ch15.htm

Java_by_Example,初级经典例子哦,珍藏版本

    name="MyApplet">

</applet>

</PRE>

</BLOCKQUOTE>

<HR>

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

The Five Stages of an Applet's Life Cycle</FONT></A></H2>

<P>

Every Java applet you create inherits a set of default behaviors

from the <TT>Applet</TT> class. In most cases, these default behaviors

do nothing, unless you override some of <TT>Applet</TT>'s methods

in order to extend the applet's basic functionality. However,

although a simple applet like <TT>MyApplet</TT> in Listing 15.1

doesn't seem to do much, a lot is going on in the background.

Some of this activity is important to your understanding of applets,

and some of it can stay out of sight and out of mind.

<P>

Part of what goes on in a simple applet is the execution of the

applet's life cycle. There are five parts to this cycle, each

of which has a matching method that you can override to gain access

to that cycle of the applet's life. The five stages of an applet's

life cycle are listed here:

<UL>

<LI><I>Initialization stage</I>. This is the part of an applet's

life cycle in which the applet object is created and loaded. At

this point, it's appropriate to create objects needed by the applet,

as well as initialize values that must be valid when the applet

runs. The initialization stage occurs only once in the applet's

life cycle. You can tap into the initialization stage by overriding

the <TT>Applet</TT> class's <TT>init()</TT> method.

<LI><I>Start stage</I>. This stage occurs when the system starts

running the applet. The start stage can occur right after the

initialization stage or when an applet is restarted. This usually

happens when the user switches back to the applet's page after

viewing a different page in his or her Web browser. Unlike the

initialization stage, the start stage can occur several times

over the life of the applet. To provide your own start code, override

the <TT>Applet</TT> class's <TT>start()</TT> method.

<LI><I>Paint stage</I>. The paint stage occurs whenever the applet's

display must be drawn on the screen. This happens right after

the applet's start stage, as well as whenever the applet's display

must be restored or changed. This can happen when the applet is

exposed from underneath another window or when the program changes

the applet's display in some way and explicitly repaints the applet.

Almost every applet you write will have a <TT>paint()</TT> method,

which is the method you override to provide your applet with its

display.

<LI><I>Stop stage</I>. As you may have guessed, the stop stage

is the counterpart to the start stage. Java executes this stage

of the applet's life cycle when the applet is no longer visible

on the screen, such as when the user switches to a different Web

page. The default behavior for this cycle, however, is to keep

the applet running in the background. If you want to handle the

stop cycle differently, you should override the <TT>Applet</TT>

class's <TT>stop()</TT> method.

<LI><I>Destroy stage</I>. This is the counterpart to the initialization

stage and occurs when the system is about to remove the applet

from memory. Like the initialization cycle, the destroy cycle

occurs only once. If your applet has resources that need to be

cleaned up before the applet exits, this is the place to do it.

You tap into this cycle by overriding the <TT>Applet</TT> class's

<TT>destroy()</TT> method.

</UL>

<P>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

To be entirely accurate, the paint stage isn't considered an actual applet life cycle, but because an applet without a display is likely useless (not always, though), I thought I'd include the paint cycle. Truth is, the <TT>paint()</TT> method isn't even defined in the <TT>Applet</TT> class. Rather, <TT>Applet</TT> inherits <TT>paint()</TT> from the <TT>Component</TT> class, a superclass in <TT>Applet</TT>'s long chain of inheritance, which goes from <TT>Applet</TT> to <TT>Panel</TT> to <TT>Container</TT> and finally to <TT>Component</TT>.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

Example: Overriding the Life Cycle Methods</FONT></A></H2>

<P>

All this talk about life cycles and overriding methods may have

left you a little confused as to how all this actually applies

to the applets you want to create. In previous chapters, you managed

to create applets without dealing with most of this stuff because

the <TT>Applet</TT> class, from which you derived your own applet

classes, handled the life-cycle methods in the default manner

proscribed by the Java system. If you look at Listing 15.3, you'll

see a small applet that overrides all the methods needed to provide

custom behaviors for all the applet's life-cycle stages.

<HR>

<BLOCKQUOTE>

<B>Listing 15.3&nbsp;&nbsp;MyApplet2.java: Overriding the Applet

Life-Cycle Methods.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.applet.*;

import java.awt.*;



public class MyApplet2 extends Applet

{

    public void init()

    {

        // Place initialization cycle code here.

    }



    public void start()

    {

        // Place start cycle code here.

    }



    public void paint(Graphics g)

    {

        // Place paint cycle code here.

    }



    public void stop()

    {

        // Place stop cycle code here.

    }

public void destroy()

    {

        // Place destroy cycle code here.

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

Notice that in order to override the <TT>paint()</TT> method,

you must import the <TT>java.awt.*</TT> libraries, which contain

information about the <TT>Graphics</TT> class. As you learned

when writing previous applets in this book, the <TT>Graphics</TT>

class enables you to display information and graphics in an applet's

display area (or canvas, as the display area is sometimes called).

<P>

If you look for the previous methods in Java's source code, you'll

discover that the default implementations of <TT>init()</TT>,

<TT>start()</TT>, <TT>paint()</TT>, <TT>stop()</TT>, and <TT>destroy()</TT>

all do nothing at all. If you want your applet to do something

in response to these cycles, you have to provide the code yourself

by overriding the appropriate method.

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

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

<P>

In this chapter, you got a quick look at the basic applet and

the methods you can call at various stages during the applet's

life cycle. Over the rest of the chapters in this book, you'll

use this knowledge to develop applets that can do anything from

display text in various fonts to execute animation sequences with

sound.

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

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

<OL>

<LI>What is the superclass for all applets?

<LI>Why do applet classes need to be declared as <TT>public</TT>?

<LI>What are the five life-cycle stages of an applet?

<LI>How is the paint cycle different from the rest of the life-cycle

stages?

<LI>What's the difference between the initialize and start life-cycle

stages?

<LI>What do the life-cycle methods in the <TT>Applet</TT> superclass

do?

</OL>

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

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

<OL>

<LI>Write a simple do-nothing applet called <TT>TestApplet</TT>.

<LI>Override the <TT>paint()</TT> method in <TT>TestApplet</TT>

so that the applet displays the text string &quot;Hello there!.&quot;

The final applet should look like Figure 15.1. You can find the

solution for these exercises in the CHAP15 folder of this book's

CD-ROM.<BR>

<A HREF="f15-1.gif"><B> Figure 15.1 : </B><I>This is TestApplet running under Appletviewer.



</I></A><P>

</OL>

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