ch28.htm

Java_by_Example,初级经典例子哦,珍藏版本

<BLOCKQUOTE>

You can load ConnectApplet's HTML file using Appletviewer, if you like. However, you will be unable to make a connection to the requested URL. You can, however, see what happens when you enter a badly constructed URL string.</BLOCKQUOTE>



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<HR>

<BLOCKQUOTE>

<B>Listing 28.3&nbsp;&nbsp;ConnectApplet.java: An Applet That

Connects to User-Requested URLs.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;

import java.net.*;



public class ConnectApplet extends Applet

{

    TextField textField;

    boolean badURL;



    public void init()

    {

        textField = new TextField(&quot;&quot;, 40);

        Button button = new Button(&quot;Connect&quot;);



        add(textField);

        add(button);



        badURL = false;

    }



    public void paint(Graphics g)

    {

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

        g.setFont(font);



        int height = font.getSize();



        if (badURL)

            g.drawString(&quot;Bad URL!&quot;, 60, 130);

        else

        {

            g.drawString(&quot;Type the URL to which&quot;, 25, 130);

            g.drawString(&quot;you want to connect,&quot;,

                25, 130+height);

            g.drawString(&quot;and then click the Connect&quot;,

                25, 130+height*2);

            g.drawString(&quot;button.&quot;, 25, 130 + height*3);

        }

    }



    public boolean action(Event evt, Object arg)

    {

        String str = textField.getText();



        try

        {

            URL url = new URL(str);

            AppletContext context = getAppletContext();

            context.showDocument(url);

        }

        catch (MalformedURLException e)

        {

            badURL = true;

            repaint();

        }

        

        return true;

    }

}

</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>

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

package.<BR>

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

class.<BR>

    Declare the class's data fields.<BR>

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

        Create the <TT>TextField</TT> and <TT>Button</TT> controls.

<BR>

        Add the controls to the applet's layout.<BR>

        Initialize the bad URL flag.<BR>

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

        Create and set the <TT>Graphics</TT> object's font.<BR>

        Get the font's height.<BR>

        If the applet has a bad URL string...<BR>

            Display an error message.<BR>

        Or, of the URL is OK...<BR>

            Draw the applet's instructions.<BR>

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

        Get the URL string the user entered.<BR>

        Start the try block.<BR>

            Attempt to create an URL object from the string.<BR>

            Get the <TT>AppletContext</TT> object.<BR>

            Make the connection.<BR>

        Start the catch block.<BR>

            Set the bad URL flag to true.<BR>

            Repaint the applet in order to display the error message.

<BR>

        Tell Java that the applet handled the event message.

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 28.4&nbsp;&nbsp;CONNECTAPPLET.htmL: ConnectApplet's

HTML Document.<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;ConnectApplet.class&quot;

    width=300

    height=250

    name=&quot;ConnectApplet&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

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

Creating a &quot;Favorite URLs&quot; Applet</FONT></A></H2>

<P>

Nothing, of course, says that the string from which you create

an <TT>URL</TT> object must be typed in by the user at runtime.

You can hard-code the URLs you want to use right in the applet's

source code, which not only ensures that the URLs will always

be correct (unless the associated server changes), but also makes

it quick and easy to jump to whatever URL you want. Using this

idea, you can put together an applet that gives you pushbutton

control over your connections, selecting your URLs as easily as

you'd select a radio station.

<P>

The ConnectApplet2 applet, shown in Listing 28.5, is just such

an applet. In its current version, it provides four buttons that

give you instant connection to the Web sites represented by the

buttons. Want to jump to Microsoft's Web page? Give the Microsoft

button a click. Want to check out the latest news at Macmillan

Computer Publishing? Click the Macmillan button. Of course, just

as with the original ConnectApplet, you must have your Internet

connection established before you run the applet. And, you must

run the applet from a Java-compatible browser.

<P>

When you run the applet from Netscape Navigator 2.0, you see the

window shown in Figure 28.4. As you can see, the applet currently

displays four buttons, one each for the Sun, Netscape, Microsoft,

and Macmillan Web sites. Just click a button to jump to the associated

site. (Figure 28.5 shows the browser after the user has clicked

the Macmillan button.) When you're through with that site, use

the browser's Back button to return to the ConnectApplet2 applet.

Then, choose another site.

<P>

<A HREF="f28-4.gif"><B> Figure 28.4 : </B><I>ConnectApplet2 running under Netscape Navigator 2.0.



</I></A><P>

<P>

<A HREF="f28-5.gif"><B> Figure 28.5 : </B><I>After clicking the Macmillan button.



</I></A><P>

<P>

Sure, you can do the same sort of thing with an HTML document

using Web links. But, let's face it, applets are way cooler.

