ch11.htm

JAVA Developing Professional JavaApplets

be lost. Given this, it's possible that the clients might not
have received some earlier updates. However, you could increase
the rebroadcast time to a higher value, such as 30 minutes, an
hour, or more. The right figure for this will depend on a variety
of factors, including the size and sensitivity of your data.
<H4>Database</H4>
<P>
The database used in this chapter is generally the same dBASE
IV table described in the previous chapter. An UPDATED field was
added so that changes can be timestamped, thus allowing the server
to download only the state information that has changed.
<P>
A couple of other things should be mentioned about the database.
First of all, a blank version of the database can be found in
the file NEWELECT.DBF located on the CD-ROM. It has all the states,
candidates, and electoral votes. Data that changes over time,
such as the popular vote, is initialized to zero. Since the runtime
server uses ELECT.DBF, you can simply replace NEWELECT.DBF to
generate a clean database after the ELECT.DBF data is modified
for testing.
<P>
Another peculiarity of the table is that the ELECTORAL field initially
has negative numbers. The absolute value of the negative numbers
symbolizes the number of electoral votes a state carries. If the
number is positive, it indicates that the corresponding candidate
has won the state. A normalized database would have been a better
solution here (separate tables for state information and declared
winners), but it didn't seem appropriate to focus on this aspect
of the project.
<P>
Yet another curiosity is the UPDATED field. Since the native method's
ODBC driver implements only character data types, all the fields
in the database need to be of a character type. Unfortunately,
this makes timestamping a little tricky, so timestamps in this
project have the following format: <TT>000XXX</TT>.
That is, all timestamps need to be prefixed by zeroes to produce
a timestamp six characters long. So the first stamp would be 000001,
the hundredth would be 000100, and so on. If your server updates
aren't working properly, be careful how you enter the timestamps
in the database.
<P>
After sending data down the first time, the server retrieves partial
updates based on a timestamp. Although internally it is a number,
it's in the format just described in the database. Any database
rows with a timestamp higher than the last timestamp are made
part of any new partial update broadcasts. Once the database timestamp
is surpassed by the external timestamp, the corresponding row
is no longer sent down as part of a partial update. The server
will always print out the current timestamp it is working on.
Suppose it is 000003, and you want to update the data. You can
do this by changing <I>both</I> entries of the candidates in a
state and setting the timestamp to 000004. The data will then
be sent down in the next partial update broadcast and should show
up in your applet.
<H3><A NAME="AppletClient">Applet Client</A></H3>
<P>
The Election applet continually gives you updated election results
as they arrive to the server. The applet consists of basically
two parts: back-end threads to manage the arrival of datagrams
with the latest election results from the server and a front-end
that displays the results in either a graphical or text-based
format. The network-processing classes were mostly developed in
the previous two chapters; the display classes are new for this
chapter.
<H4>Class Organization</H4>
<P>
Table 11.2 lists the classes used in this chapter's Election applet.
Since many of these classes were created in the previous section,
the new classes are specified by having their names in boldface
type; the classes that were modified have their names italicized.
<BR>
<P>
<CENTER><B>Table 11.2. The Election applet classes and interfaces.</B></CENTER>
<P><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD WIDTH=186><I>Class/Interface</I></TD><TD WIDTH=404><I>Description</I>
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186><B>Candidate</B></TD><TD WIDTH=404>An accessor class that keeps vote totals for a specific candidate.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186>ClientPacketAssembler</TD><TD WIDTH=404>Tracks and assembles blocks of data packets.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186>ClientRegistration</TD><TD WIDTH=404>For registering as a new client.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186>ClientUnregistration</TD><TD WIDTH=404>For removing client registration with server.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186>DGTPClient</TD><TD WIDTH=404>To receive dynamic data updates from server.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186>Election</TD><TD WIDTH=404>The class for the Election applet that implements the LiveDataNotify interface to manage dynamic data from the server and creates components for visual display.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186><B>ElectionTable</B></TD><TD WIDTH=404>Keeps local copy of election results from the server. Uses synchronization so that incoming DGTP data does not collide with reads from visual components.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186><B>ElectionUpdater</B></TD><TD WIDTH=404>Periodically causes update of Election's visual components.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186>LiveDataNotify</TD><TD WIDTH=404>Interface that defines methods for handling dynamic delivery of data from the server.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186>PartialPacket</TD><TD WIDTH=404>Private class used by ClientPacketAssembler for assembling packets.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186><B>StateBreakdownCanvas</B></TD><TD WIDTH=404>ElectionTable observer that displays results of individual states in text or graphics format.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186><B>StateEntry</B></TD><TD WIDTH=404>Simple accessor class that keeps all election data related to a specific state.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=186><B>SummaryCanvas</B></TD><TD WIDTH=404>ElectionTable observer that displays national election totals.
