ch32.htm

Java_by_Example,初级经典例子哦,珍藏版本

        for (int x=0; x<count; ++x)

            System.out.println(message);

    }



    public static void main(String args[])

    {

        ArgApp app = new ArgApp();

        app.GetArgs(args);

        app.PrintMessage();

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

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

<BLOCKQUOTE>

Declare the <TT>ArgApp</TT> class.<BR>

    Declare the class's data fields.<BR>

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

        Extract the number of times to print the message.<BR>

        Convert the count to an integer.<BR>

        Extract the message to print.<BR>

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

        Loop for the requested number of times.<BR>

            Display the message each time through the loop.<BR>

    Define the app's<TT> main()</TT> method.<BR>

        Create an <TT>ArgApp()</TT> object.<BR>

        Call the method to extract the arguments.<BR>

        Call the method to print the message.

</BLOCKQUOTE>

<P>

The ArgApp application not only shows you how to use command-line

parameters, but also how to write a Java application that actually

does something. The important thing to notice here is that you

must first create an object of the ArgApp class before you can

call its methods and access its data fields. This is because a

class is just a template for an object; it doesn't actually become

an object until you create an instance of the class with the <TT>new</TT>

keyword.

<P>

One way to avoid having to create an object of the class is to

declare all the class's data fields and methods as <TT>static</TT>.

Such methods and data fields are accessible in memory, which is

why the SimpleApp application in Listing 31.1 runs. Its <TT>main()</TT>

method is declared as <TT>static</TT>. Of course, it's a heck

of a lot easier to just go ahead and create an object of the class

than it is to go through the source code and add <TT>static</TT>

to everything.

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

Windowed Applications</FONT></A></H2>

<P>

You're probably thinking that it's pretty much a waste of time

to write Java applications if you have to run them under the DOS

or UNIX text-based operating system. All the action these days

is in windowed applications. Yes, you can write windowed applications

with Java, as the HotJava browser proves. But, if you've written

conventional windowed applications, you're probably a little nervous

about writing similar Java applications. After all, writing such

applications with languages like C and C++ can be a nightmare.

<P>

The truth is, however, you already know almost everything you

need to know to write your own standalone applications for GUI

operating systems. You can take most any applet and convert it

to a standalone application just by adding the <TT>main()</TT>

method to the class. In <TT>main()</TT>, you have to perform some

of the tasks that Java handles for you when running an applet.

These tasks include creating, sizing, and displaying the application's

window. The following example gives you the details.

<H3><A NAME="ExampleChanginganApplettoanApplication">

Example: Changing an Applet to an Application</A></H3>

<P>

As I said in the previous paragraph, you can convert almost any

applet to an application simply by adding a <TT>main()</TT> method

that performs a few housekeeping tasks. Listing 32.3 is an applet

from a previous chapter's exercises. The previous version drew

a face image in the applet's display. The new version displays

the face in its own standalone, frame window. When you run the

application, you see the window shown in Figure 32.4.

<P>

<A HREF="f32-4.gif"><B> Figure 32.4 : </B><I>The FaceApp application draws a face in a frame window.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 32.3&nbsp;&nbsp;FaceApp.java: Converting an Applet

to an Application.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class FaceApp extends Applet

{

    public void init()

    {

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

        add(button);

    }



    public void paint(Graphics g)

    {

        // Head.

        g.drawOval(40, 40, 120, 150);





        // Eyes.

        g.drawOval(57, 75, 30, 20);

        g.drawOval(110, 75, 30, 20);



        // Pupils.

        g.fillOval(68, 81, 10, 10);

        g.fillOval(121, 81, 10, 10);



        // Nose.

        g.drawOval(85, 100, 30, 30);



        // Mouth.

        g.fillArc(60, 130, 80, 40, 180, 180);



        // Ears.

        g.drawOval(25, 92, 15, 30);

        g.drawOval(160, 92, 15, 30);

    }



    public boolean action(Event evt, Object arg)

    {

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

            System.exit(0);

 

        return true;

    }



    public static void main(String args[])

    {

        FaceApp app = new FaceApp();

