ch10.htm

Java_by_Example,初级经典例子哦,珍藏版本

</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>Applet8</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare <TT>TextField</TT> objects called <TT>textField1</TT>

and <TT>textField2.<BR>

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

method.<BR>

        Create the two <TT>TextField</TT> objects.<BR>

        Add the two <TT>TextField</TT> objects to the applet.

<BR>

        Initialize the <TT>TextField</TT> objects to &quot;1&quot;

and &quot;10.&quot;<BR>

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

<BR>

        Print a prompt for the user.<BR>

        Get the number from the first <TT>TextField</TT> object.

<BR>

        Convert the number from text to an integer.<BR>

        Get the number from the second <TT>TextField</TT> object.

<BR>

        Convert the number from text to an integer.<BR>

        Initialize the row counter.<BR>

        Initialize the loop control variable, count.<BR>

        Loop from the starting value to the ending value.<BR>

            Initialize a string to <TT>&quot;Count = &quot;</TT>.

<BR>

            Add the loop counter to the text string.<BR>

            Draw the text string in the applet's display area.

<BR>

            Increment the row counter.<BR>

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

<BR>

        Tell Java to redraw the applet's display area.<BR>

        Tell Java that the <TT>action()</TT> method finished successfully.

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 10.3&nbsp;&nbsp;APPLET8.htmL: Applet8's HTML Document.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;title&gt;Applet Test Page&lt;/title&gt;

&lt;h1&gt;Applet Test Page&lt;/h1&gt;

&lt;applet

    code=&quot;Applet8.class&quot;

    width=250

    height=350

    name=&quot;Applet8&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

When you run Applet8, type the starting value for the loop in

the first box and the ending value for the loop in the second

box. (Don't press enter until you've entered both values.) When

you do, the program's <TT>while</TT> loop runs for the values

you selected, printing the value of the loop control variable

each time through the loop. For example, in Figure 10.1, the <TT>while</TT>

loop has counted from 1 to 10 (the default values for the <TT>TextField</TT>

controls). You can type any numbers you like, however. If you

typed 15 in the first box and 30 in the last box, the <TT>while</TT>

loop will count from 15 to 30, as shown in Figure 10.2.

<P>

<A HREF="f10-2.gif"><B> Figure 10.2 : </B><I>Applet8 will count however you tell it to.

</I></A><P><P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

If you enter a pair of numbers with a wide range, the applet's output will run off the bottom of the applet. This won't hurt anything, but you'll be unable to see the entire output. In addition, if you make the starting value greater than the ending value, no output will appear in the applet's display area. This is because the <TT>while</TT> loop's conditional expression never evaluates to true.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

The <I>do-while</I> Loop</FONT></A></H2>

<P>

Java also features <TT>do-while</TT> loops. A <TT>do-while</TT>

loop is much like a <TT>while</TT> loop, except a <TT>do-while</TT>

loop evaluates its control expression at the end of the loop rather

than at the beginning. So the body of the loop-the statements

between the beginning and end of the loop-is always executed at

least once. In a <TT>while</TT> loop, the body of the loop may

or may not ever get executed. Listing 10.4 shows how a <TT>do-while</TT>

loop works.

<HR>

<BLOCKQUOTE>

<B>Listing 10.4&nbsp;&nbsp;LST10_4.TXT: Using a </B><I>do-while</I><B>

Loop.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

num = 0;



do

    ++num;

while (num &lt; 10);

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The difference between a <TT>do-while</TT> loop and a <TT>while</TT>

loop are readily apparent when you look at the listing. As you

can see, the loop's conditional expression is at the end instead

of the beginning. That is, in the example listing, <TT>num</TT>

will always be incremented at least once. After Java increments

<TT>num</TT>, it gets to the <TT>while</TT> line and checks whether

<TT>num</TT> is less than 10. If it is, program execution jumps

back to the beginning of the loop, where <TT>num</TT> gets incremented

again. Eventually, <TT>num</TT> becomes equal to 10, causing the

conditional expression to be false, and the loop ends.

<H3><A NAME="ExampleUsingaIdowhileILoop">

Example: Using a <I>do-while</I> Loop</A></H3>

<P>

Near the beginning of this chapter, you saw an example of a <TT>while</TT>

loop whose body contained multiple statements. By using braces

to mark off a program block, you can do the same thing with a

<TT>do-while</TT> loop. Listing 10.5 shows how to create a multiple

line <TT>do-while</TT> loop:

<HR>

<BLOCKQUOTE>

<B>Listing 10.5&nbsp;&nbsp;LST10_5.TXT: A Multiple-Line </B><I>do-while</I><B>

loop.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

num = 0;



do

{

    ++num;

    String s = String.valueOf(num);

    g.drawString(&quot;num is now equal to:&quot;, 20, 40);

    g.drawString(s, 20, 55); 

}

while (num &lt; 10);

</PRE>

</BLOCKQUOTE>

<HR>

<P>

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

<BLOCKQUOTE>

Initialize the loop control variable.<BR>

Begin the <TT>do-while</TT> loop.<BR>

    Increment the loop control variable.<BR>

    Create a string from the value of <TT>num.<BR>

</TT>    Display a message on the screen.<BR>

    Display the value of <TT>num</TT>.<BR>

    Determine whether to repeat the loop.

