ch2.htm

Java_by_Example,初级经典例子哦,珍藏版本

folder and type the command line <TT>APPLETVIEWER EXAMPLE1.htmL</TT>.

(The previous command line assumes that you've set your path to

the JAVA\BIN directory.) When you do, you see the display shown

in Figure 2.5. (Yep, it's the ubiquitous Duke, waving at you from

his very own applet.)

<P>

<A HREF="f2-5.gif"><B> Figure 2.5 : </B><I>The Animator applet includes several animation and sound examples, including this one featuring Duke, the Java mascot.



</I></A><P>

<P>

Animator is an example of a configurable applet. That is, by modifying

the HTML tag that loads and runs the applet, the user can display

his or her own custom animation sequence and sound effects. You'll

learn about configurable applets in <A HREF="ch26.htm" >Chapter 26</A>, &quot;Configurable

Applets.&quot; For now, though, it's enough for you to know that

Java is capable of adding both animation sequences and sound effects

to your Web pages.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD><B>NOTE</B></TD></TR>

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

If you'd like to see what the applet's HTML tag looks like, select Appletviewer's Applet,Tag command.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="TheBarChartApplet">

The BarChart Applet</A></H3>

<P>

BarChart, another configurable applet, is especially useful when

you need to graphically display data values in a Web page. To

check out BarChart, switch to the JAVA\DEMO\BARCHART folder and

type the command line <TT>APPLETVIEWER EXAMPLE1.htmL</TT>. When

you do, you see the window shown in Figure 2.6.

<P>

<A HREF="f2-6.gif"><B> Figure 2.6 : </B><I>The BarChart applet enables you to create graphs in your Web pages.



</I></A><P>

<P>

Because BarChart is configurable, you can create all sorts of

different bar charts in your Web pages just by specifying different

parameters in the applet's HTML tag. As you can see, applets can

be powerful tools for creating dynamic and useful Web pages. (Try

out Appletviewer's Applet,Tag command to see the code that specifies

how the bar chart appears.)

<H3><A NAME="OtherDemoApplets">

Other Demo Applets</A></H3>

<P>

The DEMO folder contains many sample applets that you can experiment

with using Appletviewer. All of the demo applets are run from

HTML documents with names such as EXAMPLE1.htmL, EXAMPLE2.htmL,

and so on. All demo applets have at least the EXAMPLE1.htmL document,

while others have additional examples. To run any demo applet,

change to the applet's folder and type <TT>APPLETVIEWER EXAMPLE1.htmL</TT>

(assuming that you've set your path to the Appletviewer application).

Use the <TT>DIR </TT>command to display the contents of an applet's

directory in order to discover whether the applet features additional

example HTML files.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD><B>NOTE</B></TD></TR>

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Remember that the HotJava browser cannot load and run newer applets like those that come with the latest version of the JDK.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H2><A NAME="AddingAppletstoanHTMLDocument"><FONT SIZE=5 COLOR=#Ff0000>

Adding Applets to an HTML Document</FONT></A></H2>

<P>

If you've created Web pages before, you know that you use HTML

to create a template for the page. The commands in the template

tell a Web browser how to display the Web page. When Sun Microsystems

developed Java, they also had to come up with an extension to

HTML that would enable Web pages to contain Java applets. That

extension is the <TT>&lt;applet&gt;</TT> tag, which Sun Microsystems

defines as shown in Listing 2.1.

<HR>

<BLOCKQUOTE>

<B>Listing 2.1&nbsp;&nbsp;LST2_1.TXT: The </B><I>&lt;applet&gt;</I><B>

Tag Definition.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;applet attributes&gt;

parameters

alternate-content

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

In the preceding tag, the text in normal characters is typed literally;

the text shown in italics is replaced by whatever is appropriate

for the applet you're including in the document. As you can see,

the <TT>&lt;applet&gt;</TT> tag is similar to other HTML tags

with which you may be familiar. For example, the tag starts with

<TT>&lt;applet attributes&gt;</TT> and ends with <TT>&lt;/applet&gt;</TT>,

which is not unlike the format of other HTML tags. The first and

last lines are required. Other lines in the tag are optional.

<P>

The attributes section of the <TT>&lt;applet&gt;</TT> tag contains

important information about the tag, including the associated

.CLASS file and the applet's width and height. The last line tells

the browser that it has reached the end of the tag. You can load

and run the TicTacToe applet, for example, with the <TT>&lt;applet&gt;</TT>

tag shown in Listing 2.2.