        Frame frame = new Frame(&quot;Face Window&quot;);



        app.init();

        app.start();



        frame.add(&quot;Center&quot;, app);

        frame.resize(210, 300);

        frame.show();

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

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

<BLOCKQUOTE>

Tell Java that the class uses the <TT>awt</TT> package.<BR>

Tell Java that the class uses the <TT>applet</TT> package.<BR>

Declare the <TT>FaceApp</TT> class.<BR>

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

        Create a button object and add it to the window.<BR>

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

        Draw the face image using ovals and arcs.<BR>

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

        If the user clicks the close button...<BR>

            Shut down the application.<BR>

        Tell Java the message was handled.<BR>

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

        Create the <TT>FaceApp</TT> object and the frame window.

<BR>

        Call the <TT>init()</TT> and <TT>start()</TT> methods.

<BR>

        Add the &quot;applet&quot; to the window.<BR>

        Size and show the window.

</BLOCKQUOTE>

<H3><A NAME="UnderstandingtheFaceAppApplication">

Understanding the FaceApp Application</A></H3>

<P>

If you examine Listing 32.3, you'll see that most of the FaceApp

application is written exactly like any other applet you've written

throughout this book. The big difference is the addition of the

<TT>main()</TT> method, where this program's execution starts.

Because you're no longer running the program as an applet, you

have to perform some of the start-up tasks that Java automatically

performs for applets. The first step is to create an object of

the class and a frame window to hold the object, like this:

<BLOCKQUOTE>

<PRE>

FaceApp app = new FaceApp();

Frame frame = new Frame(&quot;Face Window&quot;);

</PRE>

</BLOCKQUOTE>

<P>

Next, you have to call the methods that get the object going.

These methods are <TT>init()</TT> and <TT>start()</TT>, which

are usually called by Java:

<BLOCKQUOTE>

<PRE>

app.init();

app.start();

</PRE>

</BLOCKQUOTE>

<P>

Note that, although the FaceApp class doesn't show a <TT>start()</TT>

method, it does inherit it from the <TT>Applet</TT> class. It's

true that even the inherited <TT>start()</TT> method doesn't do

anything, but you might as well be consistent and call it anyway.

That way, you won't forget that some applets do override the <TT>start()</TT>

method, without which they won't run properly.

<P>

Now, you can add the FaceApp object to the frame window, like

this:

<BLOCKQUOTE>

<PRE>

frame.add(&quot;Center&quot;, app);

</PRE>

</BLOCKQUOTE>

<P>

Finally, you must be sure to resize and show the window, like

this:

<BLOCKQUOTE>

<PRE>

frame.resize(210, 300);

frame.show();

</PRE>

</BLOCKQUOTE>

<P>

If you fail to resize the application's window, all you'll see

is the title bar, forcing the user to resize the window by hand.

More importantly, if you fail to call <TT>show()</TT>, the application's

window won't even appear on the screen. That's sure to leave your

application's user feeling abused.

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

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

<P>

Although Java is used almost exclusively for creating applets,

it is possible to use Java to create standalone applications.

These applications can be DOS-based (or UNIX, etc.) or be written

for a windowed operating system such as Windows 95. The easiest

way to create a windowed application is to write the program as

if it were an applet and then add a <TT>main()</TT> method that

creates the application object and the frame window that'll contain

that object.

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

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

<OL>

<LI>What method do you find in an application that you don't find

in an applet?

<LI>How do you run a Java standalone application?

<LI>What is the single parameter received by the <TT>main()</TT>

method?

<LI>How can you extract parameters from an application's command

line?

<LI>What do you have to add to an applet to convert it to an application?

<LI>Why do you have to instantiate an object from a class before

you can call its methods?

<LI>What do you have to do to create and display an application's

window?

</OL>

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

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

<OL>

<LI>Write a stand-alone application that accepts two numbers as

parameters and prints out the numbers' sum.

<LI>Choose any applet from this book and convert it to a stand-alone

application.

<LI>Convert the ThreadApplet5 applet from <A HREF="ch31.htm" >Chapter 31</A>'s exercises

into a stand-alone application. (You can find the solution for

this exercise in the CHAP32 folder of this book's CD-ROM.)

</OL>

<HR>











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