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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 6</FONT></H1><H1><FONT SIZE=6 COLOR=#Ff0000>Simple Input and Output</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#WindowsandGraphics">Windows and Graphics</A><LI><A HREF="#DisplayingTextinanApplet">Displaying Text in an Applet</A><UL><LI><A HREF="#ExampleCreatingandRunningApplet">Example: Creating and Running Applet1</A><LI><A HREF="#HowAppletWorks">How Applet1 Works</A></UL><LI><A HREF="#GettingInputfromtheUser">Getting Input from the User</A><UL><LI><A HREF="#HowAppletWorks1">How Applet2 Works</A><LI><A HREF="#ExampleRetrievingtextfromaITextFieldIcontrol">Example: Retrieving text from a <I>TextField</I> control</A><LI><A HREF="#HowAppletWorks2">How Applet3 Works</A></UL><LI><A HREF="#DisplayingNumericalValues">Displaying Numerical Values</A><LI><A HREF="#Summary">Summary</A><LI><A HREF="#ReviewQuestions">Review Questions</A><LI><A HREF="#ReviewExercises">Review Exercises</A></UL><HR><P>In the previous chapter, you learned how you can store varioustypes of values in your Java programs. It may have occurred toyou that it doesn't do much good to store data in a computer'smemory if you have no way of actually seeing that data. Also,you need a way to request and receive data from users of yourprograms. Both of these tasks-displaying and retrieving data-areaccomplished through simple input and output, or I/O (as it'scommonly called). In this chapter, you'll learn how Java enablesyou to perform simple I/O in your applets.<H2><A NAME="WindowsandGraphics"><FONT SIZE=5 COLOR=#Ff0000>Windows and Graphics</FONT></A></H2><P>Because you'll be writing your applets for a graphical user interface(GUI) such as Windows, you can't use the same kinds of data input/outputthat you may have been accustomed to using under another operatingsystem, such as MS-DOS. This is because, under a system such asWindows, everything that appears on the screen is graphical (unlikeMS-DOS, which displays plain old text).<P>Still, displaying graphical text doesn't require a whole lot ofwork, as long as you're not concerned with things like fonts andthe size of the text string you want to display. You have to remember,though, that almost all graphical text is proportional, meaningthat each letter in a line of graphical text takes up a differentamount of space. Under an operating system like MS-DOS, most textoutput uses non-proportional characters. The difference is illustratedin Figure 6.1.<P><A HREF="f6-1.gif"><B> Figure 6.1 : </B><I>In a proportional font, each character takes <r>up only as much space as it needs.</I></A><P><P>Look at the letter "I" in the proportional font. Youcan see that it takes up much less space than other letters inthe word. On the other hand, the letter "I" in the non-proportionalfont, as well as the hyphen, takes up exactly the same amountof space as every other letter.<P>The point is that, because of the different fonts that are usedto print text in a graphical user interface, you can't assumemuch about the size of the text that'll be used. There are waysto figure out the actual size of a text string, but you don'thave to be concerned with these details for now. Just be awarethat text output in your applets is going to be graphical.<H2><A NAME="DisplayingTextinanApplet"><FONT SIZE=5 COLOR=#Ff0000>Displaying Text in an Applet</FONT></A></H2><P>The easiest thing to display is a line of text. But because thetext output will be graphical, you need to use one of Java's graphical-textfunctions. The most commonly used is <TT>drawString()</TT>, whichis part of the <TT>Graphics</TT> class contained in the <TT>awt</TT>package. Listing 6.1 shows a simple applet that uses this methodto display a single line of text. Figure 6.1 shows what the appletlooks like when viewed with the Appletviewer application. Finally,Listing 6.2 is an HTML document that tests the Applet1 applet.<P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD><B>NOTE</B></TD></TR><TR VALIGN=TOP><TD><BLOCKQUOTE>A <I>package</I> is nothing more than a collection of related classes. The <TT>awt</TT> (abstract windows toolkit) package contains all the classes that handle graphical windows. You'll learn a lot more about classes, packages, and the <TT>awt</TT> later in this book.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><HR><BLOCKQUOTE><B>Listing 6.1 Applet1.java: An Applet That Displaysa Single Line of Text.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>import java.awt.*;import java.applet.*;public class Applet1 extends Applet{ public void paint(Graphics g) { g.drawString("Hello from Java!", 60, 75); }}</PRE></BLOCKQUOTE><HR><P><IMG ALIGN=RIGHT SRC="pseudo.gif" HEIGHT=94 WIDTH=94 BORDER=1><BLOCKQUOTE>Tell Java that the program uses classes in the <TT>awt</TT> package.<BR>Tell Java that the program uses classes in the <TT>applet</TT>package.<BR>Derive the <TT>Applet1</TT> class from Java's <TT>Applet</TT>class.<BR> Override the <TT>Applet</TT> class's <TT>paint()</TT> method.<BR> Draw a text string on the applet's surface.</BLOCKQUOTE><BLOCKQUOTE><A HREF="f6-2.gif"><B> Figure 6.2 : </B><I>When you run the Applet1 applet, you'll see a single line of text in the applet's display.</I></A><P></BLOCKQUOTE><HR><BLOCKQUOTE><B>Listing 6.2 APPLET1.htmL: An HTML Document for TestingApplet1.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE><title>Applet Test Page</title><h1>Applet Test Page</h1><applet code="Applet1.class" width=200 height=150 name="Applet1">⌨️ 快捷键说明
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