chap13.htm

Thinking in Java, 2nd edition

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Swing also supports a rather radical
feature called &#8220;pluggable look and feel,&#8221; which means that the
appearance of the UI can be dynamically changed to suit the expectations of
users working under different platforms and operating systems. It&#8217;s even
possible (albeit difficult) to invent your own look and feel.

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<A NAME="Heading415"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
The basic applet<BR><A NAME="Index1559"></A></H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">One of Java&#8217;s design goals is to
create <I>applets</I>, which are little programs that run inside a Web browser.
Because they must be safe, applets are limited in what they can accomplish.
However, applets are a powerful tool that support client-side programming, a
major issue for the Web.

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<A NAME="Heading416"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
Applet
restrictions<BR><A NAME="Index1560"></A><A NAME="Index1561"></A><A NAME="Index1562"></A></H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Programming within an applet is so
restrictive that it&#8217;s often referred to as being &#8220;inside the
sandbox,&#8221; since you always have someone&#8212;that is, the Java run-time
security system&#8212;watching over you. 

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">However, you can also step outside the
sandbox and write regular applications rather than applets, in which case you
can access the other features of your OS. We&#8217;ve been writing regular
applications all along in this book, but they&#8217;ve been <I>console
applications</I> without any graphical components. Swing can also be used to
build GUI interfaces for regular applications.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can generally answer the question of
what an applet is able to do by looking at what it is <I>supposed</I> to do:
extend the functionality of a Web page in a browser. Since, as a Net surfer, you
never really know if a Web page is from a friendly place or not, you want any
code that it runs to be safe. So the biggest restrictions you&#8217;ll notice
are probably: <A NAME="Index1563"></A>

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<OL>
<LI><FONT FACE="Verdana"><I>	</I></FONT><FONT FACE="Georgia"><I>An applet
can&#8217;t touch the local disk</I>. This means writing <I>or</I> reading,
since you wouldn&#8217;t want an applet to read and transmit private information
over the Internet without your permission. Writing is prevented, of course,
since that would be an open invitation to a virus. Java offers <I>digital
signing</I> for applets. Many applet restrictions are relaxed when you choose to
allow <I>trusted applets</I> (those signed by a trusted source) to have access
to your machine.

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</FONT><LI><FONT FACE="Verdana"><I>	</I></FONT><FONT FACE="Georgia"><I>Applets
can take longer to display, </I>since you must download the whole thing every
time, including a separate server hit for each different class. Your browser can
cache the applet, but there are no guarantees. Because of this, you should
always package your applets in a JAR (Java ARchive) file that combines all the
applet components (including other <B>.class</B> files as well as images and
sounds) together into a single compressed file that can be downloaded in a
single server transaction. &#8220;Digital signing&#8221; is available for each
individual entry in the JAR file.

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</FONT><A NAME="_Toc375545465"></A><A NAME="_Toc481064784"></A></OL><A NAME="Heading417"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
Applet advantages</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">If you can live within the restrictions,
applets have definite advantages, especially when building
<A NAME="Index1564"></A>client/server or other networked applications:
<A NAME="Index1565"></A><A NAME="Index1566"></A>

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<OL>
<LI><FONT FACE="Verdana"><I>	</I></FONT><FONT FACE="Georgia"><I>There is no
installation issue.</I> An applet has true platform independence (including the
ability to easily play audio files, etc.) so you don&#8217;t need to make any
changes in your code for different platforms nor does anyone have to perform any
&#8220;tweaking&#8221; on installation. In fact, installation is automatic every
time the user loads the Web page that contains applets, so updates happen
silently and automatically. In traditional client/server systems, building and
installing a new version of the client software is often a nightmare.

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</FONT><LI><FONT FACE="Verdana"><I>	</I></FONT><FONT FACE="Georgia"><I>You
don&#8217;t have to worry about bad code causing damage to someone&#8217;s
system,</I> because of the security built into the core Java language and applet
structure. This, along with the previous point, makes Java popular for so-called
<I>intranet</I> client/server applications that live only within a company or
restricted arena of operation where the user environment (Web browser and
add-ins) can be specified and/or controlled.

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</FONT></OL><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Because
applets are automatically integrated with HTML, you have a built-in
platform-independent documentation system to support the applet. It&#8217;s an
interesting twist, since we&#8217;re used to having the documentation part of
the program rather than vice versa.

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<A NAME="Heading418"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
Application frameworks</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Libraries are often grouped according to
their functionality. Some libraries, for example, are used as is, off the shelf.
The standard Java library <B>String</B> and <B>ArrayList</B> classes are
examples of these. Other libraries are designed specifically as building blocks
to create other classes. A certain category of library is the
<A NAME="Index1567"></A><A NAME="Index1568"></A><I>application framework</I>,
whose goal is to help you build applications by providing a class or set of
classes that produces the basic behavior that you need in every application of a
particular type. Then, to customize the behavior to your own needs, you inherit
from the application class and override the methods of interest. The application
framework&#8217;s default control mechanism will call your overridden methods at
the appropriate time. An application framework is a good example of
&#8220;separating the things that change from the things that stay the
same,&#8221; since it attempts to localize all the unique parts of a program in
the overridden
methods</FONT><A NAME="fnB62" HREF="#fn62">[62]</A><FONT FACE="Georgia">.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Applets are built using an application
framework. You inherit from class <B>JApplet</B> and override the appropriate
methods. There are a few methods that control the creation and execution of an
applet on a Web page: </FONT><BR></P></DIV>
<DIV ALIGN="CENTER"><TABLE BORDER>
<TR VALIGN="TOP">
<TH WIDTH=72 COLSPAN=1 ROWSPAN=1 VALIGN="TOP">
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia"><B>Method</B></FONT><BR></P></DIV>
</TH>
<TH WIDTH=288 COLSPAN=1 ROWSPAN=1 VALIGN="TOP">
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia"><B>Operation</B></FONT><BR></P></DIV>
</TH>
</TR>
<TR VALIGN="TOP">
<TD>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia"><B>init(&#160;)</B></FONT><BR></P></DIV>
</TD>
<TD>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Automatically called to perform
first-time initialization of the applet, including component layout.
You&#8217;ll always override this method.</FONT><BR></P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia"><B>start(&#160;)</B></FONT><BR></P></DIV>
</TD>
<TD>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Called every time the applet moves into
sight on the Web browser to allow the applet to start up its normal operations
(especially those that are shut off by <B>stop(&#160;)</B>). Also called after
<B>init(&#160;)</B>.</FONT><BR></P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia"><B>stop(&#160;)</B></FONT><BR></P></DIV>
</TD>
<TD>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Called every time the applet moves out of
sight on the Web browser to allow the applet to shut off expensive operations.
Also called right before <B>destroy(&#160;)</B>.</FONT><BR></P></DIV>
</TD>
</TR>
<TR VALIGN="TOP">
<TD>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia"><B>destroy(&#160;)</B></FONT><BR></P></DIV>
</TD>
<TD>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Called when the applet is being unloaded
from the page to perform final release of resources when the applet is no longer
used</FONT><BR></P></DIV>
</TD>
</TR>
</TABLE></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">With this information you are ready to
create a simple applet:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c13:Applet1.java</font>
<font color=#009900>// Very simple applet.</font>
<font color=#0000ff>import</font> javax.swing.*;
<font color=#0000ff>import</font> java.awt.*;

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Applet1 <font color=#0000ff>extends</font> JApplet {
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> init() {
    getContentPane().add(<font color=#0000ff>new</font> JLabel(<font color=#004488>"Applet!"</font>));
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Note that applets are not required to