ch2.htm

Java_by_Example,初级经典例子哦,珍藏版本

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

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<BLOCKQUOTE>

To load an HTML document into Netscape Navigator 2.0, select the File, Open File command or press Ctrl+O. Then select the file in the dialog box that appears.</BLOCKQUOTE>



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<P>

<H3><A NAME="AppletParameters">

Applet Parameters</A></H3>

<P>

As you know, many Java applets are configurable, meaning that

the applet user can specify certain values that the applet will

use when it starts. A good example is the BarChart applet you

saw earlier in this chapter. When you need to specify parameters

for an applet, you use the <TT>&lt;param&gt;</TT> tag. The <TT>&lt;param&gt;</TT>

tags, one for each parameter you want to set, are placed after

the starting <TT>&lt;applet&gt;</TT> tag and before the ending

<TT>&lt;/applet&gt;</TT> tag. For example, Listing 2.5 shows parameters

being set for the BarChart applet. Figure 2.8 shows the resultant

bar chart. As you can see from the listing, each parameter has

two parts, the parameter name and the value to which it should

be set. You'll learn more about applet parameters in <A HREF="ch26.htm" >Chapter 26</A>,

&quot;Configurable Applets.&quot;

<P>

<A HREF="f2-8.gif"><B> Figure 2.8 : </B><I>This is the bar chart created by the parameters in Listing 2.5.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 2.5&nbsp;&nbsp;LST2_5.TXT: Using Parameters with Applets.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;applet

    code=&quot;Chart.class&quot;

    width=251

    height=125&gt;

&lt;param name=title value=&quot;Sales&quot;&gt;

&lt;param name=orientation value=&quot;vertical&quot;&gt;

&lt;param name=scale value=&quot;5&quot;&gt;

&lt;param name=columns value=&quot;3&quot;&gt;

&lt;param name=c1_style value=&quot;solid&quot;&gt;

&lt;param name=c1 value=&quot;10&quot;&gt;

&lt;param name=c1_color value=&quot;blue&quot;&gt;

&lt;param name=c1_label value=&quot;Jan&quot;&gt;

&lt;param name=c2_style value=&quot;solid&quot;&gt;

&lt;param name=c2 value=&quot;12&quot;&gt;

&lt;param name=c2_color value=&quot;green&quot;&gt;

&lt;param name=c2_label value=&quot;Feb&quot;&gt;

&lt;param name=c3_style value=&quot;solid&quot;&gt;

&lt;param name=c3 value=&quot;15&quot;&gt;

&lt;param name=c3_color value=&quot;red&quot;&gt;

&lt;param name=c3_label value=&quot;Mar&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<H3><A NAME="NonJavaBrowsers">

Non-Java Browsers</A></H3>

<P>

You may wonder what happens when a browser that's not Java-compatible

finds an applet in an HTML document. In this case, as is standard

behavior for browsers, the non-Java browser simply ignores the

tags it doesn't recognize. However, you may want to provide a

more user-friendly response to users who are trying to view your

applets with non-Java browsers. You can do this easily by placing

alternate content right before the ending <TT>&lt;/applet&gt;</TT>

tag. Listing 2.6, for example, shows the HTML script for running

the TicTacToe applet with alternate content for browsers that

don't support Java.

<HR>

<BLOCKQUOTE>

<B>Listing 2.6&nbsp;&nbsp;LST2_6.TXT: Supplying Alternate Content

for TicTacToe.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;applet

    code=TicTacToe.class

    width=120

    height=120&gt;

&lt;b&gt;If you had a Java-compatible browser,

you'd be playing TicTacToe right now!&lt;/b&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The alternate content you provide can comprise any standard HTML

commands and is ignored by Java-compatible browsers. That is,

the alternate content appears only in non-Java browsers.

<H3><A NAME="ExampleAJavaPoweredHomePage">

Example: A Java-Powered Home Page</A></H3>

<P>

In the previous section, you saw a sample HTML document that contains

an applet. You also saw what this document looks like when loaded

into Netscape Navigator 2.0. Now you'll use what you've learned

to create your own &quot;appletized&quot; home page for Netscape

Navigator. To do this, you must first write your home page's HTML

file. Then you must set Netscape Navigator's home-page property,

which determines the document the browser displays at startup.

