ch9.htm

Java_by_Example,初级经典例子哦,珍藏版本

<BLOCKQUOTE>

<PRE>

if (x == 1)

    y = 1;

if (x == 2)

    y = 2;

if (x == 3)

    y = 3;

else

    y = 0;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

You could easily rewrite the preceding <TT>if </TT>statement as

a <TT>switch</TT> statement, as shown in Listing 9.9:

<HR>

<BLOCKQUOTE>

<B>Listing 9.9&nbsp;&nbsp;LST9_9.TXT: Changing an </B><I>if</I><B>

to a </B><I>switch<B>.<BR>

</B></I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

switch(x)

{

    case 1:

        y = 1;

        break;

    case 2:

        y = 2;

        break;

    case 3:

        y = 3;

        break;

    default:

        y = 0;

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The first line of a <TT>switch</TT> statement is the keyword <TT>switch</TT>

followed by the variable whose value will determine the outcome.

This variable is called the control variable. Inside the main

<TT>switch</TT> statement (which begins and ends with curly braces)

are a number of <TT>case</TT> clauses, one for each possible value

of the <TT>switch</TT> control variable (the <TT>x</TT>, in this

case). In the above example, if <TT>x</TT> equals 1, Java jumps

to the <TT>case 1</TT> and sets <TT>y</TT> equal to 1. Then, the

<TT>break</TT> statement tells Java that it should skip over the

rest of the <TT>switch</TT> statement.

<P>

If <TT>x</TT> is 2, the same sort of program flow occurs, except

Java jumps to the <TT>case 2</TT>, sets <TT>y</TT> equal to 2,

and then breaks out of the <TT>switch</TT>. If the <TT>switch</TT>

control variable is not equal to any of the values specified by

the various <TT>case</TT> clauses, Java jumps to the <TT>default</TT>

clause. The <TT>default</TT> clause, however, is optional. You

can leave it out if you want, in which case Java will do nothing

if there's no matching <TT>case</TT> for the control variable.

<H3><A NAME="ExampleUsingtheIbreakIStatementCorrectly">

Example: Using the <I>break</I> Statement Correctly</A></H3>

<P>

One tricky thing about <TT>switch</TT> statements is the various

ways that you can use the <TT>break</TT> statement to control

program flow. Look at Listing 9.10:

<HR>

<BLOCKQUOTE>

<B>Listing 9.10&nbsp;&nbsp;LST9_10.TXT: Using </B><I>break</I><B>

to Control Program Flow.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

switch(x)

{

    case 1:

        y = 1;

    case 2:

        y = 2;

        break;

    case 3:

        y = 3;

        break;

    default:

        y = 0;

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

In this example, funny things happen, depending on whether the

control variable <TT>x</TT> equals 1 or 2. In the former case,

Java first jumps to <TT>case 1</TT> and sets <TT>y</TT> equal

to 1. Then, because there is no <TT>break</TT> before the <TT>case

2</TT> clause, Java continues executing statements, dropping through

the <TT>case 2</TT> and setting <TT>y</TT> equal to 2. Ouch! The

moral of the story is: Make sure you have <TT>break</TT> statements

in the right places.

<P>

If the outcome of Listing 9.10 was really what you wanted to happen,

you'd probably rewrite the <TT>switch</TT> statement is to look

like Listing 9.11:

<HR>

<BLOCKQUOTE>

<B>Listing 9.11&nbsp;&nbsp;LST9_11.TXT: Rewriting Listing 9.10.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

switch(x)

{

    case 1:

    case 2:

        y = 2;

        break;

    case 3:

        y = 3;

        break;

    default:

        y = 0;

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

Here, just as in Listing 9.10, <TT>y</TT> will end up equal to

2 if <TT>x</TT> equals 1 or 2. You'll run into this type of <TT>switch</TT>

statement a lot in Java programs. If you're a C or C++ programmer,

you've already seen a lot of this sort of thing, so you should

feel right at home.

<H3><A NAME="ExampleUsingtheIswitchIStatementinaProgram">

Example: Using the <I>switch</I> Statement in a Program</A></H3>

<P>

You've seen that a <TT>switch</TT> statement is similar in many

ways to an <TT>if</TT> statement. In fact, <TT>if</TT> and <TT>switch</TT>

statements can easily be converted from one to the other. Listing

9.12 is a new version of Applet6 (called Applet7) that incorporates

the menu example by using a <TT>switch</TT> statement instead

of an <TT>if</TT> statement.

<HR>

<BLOCKQUOTE>

<B>Listing 9.12&nbsp;&nbsp;APPLET7.JAVA: Using a </B><I>switch</I><B>

Statement in a Program.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet7 extends Applet

{

    TextField textField1;



    public void init()

    {

        textField1 = new TextField(5);

        add(textField1);

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

    }



    public void paint(Graphics g)

    {

        g.drawString(&quot;Type a menu choice in the above box.&quot;, 20, 50);

        g.drawString(&quot;1. Red&quot;, 40, 75);

        g.drawString(&quot;2. Blue&quot;, 40, 95);

        g.drawString(&quot;3. Green&quot;, 40, 115);

        String s = textField1.getText();

        int choice = Integer.parseInt(s);



        switch(choice)

        {

            case 1:

                g.setColor(Color.red);

                break;

            case 2:

                g.setColor(Color.blue);

                break;

            case 3:

                g.setColor(Color.green);

                break;

            default:

                g.setColor(Color.black);

        }



        

if ((choice &gt;= 1) &amp;&amp; (choice &lt;= 3))

            g.drawString(&quot;This is the color you chose.&quot;, 60, 140);

        else

            g.drawString(&quot;Invalid menu selection.&quot;, 60, 140);

    }



    public boolean action(Event event, Object arg)

    {

        repaint();

        return true;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

When you run this program, you'll see the same display as the

one you saw with Applet6. In fact, from the user's point of view,

the program works exactly the same as the previous version. The

differences are completely internal.

