ch26.htm

Java_by_Example,初级经典例子哦,珍藏版本

<PARAM NAME=typesize VALUE=18>

<PARAM NAME=xpos VALUE=60>

<PARAM NAME=ypos VALUE=150>

</PRE>

</BLOCKQUOTE>

<HR>

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

Default Parameter Values</FONT></A></H2>

<P>

You might have noticed by now that there's a big problem with

the ConfigApplet and ConfigApplet2 applets. Neither applet checks

to ensure that the parameters it tries to retrieve exist. For

example, what happens when the user forgets to include the <TT>text</TT>

parameter?

<P>

Relying on other people to provide your applet with the data it

needs is a dangerous practice. Your applet should always check

the validity of values returned from the <TT>getParameter()</TT>

method. At the very least, you should be sure that the returned

value is not <TT>null</TT>, which is the value <TT>getParameter()</TT>

returns when a particular parameter doesn't exist (that is, the

user forget to define it in the HTML document or deliberately

left it out assuming that the applet will automatically use a

default value for the missing one).

<P>

To ensure that your applet is in a runnable state after retrieving

parameters, you must always check the parameter's values and supply

default values for those parameters that are missing or invalid.

For example, to make sure that your applet has a text string to

display, you might use lines like this:

<BLOCKQUOTE>

<PRE>

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

if (str == null)

    str = &quot;Default Text&quot;;<BR>

</PRE>

</BLOCKQUOTE>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

If you decide to release your applets so other people can use them in their Web pages, be sure that you include a separate documentation file that describes the applet's parameters and shows how to use them.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="ExampleUsingDefaultParametersinanApplet">

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

<P>

You can now extend the ConfigApplet2 so that it provides default

values for each parameter. When you've done this, the applet can

run without generating errors no matter what parameters the user

chooses to include or ignore. Listing 26.7 is the new version,

called ConfigApplet3.

<P>

Notice that although the program now checks for missing parameters,

it doesn't limit the values to any ranges or otherwise check their

validity. Because the <TT>text</TT> parameter will always be a

string, there's really nothing you need to check for (except <TT>null</TT>).

However, you may want to limit the font size or make sure that

the display location is inside the applet's window. Listing 26.8

is the HTML document used to load and run the applet as it's displayed

in Figure 26.4.

<P>

<A HREF="f26-4.gif"><B> Figure 26.4 : </B><I>This is ConfigApplet3 running under Appletviewer.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 26.7&nbsp;&nbsp;ConfigApplet3.java: This Applet Provides

Default Values for All Parameters.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class ConfigApplet3 extends Applet

{

    String str;

    Point position;



    public void init()

    {

        HandleTextParam();

        HandleTypeSizeParam();

        HandlePositionParam();

    }



    public void paint(Graphics g)

    {

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

    }



    protected void HandleTextParam()

    {

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

        if (str == null)

            str = &quot;Default Text&quot;;

    }



    protected void HandleTypeSizeParam()

    {

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

        if (s == null)

            s = &quot;24&quot;;

        int typeSize = Integer.parseInt(s);



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

        setFont(font);

    }



    protected void HandlePositionParam()

    {

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

        if (s == null)

            s = &quot;20&quot;;

        int xpos = Integer.parseInt(s);



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

        if (s == null)

            s = &quot;50&quot;;

        int ypos = Integer.parseInt(s);



        position = new Point(xpos, ypos);

    }

}

</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>ConfigApplet3</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare the class's data fields.<BR>

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

        Call the methods that retrieve and validate the parameters.

<BR>

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

        Display the given string.<BR>

    Define the <TT>HandleTextParam()</TT> method.<BR>

        Retrieve the text parameter.<BR>

        If the parameter is null, set str to the default text.

<BR>

    Define the <TT>HandleTypeSizeParam()</TT> method.<BR>

        Retrieve the typesize parameter.<BR>

        If the parameter is null, set the parameter string to

&quot;24.&quot;<BR>

        Convert the parameter string to an integer.<BR>

        Create and set the font.<BR>

    Define the <TT>HandlePositionParam()</TT> method.<BR>

        Retrieve the xpos parameter.<BR>

        if xpos is null, set the parameter string to &quot;20.&quot;

<BR>

        Convert the parameter string to an integer.<BR>

        Retrieve the ypos parameter.<BR>

        if ypos is null, set the parameter string to &quot;50.&quot;

<BR>

        Convert the parameter string to an integer.<BR>

        Create the Point object with the position values.

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 26.8&nbsp;&nbsp;CONFIGAPPLET3.htmL: The HTML Document

for ConfigApplet3.<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;ConfigApplet3.class&quot;

    width=640

    height=200

    name=&quot;ConfigApplet3&quot;&gt;

    &lt;PARAM NAME=text VALUE=&quot;Hi there!&quot;&gt;

    &lt;PARAM NAME=typesize VALUE=144&gt;

    &lt;PARAM NAME=xpos VALUE=40&gt;

    &lt;PARAM NAME=ypos VALUE=140&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

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

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

<P>

By supporting parameters, your applets are more flexible, which

makes it easier for other people to incorporate them into their

Web-page designs. Even if you don't plan to release your applets,

using parameters can make your applets more powerful and your

own Web pages easier to fine tune. Using the <TT>&lt;PARAM&gt;</TT>

tag is more sensible than having to reprogram and recompile an

applet every time you want it to do something slightly different.

Keep in mind, though, that all parameters must have default values

built into the applet's source code. Otherwise, you could end

up with an error-ridden applet, something that won't do much for

your reputation as a Java guru.

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

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

<OL>

<LI>What kind of applet user is likely to appreciate parameterized

applets?

<LI>What are the two parts of the <TT>&lt;PARAM&gt;</TT> tag?

<LI>How can your applet retrieve the value of a parameter?

<LI>Where do you specify the values for an applet's parameters?

<LI>Do you need to recompile an applet in order to take advantage

of new parameters?

<LI>How many parameters can you have in a single applet?

<LI>Why do you need to convert the values of some parameters?

<LI>Why is it important to supply default values for all parameters?

</OL>

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

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

<OL>

<LI>Write an applet that displays a rectangle on the screen. The

rectangle's size should be specified using parameters.

<LI>Write an applet that uses a parameter to display a red, green,

or blue background.

<LI>Modify ConfigApplet2 so that it not only checks for null parameters,

but also checks for the parameters' validity. The type size should

always be in the range of 10 to 72 points and the text should

always be positioned so that it's never printed completely out

of view. (You can find the solution to this exercise, called ConfigApplet4,

in the CHAP26 folder of this book's CD-ROM.)

</OL>

<HR>











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