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SHAPE ="RECT" HREF ="/reference/author.html" COORDS ="361,0,440,29"><AREA SHAPE ="RECT" HREF ="/reference/basic.html" COORDS ="254,0,360,29"><AREA SHAPE ="RECT" HREF ="/reference/help.html" COORDS ="183,0,251,29"><AREA SHAPE ="RECT" HREF ="/reference/cool.html" COORDS ="100,0,181,29"><AREA SHAPE ="RECT" HREF ="/reference/mybook.html" COORDS ="0,0,100,29"></MAP><MAP NAME ="header_r1.gif.map"><AREA SHAPE ="RECT" HREF ="/reference/personalize.html" COORDS ="339,0,439,29"></MAP><p><font face="Arial,Helvetica" size="-1" color="#006666"><A HREF="/reference/r_library.html"><B>All Categories</A> :</B><b><A HREF="/reference/r_java.html">Java</A></b></font><p><META><H1><FONT SIZE=6 COLOR=#Ff0000>Chapter 15</FONT></H1><H1><FONT SIZE=6 COLOR=#Ff0000>Writing a Simple Applet</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#TheSimplestJavaApplet">The Simplest Java Applet</A><LI><A HREF="#TheFiveStagesofanAppletsLifeCycle">The Five Stages of an Applet's Life Cycle</A><LI><A HREF="#ExampleOverridingtheLifeCycleMethods">Example: Overriding the Life Cycle Methods</A><LI><A HREF="#Summary">Summary</A><LI><A HREF="#ReviewQuestions">Review Questions</A><LI><A HREF="#ReviewExercises">Review Exercises</A></UL><HR><P>Throughout Part II of this book, you've been writing applets thatdemonstrate the various features of the Java language. Along theway, you learned a few things about how applet works, includinghow Java calls the <TT>init()</TT> method when the applet starts,how the <TT>paint()</TT> method draws the applet's display, andhow Java calls the <TT>action()</TT> method in response to someaction by the user (for example, pressing Enter when typing ina <TT>TextField</TT> object). In this chapter, you'll extend yourknowledge of applets by looking more closely at the constructionof an applet, as well as discovering some other methods that areimportant to Java applets.<H2><A NAME="TheSimplestJavaApplet"><FONT SIZE=5 COLOR=#Ff0000>The Simplest Java Applet</FONT></A></H2><P>The Java programming language and libraries enable you to createapplets that are as simple or as complex as you like. In fact,you can write the simplest Java applet in only a few lines ofcode, as shown in Listing 15.1.<HR><BLOCKQUOTE><B>Listing 15.1 MyApplet.java: The Simplest Java Applet.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>import java.applet.*;public class MyApplet extends Applet{}</PRE></BLOCKQUOTE><HR><P>The first line of Listing 15.1 tells the Java compiler that thisapplet will be using some or all of the classes defined in the<TT>applet</TT> package (the asterisk acts as a wildcard, justas in DOS file names). All of the basic capabilities of an appletare provided for in these classes, which is why you can createa usable applet with so few lines of code.<P>The second line of code declares a new class called <TT>MyApplet</TT>.This new class is declared as <TT>public</TT> so that the classcan be accessed when the applet is run in a Web browser or inthe Appletviewer application. If you fail to declare the appletclass as <TT>public</TT>, the code will compile fine, but theapplet will refuse to run. In other words, all applet classesmust be <TT>public</TT>.<P>As you can see, you take advantage of object-oriented programming(OOP) inheritance to declare your applet class by subclassingJava's <TT>Applet</TT> class. This inheritance works exactly thesame as when you created your own classes in <A HREF="ch14.htm" >Chapter 14</A>, "Classes."The only difference is that <TT>Applet</TT> is a class that'sincluded with the Java Developer's Kit (JDK), rather than a classyou created yourself.<P>You can actually compile the applet shown in Listing 15.1. Whenyou do, you'll have the <TT>MyApplet.class</TT> file, which isthe byte-code file that can be executed by the Java system. Torun the applet, just create an HTML document like the one shownin Listing 15.2. You've already used similar HTML documents withthe many applets you created in part II of this book. However,if you need a refresher course on using the <TT><applet></TT>tag, turn back to <A HREF="ch2.htm" >Chapter 2</A> "Running Java Applets."If you were to run the MyApplet applet, however, you wouldn'tsee anything much in Appletviewer or in your Web browser.<HR><BLOCKQUOTE><B>Listing 15.2 MYAPPLET.htmL: MyApplet's HTML Document.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE><title>Applet Test Page</title><h1>Applet Test Page</h1><applet code="MyApplet.class" width=250 height=250⌨️ 快捷键说明
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