ch11.htm

Java_by_Example,初级经典例子哦,珍藏版本

&quot;Moe Howard.&quot;<BR>

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

<BR>

        Display a prompt for the user.<BR>

        Get the name from the <TT>TextField</TT> object.<BR>

        Loop ten times.<BR>

            Draw the name in the applet's display area.<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 11.4&nbsp;&nbsp;APPLET10.htmL: The HTML Document That

Runs Applet10.<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;Applet10.class&quot;

    width=250

    height=250

    name=&quot;Applet10&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The Applet10 applet is well explained in the pseudocode following

Listing 11.3. However, notice how the loop counter <TT>x</TT>

works in this program. Although the <TT>for</TT> loop executes

10 times, the largest value stored in <TT>x</TT> will be 9. This

is because the loop counter starts at 0, and counts 0, 1, 2, 3,

4, 5, 6, 7, 8, and 9. You might at first expect <TT>x</TT> to

end up being equal to 10, Watch out for this sort of thing in

your Java programs, especially if you use the value of the loop

counter within the body of the loop or elsewhere in the program.

Applet10 uses the loop counter to determine the row at which to

print the next name. Keep in mind that, because <TT>x</TT> is

declared as part of the <TT>for</TT> loop's program block, it

can't be accessed outside of that block.

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

Changing the Increment Value</FONT></A></H2>

<P>

The previous example of a <TT>for</TT> loop increments the loop

counter by 1. But suppose you want a <TT>for</TT> loop that counts

from 5 to 50 by fives? This could be useful if you need to use

the loop counter to display a value, which needs to be incremented

by a different number.  You can do this by changing the sections

of the <TT>for</TT> loop, like this.

<BLOCKQUOTE>

<PRE>

for (x=5; x&lt;=50; x+=5)

</PRE>

</BLOCKQUOTE>

<P>

This loop doesn't start counting at 1. Rather, the loop variable

begins with a value of 5. Then, thanks to the <TT>x+=5</TT> statement,

the loop variable is incremented by 5 each time through the loop.

Therefore, <TT>x</TT> goes from 5 to 10, from 10 to 15, and so

on up to 50, resulting in ten loops.

<P>

You can also use a <TT>for</TT> loop to count backwards, like

this:

<BLOCKQUOTE>

<PRE>

for (x=50; x&gt;=5; x-=5)

</PRE>

</BLOCKQUOTE>

<P>

Notice that in the initialization part of the <TT>for</TT> statement,

the higher value is used. Notice also that the increment clause

uses a decrement operator, which causes the loop count to be decremented

(decreased) rather than incremented.

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

Example: Looping with Different Increments</FONT></A></H2>

<P>

If you're a little confused, take a look at Listing 11.5, which

shows the Java source code for a small applet called Applet11.

This applet's <TT>paint()</TT> method contains two <TT>for</TT>

loops, one that counts upward by fives and one that counts downward

by fives. Each time through the loop, the program prints out the

value of the loop control variable, so that you can see exactly

what's going on inside the loops. To run this applet, use the

HTML document from the previous applet, but replace all occurrences

of Applet10 with Applet11. Figure 11.3 shows Appletviewer running

the Applet11 applet.

<P>

<A HREF="f11-3.gif"><B> Figure 11.3 : </B><I>You can use for loops to count forward or backward by any amount that you like.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 11.5&nbsp;&nbsp;Applet11.java: Using Different Increments

with </B><I>for</I><B> Loops.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet11 extends Applet

{

    public void paint(Graphics g)

    {

        int row = 0;



        for (int x=5; x&lt;=40; x+=5)

        {

            String s = &quot;Loop counter = &quot;;

            s += String.valueOf(x);

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

            ++row;

        }



        for (int x=40; x&gt;=5; x-=5)

        {

            String s = &quot;Loop counter = &quot;;

            s += String.valueOf(x);

            

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

            ++row;

        }

    }

}

</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>Applet11</TT> class from Java's <TT>Applet</TT>

class.<BR>

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

<BR>

        Initialize the row counter.<BR>

        Begin the first <TT>for</TT> loop.<BR>

            Create the basic display string.<BR>

            Add the loop counter value to the display string.

<BR>

            Show the string in the applet's display area.<BR>

            Increment the row counter.<BR>

        Start the second <TT>for</TT> loop.<BR>

            Create the basic display string.<BR>

            Add the loop counter value to the display string.

