ch26.htm

Java_by_Example,初级经典例子哦,珍藏版本

<P>

When the applet runs, it has to find out what greeting to display.

So, in the applet's <TT>init()</TT> method is the following line:

<BLOCKQUOTE>

<PRE>

String str = getParameter(&quot;greeting&quot;);

</PRE>

</BLOCKQUOTE>

<P>

Now that the applet has the text stored in the <TT>str</TT> variable,

it can manipulate and display it any way it needs to.

<H3><A NAME="ExampleUsingaParameterinanApplet">

Example: Using a Parameter in an Applet</A></H3>

<P>

Now that you know how to create HTML documents that set parameters,

as well as how to obtain those parameters from within your applet,

you'd probably like a real parameterized applet with which you

can experiment. Listing 26.2 is an applet called ConfigApplet,

which takes a single parameter. This parameter is the text string

to display. Listing 26.3 is the HTML document that loads and runs

the applet. Notice the <TT>&lt;PARAM&gt;</TT> tag. When you run

the applet with Appletviewer, you see the window shown in Figure

26.1.

<P>

<A HREF="f26-1.gif"><B> Figure 26.1 : </B><I>This applet's display string is given as a parameter.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 26.2&nbsp;&nbsp;ConfigApplet.java: An Applet with a

Single Parameter.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class ConfigApplet extends Applet

{

    String str;



    public void init()

    {

        str = getParameter(&quot;text&quot;);



        Font font = new Font(&quot;TimesRoman&quot;, Font.BOLD, 24);

        setFont(font);

    }



    public void paint(Graphics g)

    {

        g.drawString(str, 50, 50);

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<IMG ALIGN=RIGHT SRC="pseudo.gif" HEIGHT=94 WIDTH=94 BORDER=1>

<BLOCKQUOTE>

Tell Java that the applet uses the classes in the <TT>awt</TT>

package.<BR>

Tell Java that the applet uses the classes in the <TT>applet</TT>

package.<BR>

Derive the <TT>ConfigApplet</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare the class's data field.<BR>

    Override the <TT>init()</TT> method.<BR>

        Retrieve the value of the text parameter.<BR>

        Create and set the font for the applet.<BR>

    Override the <TT>paint()</TT> method.<BR>

        Display the given string.

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 26.3&nbsp;&nbsp;CONFIGAPPLET.htmL: HTML Document for

ConfigApplet.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;title&gt;Applet Test Page&lt;/title&gt;

&lt;h1&gt;Applet Test Page&lt;/h1&gt;

&lt;applet

    code=&quot;ConfigApplet.class&quot;

    width=250

    height=150

    name=&quot;ConfigApplet&quot;&gt;

    &lt;PARAM NAME=text VALUE=&quot;Display Text&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

Once you get ConfigApplet compiled, try running the applet several

times, each time changing the parameter in the HTML document to

a new text string. This will give you a good example of how parameters

work from the HTML document writer's point of view. Changing the

value of the parameter in the HTML document is all you need to

do to display a different text string. You don't have to change

the applet's source code at all.

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

Multiple Parameters</FONT></A></H2>

<P>

When you're writing an application that others may use in their

Web pages, it's important that you make the applet as flexible

as possible. One way to do this is to use parameters for any applet

value that the user might like to customize. Adding multiple parameters

is just a matter of adding additional <TT>&lt;PARAM&gt;</TT> tags

to the HTML document and then retrieving the values of the parameters

in the applet. In the next example, you take a look at ConfigApplet2,

which gives the user much more control over how the applet displays

the text string.

<H3><A NAME="ExampleUsingMultipleParametersinanApplet">

Example: Using Multiple Parameters in an Applet</A></H3>

<P>

Suppose that you want to rewrite ConfigApplet so that the user

can customize not just the text string the applet will display,

but also the position of the text and the size of the font used

to print the text. To do this, you need to create four parameters,

one each for the text to display, the X position of the text,

the Y position of the text, and the point size of the font. Listing

26.4 is an HTML document that loads and runs the ConfigApplet2

applet, which is a new version of ConfigApplet. Notice that the

HTML document now specifies four parameters for the applet. You

can specify as many parameters as you need for an applet.