<HR>

<BLOCKQUOTE>

<B>Listing 28.5&nbsp;&nbsp;ConnectApplet2.java: A &quot;Favorite

URLs&quot; Applet.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;

import java.net.*;



public class ConnectApplet2 extends Applet

{

    boolean badURL;



    public void init()

    {

        GridLayout layout = new GridLayout(2, 2, 10, 10);

        setLayout(layout);



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

        setFont(font);



        Button button = new Button(&quot;Sun&quot;);

        add(button);

        button = new Button(&quot;Netscape&quot;);

        add(button);

        button = new Button(&quot;Microsoft&quot;);

        add(button);

        button = new Button(&quot;Macmillan&quot;);

        add(button);



        badURL = false;

    }



    public void paint(Graphics g)

    {

        if (badURL)

            g.drawString(&quot;Bad URL!&quot;, 60, 130);

    }



    public boolean action(Event evt, Object arg)

    {

        String str;



        if (arg == &quot;Sun&quot;)

            str = &quot;http://www.sun.com&quot;;

        else if (arg == &quot;Netscape&quot;)

            str = &quot;http://www.netscape.com&quot;;

        else if (arg == &quot;Microsoft&quot;)

            str = &quot;http://www.microsoft.com&quot;;

        else

            str = &quot;http://www.mcp.com&quot;;



        try

        {

            URL url = new URL(str);

            AppletContext context = getAppletContext();

            context.showDocument(url);

        }

        catch (MalformedURLException e)

        {

            badURL = true;

            repaint();

        }

        

        return true;

    }

}

</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>

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

package.<BR>

Derive the <TT>ConnectApplet2</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare the class's data field.<BR>

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

        Create and set the applet's layout manager.<BR>

        Create and set the applet's font.<BR>

        Add four button controls to the layout.<BR>

        Initialize the bad URL flag.<BR>

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

        If the applet has a bad URL string...<BR>

            Display an error message.<BR>

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

        Declare a local string variable.<BR>

        Get the <TT>URL</TT> string the user requested.<BR>

        Start the try block.<BR>

            Create an <TT>URL</TT> object from the string.<BR>

            Get the <TT>AppletContext</TT> object.<BR>

            Make the connection.<BR>

        Start the catch block.<BR>

            Set the bad URL flag to true.<BR>

            Repaint the applet in order to display the error message.

<BR>

        Tell Java that the applet handled the event message.

</BLOCKQUOTE>

<P>

In Listing 28.5, notice how, even though the URLs are hard-coded

into the program, the <TT>action()</TT> method still surrounds

the call to the <TT>URL</TT> constructor with the <TT>try</TT>

and <TT>catch</TT> program blocks. This is because Java insists

that the applet handle the exception should the <TT>URL</TT> class

throw it. If you remove the exception handling, the applet won't

compile. Anyway, having a little extra protection never hurts.

Handling the exception is a good way to test whether your hard-coded

URLs are valid. I've never known a programmer yet who didn't need

to be protected from himself!

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

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

<P>

Although a running applet has to deal with many security considerations,

it can usually connect to other Web sites. To do this, the applet

creates an <TT>URL</TT> object representing the site to which

the applet should connect. The applet then instructs the browser

containing the applet to make the connection, by calling the <TT>AppletContext</TT>

object's <TT>showDocument()</TT> method. In spite of the telecommunications

limitations inherent in applets, you can easily create Internet-aware

applets.

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

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

<OL>

<LI>What is the single argument accepted by the version of the

URL constructor you studied in this chapter?

<LI>What is an AppletContext object?

<LI>How do you obtain a AppletContext object?

<LI>How can you be sure you have a valid URL object before trying

to connect to the URL?

<LI>What are the two types of program blocks that handle exceptions?

<LI>How do you connect to the URL represented by an URL object?

<LI>What type of exception is thrown by the <TT>URL</TT> class?

</OL>

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

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

<OL>

<LI>Write an applet that accepts<!-- reference library footer #1-->
</CENTER>
<IMG SRC="/images/rule.gif" WIDTH="460" HEIGHT="5" VSPACE="5"
ALT="Ruler image"><br>

<FONT SIZE="-1">

Contact <a href="mailto:[e-mail]">
[e-mail]</a> with questions or comments.<br>


<a href="/legal/">
Copyright 1998</a> 
<a href="http://www.earthweb.com" target="_top">
EarthWeb Inc.</a>,  All rights reserved.<BR>
PLEASE READ THE <a href="/reference/usage.html">ACCEPTABLE USAGE STATEMENT</a>.
<BR>
Copyright 1998 Macmillan Computer Publishing. All rights reserved.
</FONT>
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