Just follow the steps below to accomplish these tasks:

<OL>

<LI>Create a folder called HOMEPAGE in your C: drive's root directory.

<LI>Type Listing 2.7 and save it as an ASCII file called HOMEPAGE.htmL

in your HOMEPAGE directory. (If you don't want to type the listing,

just copy it from the CHAP02 directory of this book's CD-ROM.)

<LI>Copy the entire TICTACTOE folder (not just the folder's contents)

from your JAVA\DEMO folder to your HOMEPAGE folder.

<LI>Start up Netscape Navigator 2.0, and click Navigator's Stop

button to stop loading the currently set home page.

<LI>Select Options, General Preferences from Navigator's menu

bar. The Preferences property sheet appears (Figure 2.9).<BR>

<A HREF="f2-9.gif"><B> Figure 2.9 : </B><I>You can set your own starting <r>home page in Navigator's Preferences property sheet.



</I></A><P>

<LI>Enter <TT>C:\HOMEPAGE\HOMEPAGE.htmL</TT> into the Start With

text box.

<LI>Click the Home button to load your new TicTacToe home page

(Figure 2.10).<BR>

<A HREF="f2-10.gif"><B> Figure 2.10 : </B><I>Here's your new TicTacToe home page, up and running.



</I></A><P>

</OL>

<HR>

<BLOCKQUOTE>

<B>Listing 2.7&nbsp;&nbsp;HOMEPAGE.htmL: An HTML Script for a

Home Page.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;title&gt;My Home Page&lt;/title&gt;

&lt;h1&gt;The TicTacToe Home Page&lt;/h1&gt;

This may be a dumb home page, but it gives

you a chance to play the TicTacToe applet

every time you start Netscape Navigator 2.0!&lt;h&gt;

&lt;hr&gt;

&lt;applet

    codebase=TicTacToe

    code=TicTacToe.class

    width=120

    height=120

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

    name=TicTacToe&gt;

&lt;/applet&gt;

&lt;hr&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

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<BLOCKQUOTE>

When running under Netscape Navigator 2.0, the TicTacToe applet seems to respond slowly to mouse clicks at first. If the applet doesn't respond right away, try moving your mouse pointer off the applet. This trick wakes things up most of the time.</BLOCKQUOTE>



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<P>

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

Summary</FONT></A></H2>

<P>

As you learned in this chapter, Java applets are relatively easy

to add to your HTML documents. However, folks without Web browsers

aren't completely left out of the fun. They can create and view

applets using the handy Appletviewer application that comes as

part of the JDK. Because HTML was designed long before there were

Java applets, Sun Microsystems had to create an extension to HTML

in order to accommodate applets in Web pages. The extension takes

the form of the <TT>&lt;applet&gt;</TT> tag, which enables you

to not only provide values for an applet's attributes, but also

to include a list of parameters and even offer alternate content

for non-Java browsers.

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

Review Questions</FONT></A></H2>

<OL>

<LI>How can someone without a Java-compatible browser run applets?

<LI>What are the three required attributes for the <TT>&lt;applet</TT>&gt;

tag?

<LI>What does the optional <TT>codebase</TT> attribute do?

<LI>Name two other optional applet attributes.

<LI>Why would you use parameters with an applet?

<LI>How can your applet-enhanced HTML documents accommodate non-Java

browsers?

</OL>

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

Review Exercises</FONT></A></H2>

<OL>

<LI>Use Appletviewer to view the BouncingHeads applet (or any

other demo applet in which you're interested).

<LI>Change the size of the BarChart applet in the HTML code from

Listing 2.5.

<LI>Modify the home page you created earlier in this chapter so

that it displays both the NervousText and TicTacToe demo applets.

The code for NervousText is found in the file NervousText.class.

In addition to the applet's standard attributes, you'll also need

to include one parameter called <TT>text</TT>, whose value is

the text you want displayed.

</OL>

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