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

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

<P>

Making decisions based on the state of data is an important part

of a computer program, and now that you have this chapter behind

you, you've made a giant leap forward in your Java programming

career. You now have enough Java programming savvy to write simple,

yet useful, applets. Of course, that's not to say that there isn't

a lot left to learn. But by now you should have a good idea of

what a Java applet looks like and how it works. The remainder

of this book will fill out the details that make Java applets

so powerful.

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

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

<OL>

<LI>What is program flow?

<LI>Define conditional and unconditional branching.

<LI>What are two ways to control program flow?

<LI>In the following Java example, if <TT>num</TT> equals 5, will

the second line execute?<BR>

<TT>if (choice == 3)<BR>

<FONT FACE="Courier New">    num2 = choice;</FONT></TT>

<LI>Are there times when you don't need braces with an <TT>if</TT>

statement? Explain.

<LI>What's the difference between a logical expression and a Boolean

expression?

<LI>In Listing 9.13, what happens when <TT>choice</TT> equals

5?

<LI>Compare and contrast the <TT>if </TT>and <TT>switch</TT> statements.

<LI>What value is assigned to <TT>num</TT> in Listing 9.14 when

<TT>choice</TT> equals 2?

</OL>

<HR>

<BLOCKQUOTE>

<B>Listing 9.13&nbsp;&nbsp;LST9_13.LST: Listing for Question 7.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

if (choice == 1)

{

    num = 1;

    num2 = 10;

}

else if (choice == 2)

{

    num = 2;

    num2 = 20;

}

</PRE>

</BLOCKQUOTE>

<HR>

<HR>

<BLOCKQUOTE>

<B>Listing 9.14&nbsp;&nbsp;LST9_14.TXT: Listing for Question 10.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

switch(choice)

{

    case 1:

        num = 1;

        break;

    case 2:

        num = 2;

    case 3:

        num = 3;

        break;

    default:

        num = 0;

}

</PRE>

</BLOCKQUOTE>

<HR>

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

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

<OL>

<LI>Write an <TT>if</TT> statement that sets the variable <TT>num</TT>

to 1 when <TT>choice</TT> equals 10.

<LI>Write an <TT>if</TT> statement that sets the variable <TT>num</TT>

to the same value as the control variable <TT>choice</TT> when

<TT>choice</TT> equals 5, but sets <TT>num</TT> to 0 in every

other case.

<LI>Write an <TT>if</TT> statement that sets the variable <TT>num</TT>

to twice the value of the control variable <TT>choice</TT>. Valid

values for <TT>choice</TT> are 3, 4, 5, and 6.

<LI>Convert the <TT>if</TT> statement in exercise 3 to a <TT>switch</TT>

statement.

<LI>Modify Applet7 so that it displays the text string &quot;One,&quot;

&quot;Two,&quot; &quot;Three,&quot; or &quot;Four&quot; when the

user enters 1, 2, 3, or 4, respectively. Have the program display

&quot;Invalid value&quot; for any other user entry. Figure 9.2

shows what the final applet, called SwitchApplet, should look

like. (You can find the solution to this programming problem in

the CHAP09 folder of this book's CD-ROM.)<BR>

<A HREF="f9-2.gif"><B> Figure 9.2 : </B><I>The SwitchApplet applet converts numbers to words.



</I></A><P>

</OL>

<HR>











<HR WIDTH="100%"></P></CENTER>

<!-- reference library footer #1-->
</CENTER>
<IMG SRC="/images/rule.gif" WIDTH="460" HEIGHT="5" VSPACE="5"
ALT="Ruler image"><br>

<FONT SIZE="-1">

Contact <a href="mailto:[e-mail]">
[e-mail]</a> with questions or comments.<br>


<a href="/legal/">
Copyright 1998</a> 
<a href="http://www.earthweb.com" target="_top">
EarthWeb Inc.</a>,  All rights reserved.<BR>
PLEASE READ THE <a href="/reference/usage.html">ACCEPTABLE USAGE STATEMENT</a>.
<BR>
Copyright 1998 Macmillan Computer Publishing. All rights reserved.
</FONT>
</BLOCKQUOTE>


<!--outer table--><TD VALIGN="TOP">

<!--right side ads -->
<a target="resource window" href="http://adserver.developer.com/cgi-bin/accipiter/adclick.exe/AREA=DCAD1.REF" alt="Click here for more info"><img src="http://adserver.developer.com/cgi-bin/accipiter/adserver.exe/AREA=DCAD1.REF" alt="Click here for more info" height="88" width="88" border="0"></a>
<P>
<a target="resource window" href="http://adserver.developer.com/cgi-bin/accipiter/adclick.exe/AREA=DCAD2.REF" alt="Click here for more info"><img src="http://adserver.developer.com/cgi-bin/accipiter/adserver.exe/AREA=DCAD2.REF" alt="Click here for more info" height="88" width="88" border="0"></a>
<P>



</td>
</tr>
</table>
</BODY>
</HTML>