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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 10</FONT></H1><H1><FONT SIZE=6 COLOR=#Ff0000>The <I>while</I> and <I>do-while</I>Loops</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#TheIwhileILoop">The <I>while</I> Loop</A><UL><LI><A HREF="#ExampleUsingaIwhileILoop">Example: Using a <I>while</I> Loop</A><LI><A HREF="#ExampleUsingaIwhileILoopinaProgram">Example: Using a <I>while</I> Loop in a Program</A></UL><LI><A HREF="#TheIdowhileILoop">The <I>do-while</I> Loop</A><UL><LI><A HREF="#ExampleUsingaIdowhileILoop">Example: Using a <I>do-while</I> Loop</A><LI><A HREF="#ExampleUsingaIdowhileILoopinaProgram">Example: Using a <I>do-while</I> Loop in a Program</A></UL><LI><A HREF="#Summary">Summary</A><LI><A HREF="#ReviewQuestions">Review Questions</A><LI><A HREF="#ReviewExercises">Review Exercises</A></UL><HR><P>A computer handles repetitive operations especially well-it nevergets bored, and it can perform a task as well the 10,000th timeas it did the first. Consider, for example, a disk file containing10,000 names and addresses. If you tried to type labels for allthose people, you'd be seeing spots before your eyes in no time.On the other hand, a printer (with the aid of a computer) cantirelessly spit out all 10,000 labels-and with nary a complaintto the union.<P>Every programming language must have some form of looping commandto instruct a computer to perform repetitive tasks. Java featuresthree types of looping: <TT>for</TT> loops, <TT>while</TT> loops,and <TT>do-while</TT> loops. In this chapter, you learn aboutthe latter two types of loops. In the next chapter, you'll cover<TT>for</TT> loops.<P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD><B>NOTE</B></TD></TR><TR VALIGN=TOP><TD><BLOCKQUOTE>In computer programs, looping is the process of repeatedly running a block of statements. Starting at the top of the block, the statements are executed until the program reaches the end of the block, at which point the program goes back to the top and starts over. The statements in the block may be repeated any number of times, from none to forever. If a loop continues on forever, it is called an infinite loop.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><H2><A NAME="TheIwhileILoop"><FONT SIZE=5 COLOR=#Ff0000>The <I>while</I> Loop</FONT></A></H2><P>One type of loop you can use in your programs is the <TT>while</TT>loop, which continues running until its control expression becomesfalse. The control expression is a logical expression, much likethe logical expressions you used with <TT>if</TT> statements.In other words, any expression that evaluates to <TT>true</TT>or <TT>false</TT> can be used as a control expression for a <TT>while</TT>loop. Here's an example of simple <TT>while</TT> loop:<BLOCKQUOTE><PRE>num = 1;while (num < 10) ++num;</PRE></BLOCKQUOTE><P>Here the loop's control variable <TT>num</TT> is first set to1. Then, at the start of the <TT>while</TT> loop, the programcompares the value in <TT>num</TT> with 10. If <TT>num</TT> isless than 10, the expression evaluates to <TT>true</TT>, and theprogram executes the body of the loop, which in this case is asingle statement that increments <TT>num</TT>. The program thengoes back and checks the value of <TT>num</TT> again. As longas <TT>num</TT> is less than 10, the loop continues. But once<TT>num</TT> equals 10, the control expression evaluates to <TT>false</TT>and the loop ends.<P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD><B>NOTE</B></TD></TR><TR VALIGN=TOP><TD><BLOCKQUOTE>Notice how, in the previous example of a <TT>while</TT> loop, the program first sets the value of the control variable (<TT>num</TT>) to 1. Initializing your control variable before entering the <TT>while</TT> loop is extremely important. If you don't initialize the variable, you don't know what it might contain, and therefore the outcome of the loop is unpredictable. In the above example, if <TT>num</TT> happened to be greater than 10, the loop wouldn't happen at all. Instead, the loop's control expression would immediately evaluate to <TT>false</TT>, and the program would branch to the statement after the curly braces. Mistakes like this make programmers growl at their loved ones.