</BLOCKQUOTE>

<H3><A NAME="ExampleUsingaIdowhileILoopinaProgram">

Example: Using a <I>do-while</I> Loop in a Program</A></H3>

<P>

Now it's time to put your knowledge of <TT>do-while</TT> loops

to the test. If you haven't yet picked up on the pattern, you

may be surprised to learn that the next applet is called Applet9.

Applet9 looks and acts a lot like Applet8. In fact, as far as

the user is concerned, they are almost exactly the same program.

However, the clever among you may have already guessed that Applet9

uses a <TT>do-while</TT> loop in place of the <TT>while</TT> loop.

This means that regardless of the values the user enters into

the applet's <TT>TextField</TT> controls, the applet will always

display at least one line of text. Listing 10.6 is the applet.

Modify the HTML document from Listing 10.3, replacing all occurrences

of Applet8 with Applet9, in order to run the new version of the

applet.

<HR>

<BLOCKQUOTE>

<B>Listing 10.6&nbsp;&nbsp;Applet9.java: Source Code for Applet9.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet9 extends Applet

{

    TextField textField1;

    TextField textField2;



    public void init()

    {

        textField1 = new TextField(5);

        textField2 = new TextField(5);

        add(textField1);

        add(textField2);

        textField1.setText(&quot;1&quot;);

        textField2.setText(&quot;10&quot;);

    }



    public void paint(Graphics g)

    {

        g.drawString(&quot;Enter start and end values above.&quot;, 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;



        do

        {

            s = &quot;Count = &quot;;

            s += String.valueOf(count++);

            g.drawString(s, 80, row * 15 + 70);

            ++row;

        }



        while (count &lt;= end);

    }



    public boolean action(Event event, Object arg)

    {

        repaint();

        return true;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

As long as the user always enters a lower value in the first box

and a higher value in the second, the Applet9 applet will perform

exactly as Applet8 did. However, the difference appears when the

starting value is greater than the ending value. Applet8 produced

no text output at all under these conditions, because the <TT>while</TT>

loop evaluates the condition before it executes the body of the

loop. As Figure 10.3 shows, Applet9 always displays at least one

line of text output, because the loop is evaluated after its body

is executed rather than before. In the figure, the user has entered

a starting value that's higher than the ending value.

<P>

<A HREF="f10-3.gif"><B> Figure 10.3 : </B><I>The do-while loop always executes at least once.



</I></A><P>

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

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

<P>

By using loops, you can easily program your applets to perform

repetitive operations. Although loops may seem a little strange

to you at this point, you'll find more and more uses for them

as you write your own applets. Just remember that a <TT>while</TT>

loop may or may not ever execute depending on how the conditional

expressions works out. On the other hand, a <TT>do-while</TT>

loop always executes at least once because its conditional is

evaluated at the end of the loop rather than at the start. In

the next chapter, you'll learn about one last type of loop, called

a <TT>for</TT> loop. As you'll see, you use a <TT>for</TT> loop

when you know exactly how many times a loop must execute.

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

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

<OL>

<LI>When do you use loops in a program?

<LI>What is the body of a loop?

<LI>How does Java determine when to stop looping?

<LI>How many times is a <TT>while</TT> loop guaranteed to execute?

<LI>Why is it important to initialize a loop control variable?

<LI>What's an infinite loop?

<LI>Compare and contrast <TT>while</TT> and <TT>do-while</TT>

loops?

<LI>How many times will the loop shown in Listing 10.7 execute?

What will <TT>count</TT> equal when the loop finishes?

</OL>

<HR>

<BLOCKQUOTE>

<B>Listing 10.7&nbsp;&nbsp;LST10_7.TXT: The Loop for Review Question

8.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

int count = 10;



do

{

    ++count;

}

while (count &lt;= 15);

</PRE>

</BLOCKQUOTE>

<HR>

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

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

<OL>

<LI>Write a <TT>while</TT> loop that executes 20 times.

<LI>Convert the <TT>while</TT> loop you wrote in exercise 1 to

a <TT>do-while</TT> loop.

<LI>Write a <TT>while</TT> loop that will result in an infinite

loop.

<LI>Write a <TT>do-while</TT> loop that can never execute more

than once.

<LI>Write a <TT>while</TT> loop that counts backwards from 20

to 10.

<LI>Modify Applet8 so that it totals all the numbers from the

user's selected starting value to the ending value. That is, if

the user enters 10 and 15 as the starting and ending values, the

applet should sum 10, 11, 12, 13, 14, and 15, and then display

the result. Name the program <TT>WhileApplet.java</TT>. Figure

10.4 shows what the final applet should look like. (You can find

the solution to this programming problem in the CHAP07 folder

of this book's CD-ROM.)<BR>

<A HREF="f10-4.gif"><B> Figure 10.4 : </B><I>This is what the WhileApplet applet should <r>look like.



</I></A><P>

</OL>

<HR>











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