<HR>

<BLOCKQUOTE>

<B>Listing 2.2&nbsp;&nbsp;LST2_2.TXT: A Tag for Loading and Running

TicTacToe.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;applet

    code=TicTacToe.class

    width=120

    height=120&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

In the preceding example, the <TT>code</TT> attribute is the name

of the .CLASS file for the applet. If you remember, the .CLASS

file holds the applet's byte-code representation, which can be

run by the Java interpreter. The <TT>width</TT> and <TT>height</TT>

attributes control the size of the applet.

<P>

The TicTacToe tag above is the simplest <TT>&lt;applet&gt;</TT>

tag you can write. That is, the <TT>code</TT>, <TT>width</TT>,

and <TT>height</TT> attributes are all required, as is the final

<TT>&lt;/applet&gt;</TT> line.

<H3><A NAME="OptionalAttributesforApplets">

Optional Attributes for Applets</A></H3>

<P>

There are several optional attributes you can use with the <TT>&lt;applet&gt;</TT>

tag. The first is <TT>codebase</TT>, which specifies the applet's

base folder or URL (Uniform Resource Locator). This folder or

URL is used in combination with the file specified in the <TT>code</TT>

attribute to find the applet's code. In the case of a folder,

the <TT>codebase</TT> attribute is relative to the location of

the HTML document containing the applet's tag. In Listing 2.2,

because the <TT>codebase</TT> attribute is missing, the Web browser

will look for the applet's files in the same folder as the HTML

document. The <TT>&lt;applet&gt;</TT> tag in Listing 2.2 looks

like Listing 2.3 when using the <TT>codebase</TT> attribute. (Don't

try to use run this HTML code, as it's only an example of the

attribute's format and may not run.)

<HR>

<BLOCKQUOTE>

<B>Listing 2.3&nbsp;&nbsp;LST2_3.TXT: Using the </B><I>codebase</I><B>

Attribute.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;applet

    codebase=tictactoe

    code=TicTacToe.class

    width=120

    height=120&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The preceding tag tells the browser that the TicTacToe.class file

is located in a folder called TICTACTOE. This folder must be on

the same level in the directory tree as the HTML file. That is,

if the HTML file is in the folder JAVA\DEMO, then the path for

the .CLASS file should be JAVA\DEMO\TICTACTOE\TicTacToe.class.

You can also use an URL, such as <TT><A TARGET="resource window" HREF="http://www.provider.com/my_pages/tictactoe">http://www.provider.com/my_pages/tictactoe</A></TT>,

for the <TT>codebase</TT> attribute. This causes the applet to

be loaded from the specified site.

<P>

Other optional attributes you can use with the <TT>&lt;applet&gt;</TT>

tag are <TT>alt</TT>, <TT>align</TT>, <TT>name</TT>, <TT>hspace</TT>,

and <TT>vspace</TT>. The <TT>alt</TT> attribute enables you to

specify text that will be displayed by text-only browsers, whereas

the <TT>name</TT> attribute gives the applet a symbolic name that's

used to reference the applet (used when you need to communicate

between applets).

<P>

The <TT>align</TT>, <TT>hspace</TT>, and <TT>vspace</TT> attributes

all work together to position the applet within the text flow

of the HTML document. These attributes work exactly as they do

with the <TT>&lt;img&gt;</TT> tag that's used to display images

in Web pages. The <TT>align</TT> attribute can be one of these

values: <TT>left</TT>, <TT>right</TT>, <TT>middle</TT>, <TT>absmiddle</TT>,

<TT>bottom</TT>, <TT>absbottom</TT>, <TT>baseline</TT>, <TT>top</TT>,

or <TT>texttop</TT>. The <TT>hspace</TT> and <TT>vspace</TT> attributes

control the amount of white space around the applet when <TT>align</TT>

is set to <TT>left</TT> or <TT>right</TT>.

<P>

Listing 2.4 shows the script for a simple Web page using the <TT>&lt;applet&gt;</TT>

tag. Figure 2.7 shows Netscape Navigator 2.0 displaying the page.

<P>

<A HREF="f2-7.gif"><B> Figure 2.7 : </B><I>This is the Web page created by Listing 2.4.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 2.4&nbsp;&nbsp;LST2_4.TXT: A Simple HTML Document Using

the </B><I>&lt;applet&gt;<B> </B></I><B>Tag.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;title&gt;TicTacToe&lt;/title&gt;

&lt;hr&gt;

This is a bunch of text whose sole purpose is to demonstrate

the placement

&lt;applet

    codebase=TicTacToe

    code=TicTacToe.class

    width=120

    height=120

    alt=&quot;This is the TicTacToe applet.&quot;

    name=TicTacToe

    align=middle&gt;

&lt;/applet&gt; 

of the TicTacToe applet within the text flow of an HTML document.

&lt;hr&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<CENTER>