</TD></TR>
</TABLE></CENTER>
<P>
<H4>How It Works</H4>
<P>
Figure 11.5 illustrates the workflow of the Election applet. The
DGTPClient object receives datagrams from the server indicating
the latest election results. This data may be a partial update
of just the states that have changed, or it may be a broadcast
of the full election results. In either case, the Election object
receives the new data since it implements the LiveDataNotify interface.
<P>
<A HREF="f11-5.gif" ><B>Figure 11.5 : </B><I>Data flow of the Election applet.</I></A>
<P>
The Election object parses the data and passes it to the ElectionTable
object. The ElectionTable class has only one instance since it
has a private constructor; objects that use the data must get
a reference to the table object from a public ElectionTable method.
The Election object is the only object that updates the table
and does so when it's notified by the DGTPClient with new data.
When you get the first full batch of election data, with all the
states identified, the ElectionTable object creates a private
table of the individual state and summary totals. This table is
updated by any calls that follow.
<P>
ElectionTable is an instance of the Observable class. It notifies
its two Observers, the StateBreakdownCanvas and SummaryCanvas,
whenever its data change. These two Canvas objects implement the
Observer interface and are mainly responsible for displaying the
election information. The ElectionUpdater thread runs in the background,
periodically forcing refreshes of the Canvas objects.
<P>
Since the underlying classes that implement the network interfaces
haven't changed since the previous chapter, the discussion that
follows will focus on the classes involved in the visual interface.
<H4>The Election Class</H4>
<P>
The Election class runs the Election applet. It implements the
LiveDataNotify interface so that it can be notified whenever the
DGTPClient gets new election data packets. The Election class
also creates the two components that display the election results-the
StateBreakdownCanvas and SummaryCanvas classes. Listing 11.2 shows
the Election class's code.
<P>
Several important things happen at initialization. In the <TT>init()</TT>
method, the first step is to get an applet parameter that specifies
the port the server is listening on, then initializes the display.
It creates a panel at the top of the screen so that you can toggle
between a text-based or graphical display. The Election class
then creates the components that display the election results
and starts a simple thread, ElectionThread, that periodically
causes the applet to repaint. This is needed because paint messages
could be lost if election returns come in rapid-fire fashion.
The last step in the <TT>init()</TT>
method is to resize the applet so that it's large enough to show
all of the state returns.
<P>
The other major thing that happens at initialization occurs when
the <TT>start()</TT> method is first
called. The Election class creates an instance of the DGTPClient
class, specifying itself as the LiveDataNotify parameter-this
ensures that the Election applet is notified of all incoming datagrams.
<P>
The DGTPClient calls the Election class's <TT>recvNewData()</TT>
method when data has arrived. The Election class then parses this
data into a large two-dimensional String array, which is then
passed to the ElectionTable class that provides the data's final
storage location.
<HR>
<BLOCKQUOTE>
<B>Listing 11.2. The Election class.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>// This class starts the Election applet.
It implements<BR>
// the LiveDataNotify interface that sends the latest<BR>
// results to it.&nbsp;&nbsp;It creates objects to display the
<BR>
// returns as they arrive.<BR>
public class Election extends Applet<BR>
&nbsp;&nbsp;&nbsp;&nbsp;implements LiveDataNotify<BR>
{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;private static final int SPACING = 70;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;private boolean init = false;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;DGTPClient ct = null;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;int destPort;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;String destHost = null;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;String numUsers = &quot;Unknown at this
time&quot;;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;String users = null;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;String results[][];<BR>
&nbsp;&nbsp;&nbsp;&nbsp;int nRows = 0, nCols = 0;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// Place canvas drawing objects...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;SummaryCanvas sc;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;StateBreakdownCanvas states;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;Checkbox graphView,textView;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;Panel p;<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public void init()<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if ( init == false
)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Initialize network connections...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;init
= true;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String
strPort = getParameter(&quot;PORT&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
( strPort == null )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;ERROR:
PORT parameter is missing&quot;);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;strPort
= &quot;4545&quot;;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;destPort
= Integer.valueOf(strPort).intValue();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;destHost
= getDocumentBase().getHost();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Initialize AWT components...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Do basic setup...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;setLayout(new
BorderLayout());<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Add panel to set views...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;p
= new Panel();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;CheckboxGroup
cg = new CheckboxGroup();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;graphView
= new Checkbox(&quot;Graphical View&quot;,cg,false);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;textView
= new Checkbox(&quot;Text View&quot;,cg,true);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;p.add(graphView);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;p.add(textView);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;add(&quot;North&quot;,p);
<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Add summary information at top of applet<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sc
= new SummaryCanvas(this);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Show state breakdowns...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;states
= new StateBreakdownCanvas(this);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Create update thread...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Thread
t = new ElectionUpdater(this);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;t.start();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Resize to fit all the states...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Dimension
d = size();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;FontMetrics
fm = Toolkit.getDefaultToolkit().getFontMetrics(getFont());<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;int
height = sc.getHeight() + states.getHeight()<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+
(2 * fm.getHeight());<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;resize(d.width,height);
<BR>