the applet. It reads in the parameters to determine
what images to display. There are three rows of display for each
Catalog applet. On each row there is a field (also known as a
CatalogButton, after its class) that appears on the left-hand
side; it is effectively a button that appears as an image. The
field is complemented by a larger image that appears on its right
(called a SelectionCanvas, after its class).
<P>
The three &lt;PARAM&gt; tags&yacute; whose NAME attribute begins
with the &quot;field&quot; prefix specify the left-hand image
buttons. The corresponding VALUE attribute has four subfields
used to create the CatalogButton. The first subfield is the name
appearing on the button. The second is the image to be displayed
in the button's area. The third subfield is a style, which represents
the size of the button. Although the MEDIUM style is the only
one used in the sample applets, its existence gives you a way
of customizing the applet. The last subfield specifies the URL
that the applet goes to when you click the image button.
<P>
The three &lt;PARAM&gt; tags whose NAME attribute begins with
the &quot;image&quot; prefix specify the SelectionCanvas objects
that appear to the right of the fields. The VALUE attribute specifies
the image to be displayed in the canvas area.
<P>
The &lt;PARAM&gt; tag with the NAME attribute of &quot;data&quot;
specifies a URL containing text data that can be used to complement
the display of the CatalogButton objects. This data would be such
things as &quot;On Sale&quot; that would appear underneath the
larger font name of the button.
<HR>
<BLOCKQUOTE>
<B>Listing 6.7. The HTML of the main catalog page (index.html).
<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>&lt;title&gt;Catalog Applet&lt;/title&gt;
<BR>
&lt;hr&gt;<BR>
&lt;applet code=&quot;Catalog&quot; width=400 height=300&gt;<BR>
&lt;param name=field1 value=&quot;Computers,catalog/field1.gif,MEDIUM,computer/main.html&quot;&gt;
<BR>
&lt;param name=image1 value=&quot;catalog/selection1.gif&quot;&gt;
<BR>
&lt;param name=field2 value=&quot;Software,catalog/field1.gif,MEDIUM,software/main.html&quot;&gt;
<BR>
&lt;param name=image2 value=&quot;catalog/selection1.gif&quot;&gt;
<BR>
&lt;param name=field3 value=&quot;Accessories,catalog/field1.gif,MEDIUM,accessory/</TT><FONT FACE="ZapfDingbats">&Acirc;</FONT><TT>main.html&quot;&gt;
<BR>
&lt;param name=image3 value=&quot;catalog/selection1.gif&quot;&gt;
<BR>
&lt;param name=data value=&quot;catalog/data.txt&quot;&gt;<BR>
&lt;/applet&gt;<BR>
&lt;hr&gt;</TT>
</BLOCKQUOTE>
<HR>
<H3><A NAME="TheCatalogClass">The Catalog Class</A></H3>
<P>
Listing 6.7 gives the full listing of the Catalog class. This
subclass of Applet represents the applet loaded for every page
in the catalog project. The initialization of the applet has three
steps. Its main job is to create the CatalogButton and SelectionCanvas
objects by parsing out the parameters specified in the current
HTML. However, it also traps for a mouse click on one of the CanvasButton
objects in <TT>handleEvent()</TT>.
When this occurs, it calls the CanvasButton <TT>select()</TT>
method, which may result in the browser loading in a new URL representing
a new page in the catalog.
<P>
After the Catalog applet initializes the fonts, it gets the applet
parameter in the HTML that corresponds to the &quot;data&quot;
NAME attribute. It does this through the <TT>getParameter()</TT>
method, which will return either a String representation of the
corresponding VALUE attribute, or a null value if the name cannot
be found. The <TT>getParameter()</TT>
method is used to get all the CatalogButton and SelectionCanvas
parameters that follow.
<P>
After the &quot;data&quot; value is retrieved, it is used to derive
a URL that contains the additional text data. This is performed
in the applet's <TT>loadURLData()</TT>
method. It creates a URL object by taking the path to the text
data in relation to the base document of the current HTML:
<BLOCKQUOTE>
<TT>u = new URL(getDocumentBase(),dataPath);</TT>></TT>
</BLOCKQUOTE>
<P>
It uses this URL object to open up an input stream to the text
file, then reads the data in by Strings delimited by newlines.
This process is similar to the example discussed in the &quot;Creating
and Reading a URL&quot; section above.
<P>
The last step in the applet's initialization is to create the
three rows of CatalogButton and SelectionCanvas couplets. It uses
the <TT>getParameter()</TT> method
to get the information needed to create the canvas components.
In the <TT>createCatalogButton()</TT>
method, the parameter values are parsed with an instance of the
StringTokenizer class. Given a set of delimiters (like commas),
this class simply walks through and produces String tokens that
appear between the delimiters.
<P>
The other thing that the Catalog class does is handle the painting
of the applet. This is simple because it walks through the three
rows and displays the components based on the size of their images.
<HR>
<BLOCKQUOTE>
<B>Listing 6.7. The Catalog class.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.awt.*;<BR>
import java.lang.*;<BR>
import java.util.StringTokenizer;<BR>
import java.applet.*;<BR>
import java.net.URL;<BR>
import java.io.DataInputStream;<BR>
import java.io.BufferedInputStream;<BR>
import java.io.IOException;<BR>
import java.net.MalformedURLException;<BR>
<BR>
// This is the main class that loads the parameters<BR>
// for the current applet and sets up the images<BR>
// fields.<BR>
public class Catalog extends Applet {<BR>
&nbsp;&nbsp;&nbsp;CatalogButton button[] = new CatalogButton[3];
<BR>
&nbsp;&nbsp;&nbsp;SelectionCanvas drawing[] = new SelectionCanvas[3];
<BR>
&nbsp;&nbsp;&nbsp;// Three styles<BR>
&nbsp;&nbsp;&nbsp;private static final int SMALL_STYLE = 0;<BR>
&nbsp;&nbsp;&nbsp;private static final int MEDIUM_STYLE = 1;<BR>
&nbsp;&nbsp;&nbsp;private static final int LARGE_STYLE = 2;<BR>
&nbsp;&nbsp;&nbsp;private static final int DEFAULT_STYLE = MEDIUM_STYLE;
<BR>
&nbsp;&nbsp;&nbsp;Font styleFont[] = new Font[3];<BR>
&nbsp;&nbsp;&nbsp;Font dataFont;<BR>
&nbsp;&nbsp;&nbsp;String data[] = new String[3];<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Initialize the graphic display...<BR>
&nbsp;&nbsp;&nbsp;public void init() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// First create fonts...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;styleFont[0] = new Font(&quot;Helvetica&quot;,Font.PLAIN,16);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;styleFont[1] = new Font(&quot;Helvetica&quot;,Font.BOLD,18);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;styleFont[2] = new Font(&quot;Helvetica&quot;,Font.BOLD,24);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dataFont = new Font(&quot;TimesRoman&quot;,Font.ITALIC,14);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Get the additional data
from URL...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;loadURLData();<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Add the components...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;addComponents();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;show();<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Add the components to the display...<BR>
&nbsp;&nbsp;&nbsp;void addComponents() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Create Font for buttons