<HR>

<BLOCKQUOTE>

<B>Listing 26.4&nbsp;&nbsp;CONFIGAPPLET2.htmL: HTML Document for

ConfigApplet2.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;title&gt;Applet Test Page&lt;/title&gt;

&lt;h1&gt;Applet Test Page&lt;/h1&gt;

&lt;applet

    code=&quot;ConfigApplet2.class&quot;

    width=350

    height=200

    name=&quot;ConfigApplet2&quot;&gt;

    &lt;PARAM NAME=text VALUE=&quot;Display Text&quot;&gt;

    &lt;PARAM NAME=typesize VALUE=72&gt;

    &lt;PARAM NAME=xpos VALUE=20&gt;

0    &lt;PARAM NAME=ypos VALUE=100&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

In the above HTML document, the user is specifying that he wants

to display the text string <TT>Display Text</TT> in 72-point type

and at position 20,100. The applet, of course, must call <TT>getParameter()</TT>

to read these values into the applet. Moreover, the applet must

call <TT>getParameter()</TT> once for each parameter. After retrieving

the parameters, the applet must initialize itself such that it

displays the text as requested. Listing 26.5 is the Java source

code for ConfigApplet2, which accomplishes all these tasks. Figure

26.2 shows the applet running under Appletviewer, using the parameters

given in the HTML document in Listing 25.4.

<P>

<A HREF="f26-2.gif"><B> Figure 26.2 : </B><I>This applet accepts four parameters that determine how the text is displayed.



</I></A><P>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Because the <TT>getParameter()</TT> method always returns a string, you may have to convert some parameters before you can use them in your applet. For example, the ConfigApplet2 applet must convert its <TT>typesize</TT>, <TT>xpos</TT>, and <TT>ypos</TT> parameters from strings to integers.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<HR>

<BLOCKQUOTE>

<B>Listing 26.5&nbsp;&nbsp;ConfigApplet2.java: The ConfigApplet2

Applet.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class ConfigApplet2 extends Applet

{

    String str;

    Point position;



    public void init()

    {

        String s;



        str = getParameter(&quot;text&quot;);



        s = getParameter(&quot;typesize&quot;);

        int typeSize = Integer.parseInt(s);



        s = getParameter(&quot;xpos&quot;);

        int xpos = Integer.parseInt(s);



        s = getParameter(&quot;ypos&quot;);

        int ypos = Integer.parseInt(s);



        position = new Point(xpos, ypos);



        Font font = new Font(&quot;TimesRoman&quot;, Font.BOLD, typeSize);

        setFont(font);

    }



    public void paint(Graphics g)

    {

        g.drawString(str, position.x, position.y);

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<IMG ALIGN=RIGHT SRC="pseudo.gif" HEIGHT=94 WIDTH=94 BORDER=1>

<BLOCKQUOTE>

Tell Java that the applet uses the classes in the <TT>awt</TT>

package.<BR>

Tell Java that the applet uses the classes in the <TT>applet</TT>

package.<BR>

Derive the <TT>ConfigApplet2</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare the class's data fields.<BR>

    Override the <TT>init()</TT> method.<BR>

        Declare a local variable.<BR>

        Retrieve the value of the text parameter.<BR>

        Retrieve and convert the typesize parameter.<BR>

        Retrieve and convert the xpos parameter.<BR>

        Retrieve and convert the ypos parameter.<BR>

        Store the position in a Point object.<BR>

        Create and set the font for the applet, using typesize.

<BR>

    Override the <TT>paint()</TT> method.<BR>

        Display the given string.

</BLOCKQUOTE>

<P>

Suppose you were to change the parameters in the HTML file to

those shown in Listing 26.6. You'd then completely change the

way the text string is displayed in the applet, as shown in Figure

26.3. As you can see, parameters can have a profound effect on

the way an applet looks and acts.

<P>

<A HREF="f26-3.gif"><B> Figure 26.3 : </B><I>Here's ConfigApplet2 running with different parameters.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 26.6&nbsp;&nbsp;LST26_6.TXT: New Parameters for ConfigApplet2.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;PARAM NAME=text VALUE=&quot;New Text String&quot;&gt;