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><H3><A NAME="ExampleUsingaIwhileILoop">Example: Using a <I>while</I> Loop</A></H3><P>Although the previous example has only a single program line inthe body of the <TT>while</TT> loop, you can make a <TT>while</TT>loop do as much as you want. As usual, to add more program lines,you create a program block using braces. This program block tellsJava where the body of the loop begins and ends. For example,suppose you want to create a loop that not only increments theloop control variable, but also displays a message each time throughthe loop. You might accomplish this task as shown in Listing 10.1.<HR><BLOCKQUOTE><B>Listing 10.1 LST10_1.TXT: Using a </B><I>while</I><B>Loop.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>num = 0;while (num < 10){ ++num; String s = String.valueOf(num); g.drawString("num is now equal to:", 20, 40); g.drawString(s, 20, 55); }</PRE></BLOCKQUOTE><HR><P><IMG ALIGN=RIGHT SRC="pseudo.gif" HEIGHT=94 WIDTH=94 BORDER=1><BLOCKQUOTE>Initialize the loop control variable.<BR>Check whether <TT>num</TT> is less than 10.<BR> Increment the loop control variable.<BR> Create a string from the value of <TT>num</TT>.<BR> Display a message on the screen.<BR> Display the value of <TT>num</TT>.<BR></BLOCKQUOTE><P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD><B>NOTE</B></TD></TR><TR VALIGN=TOP><TD><BLOCKQUOTE>The <I>body </I>of a loop comprises the program lines that are executed when the loop control expression is true. Usually, the body of a loop is enclosed in braces, creating a program block.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><P>The pseudocode given after the listing illustrates how this <TT>while</TT>loop works. The thing to notice is how all the statements thatJava should execute if the loop control expression is true areenclosed by braces. As I mentioned previously, the braces createa program block, telling Java where the body of the loop beginsand ends.<P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD><B>CAUTION</B></TD></TR><TR VALIGN=TOP><TD><BLOCKQUOTE>Always initialize (set the starting value of) any variable used in a <TT>while</TT> loop's control expression. Failure to do so may result in your program skipping over the loop entirely. (Initializing a variable means setting it to its starting value. If you need a variable to start at a specific value, you must initialize it yourself.) Also, be sure to increment or decrement the control variable as appropriate in the body of a loop. Failure to do this could result in an <I>infinite loop</I>, which is when the loop conditional never yields a true result, causing the loop to execute endlessly.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><H3><A NAME="ExampleUsingaIwhileILoopinaProgram">Example: Using a <I>while</I> Loop in a Program</A></H3><P>As with most things in life, you learn best by doing. So, in thisexample, you put together an applet that uses a <TT>while</TT>loop to create its display. Listing 10.2 is the applet's sourcecode, whereas Listing 10.3 is the HTML document that loads andruns the applet. Figure 10.1 shows the Applet8 applet runningin Appletviewer. If you need a reminder on how to compile andrun an applet, follow these steps:<P><A HREF="f10-1.gif"><B> Figure 10.1 : </B><I>Applet8 running in Appletviewer.</I></A><P><OL><LI>Type the source code shown in Listing 10.2 and save it inyour CLASSES folder, naming the file <TT>Applet8.java</TT>. (Youcan copy the source code from the CD-ROM, if you like, and thussave on typing.)<LI>Compile the source code by typing <TT>javac Applet8.java</TT>at the MS-DOS prompt, which gives you the <TT>Applet8.class</TT>file.<LI>Type the HTML document shown in Listing 10.3, and save itto your CLASSES folder under the name APPLET8.htmL.<LI>Run the applet by typing, at the MS-DOS prompt, <TT>appletviewerapplet8.html</TT>.</OL><HR><BLOCKQUOTE><B>Listing 10.2 Applet8.java: An Applet That Uses a</B><I>while</I><B> Loop.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>import java.awt.*;import java.applet.*;public class Applet8 extends Applet{ TextField textField1; TextField textField2; public void init() { textField1 = new TextField(5); textField2 = new TextField(5); add(textField1); add(textField2); textField1.setText("1"); textField2.setText("10"); } public void paint(Graphics g) { g.drawString("Enter start and end values above.", 50, 45); String s = textField1.getText(); int start = Integer.parseInt(s); s = textField2.getText(); int end = Integer.parseInt(s); int row = 0; int count = start; while (count <= end) { s = "Count = "; s += String.valueOf(count++); g.drawString(s, 80, row * 15 + 70); ++row; } } public boolean action(Event event, Object arg) { repaint(); return true; }}⌨️ 快捷键说明
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