<BR>

            Show the string in the applet's display area.<BR>

            Increment the row counter.

</BLOCKQUOTE>

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

Using Variables in Loops</FONT></A></H2>

<P>

Just as you can substitute variables for most numerical values

in a program, you can also substitute variables for the literals

in a <TT>for</TT> loop. In fact, you'll probably use variables

in your loop limits as often as you use literals, if not more.

Here's an example of how to use variables to control your <TT>for</TT>

loops:

<BLOCKQUOTE>

<PRE>

for (x=start; x&lt;=end; x+=inc)

</PRE>

</BLOCKQUOTE>

<P>

In this partial <TT>for</TT> loop, the loop control variable <TT>x</TT>

starts off at the value of the variable <TT>start</TT>. Each time

through the loop, Java increments <TT>x</TT> by the value stored

in the variable <TT>inc</TT>. Finally, when <TT>x</TT> is greater

than the value stored in <TT>end</TT>, the loop ends. As you can

see, using variables with <TT>for</TT> loops (or any other kind

of loop, for that matter) enables you to write loops that work

differently based on the state of the program.

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

Example: Controlling <I>for</I> Loops with Variables</FONT></A></H2>

<P>

For this chapter's last applet, you'll apply everything you've

learned about <TT>for</TT> loops. The Applet12 applet, shown in

Listing 11.6, enables you to experiment with different starting,

ending, and increment values for the <TT>for</TT> loop contained

in the applet's <TT>paint()</TT> method. Enter the starting value

for the loop in the first box, then enter the ending and increment

values in the second and third boxes. Don't press Enter until

you've filled in all three boxes with the values you want. When

you press Enter, the <TT>action()</TT> method takes over, telling

Java to repaint the applet's display area using the new values.

Figure 11.5 shows Applet12 running under the Appletviewer application.

<P>

<A HREF="f11-4.gif"><B> Figure 11.4 : </B><I>Applet12 enables you to experiment with different starting, ending, and increment values.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 11.6&nbsp;&nbsp;Applet12.java: Advanced </B><I>for<B>

</B></I><B>Loops.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet12 extends Applet

{

    TextField textField1;

    TextField textField2;

    TextField textField3;



    public void init()

    {

        textField1 = new TextField(5);

        textField2 = new TextField(5);

        textField3 = new TextField(5);



        add(textField1);

        add(textField2);

        add(textField3);



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

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

        textField3.setText(&quot;1&quot;);

    }



    

public void paint(Graphics g)

    {

        g.drawString(&quot;Enter loop starting, ending,&quot;, 50, 45);

        g.drawString(&quot;and increment values above.&quot;, 50, 60);



        String s = textField1.getText();

        int start = Integer.parseInt(s);

        s = textField2.getText();

        int end = Integer.parseInt(s);

        s = textField3.getText();

        int inc = Integer.parseInt(s);



        int row = 0;



        for (int x=start; x&lt;=end; x+=inc)

        {

            String s2 = &quot;Loop counter = &quot;;

            s2 += String.valueOf(x);

            g.drawString(s2, 50, row * 15 + 85);

            ++row;

        }

    }



    public boolean action(Event event, Object arg)

    {

        repaint();

        return true;

    }

}

</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>Applet12</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare three <TT>TextField</TT> objects as fields of the

class.<BR>

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

<BR>

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

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

<BR>

        Initialize the <TT>TextField</TT> objects with their starting

text.<BR>

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

<BR>

        Display a prompt to the user.<BR>

        Get the entered text and convert it to integers.<BR>

        Initialize the row counter.<BR>

        Begin the <TT>for</TT> loop.<BR>

            Create the basic display string.<BR>

            Add the loop counter value to the display string.

<BR>

            Show the 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 method finished successfully.

</BLOCKQUOTE>

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

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

<P>

The <TT>for</TT> loop is a versatile programming construct that

enables you to create loops that run from a given starting value

to a given ending value. The loop's operation also depends on

the value of the loop's increment value, which enables you to

use the loop control variable to count forward or backward by

any given amount. As you'll see, <TT>for</TT> loops are used a

lot in Java programs. (In fact, they're a standard part of virtually

all programming languages.) To be sure you understand <TT>for</TT>

loops, look over the following questions and exercises.

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

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

<OL>

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