ch6.htm

Java_by_Example,初级经典例子哦,珍藏版本

<LI>Type Listing 6.4 and save it in the CLASSES folder as an ASCII

file. Name the file <TT>Applet3.java</TT>. Note that you can copy

the listing from this book's CD-ROM if you don't want to type

it.

</OL>

<HR>

<BLOCKQUOTE>

<B>Listing 6.4&nbsp;&nbsp;Applet3.java: Source Code for the Applet3

Applet.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet3 extends Applet

{

    TextField textField;



    public void init()

    {

        textField = new TextField(20);

        add(textField);

    }



    public void paint(Graphics g)

    {

        String s = textField.getText();

        g.drawString(s, 40, 50);

    }



    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>Applet3</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare <TT>textField</TT> as an object of the <TT>TextField</TT>

class.<BR>

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

<BR>

        Create the new <TT>TextField</TT> object.<BR>

        Add the <TT>TextField</TT> object to the applet's display.

<BR>

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

<BR>

        Get the text string from the <TT>TextField</TT> control.

<BR>

        Display the text in the applet's display area.<BR>

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

<BR>

        Force the applet to redraw its display area.<BR>

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

</BLOCKQUOTE>

<OL>

<LI>Start an MS-DOS session by selecting Start/Programs/MS-DOS

Prompt. If the MS-DOS screen takes over the entire screen, press

Alt+Enter to display the MS-DOS session in a window.

<LI>Type <TT>cd c:\classes</TT> to move to your CLASSES folder.

<LI>Type <TT>javac Applet3.java</TT> to compile the Applet3 applet.

You should receive no error messages. If you do receive error

messages, carefully check your typing of Listing 6.4. Also, make

sure you typed the command line <TT>javac Applet3.java</TT> properly.

<LI>Modify Listing 6.2 by replacing all occurrences of Applet1

with Applet3 and save it as an ASCII file to your CLASSES folder.

Name the file APPLET3.htmL.

<LI>At the MS-DOS prompt, type <TT>appletviewer applet3.html</TT>.

When you do, Appletviewer will load and display the applet.

</OL>

<P>

When you run Applet3, go ahead and type some text into the <TT>TextField</TT>

control. When you press Enter, the text appears in the applet's

display area, right below the control (Figure 6.6).

<P>

<A HREF="f6-6.gif"><B> Figure 6.6 : </B><I>Applet3 can display the text you type into the TextField control.



</I></A><P>

<H3><A NAME="HowAppletWorks2">

How Applet3 Works</A></H3>

<P>

If you look at the Applet3 program code, you can see that you've

added a <TT>paint()</TT> method. This is where the text that the

user types into the <TT>TextField</TT> control gets displayed.

First, <TT>paint()</TT> extracts the text from the control, like

this:

<BLOCKQUOTE>

<PRE>

String s = textField.getText();

</PRE>

</BLOCKQUOTE>

<P>

This line declares a variable called <TT>s</TT> to be an object

of the <TT>String</TT> class, and then it sets <TT>s</TT> equal

to the text string in the control. (The <TT>String</TT> class

is another of Java's built-in classes that you'll learn more about

later. For now, just know that this class represents text strings.)

You can see in this line that you're calling the <TT>textField</TT>

object's <TT>getText()</TT> method, because of the object and

method name separated by the dot. The <TT>getText()</TT> method

simply returns the text string that's stored in the <TT>textField</TT>

control.

<P>

Displaying the string is as easy as calling on your old friend

<TT>drawString()</TT>, as follows:

<BLOCKQUOTE>

<PRE>

g.drawString(s, 40, 50);

</PRE>

</BLOCKQUOTE>

<P>

One big difference between Applet2 and Applet3 is the <TT>action()</TT>

method. Java calls this method whenever the user performs some

action with controls in the applet. In this case, the action that

<TT>action()</TT> responds to is the user's pressing Enter after

typing text. In Applet3, the <TT>action()</TT> method does nothing

more than call the applet's <TT>repaint()</TT> method, which tells

Java that you want to redraw the applet's display area. (You'll

learn more about using the <TT>action()</TT> method later in the

book.) This causes Java to call the <TT>paint()</TT> method, which

very neatly displays the control's string.

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

Displaying Numerical Values</FONT></A></H2>

<P>

In the previous chapter, you learned to declare and define a number

of different variable types, including <TT>int</TT>, <TT>byte</TT>,

<TT>short</TT>, <TT>float</TT>, and other numerical values. The

problem with numerical values in a computer is that you can't

display them without first converting them to text strings. Luckily,

this task is pretty easy to perform. You need only call the <TT>String</TT>

class's <TT>valueOf()</TT> method, as shown in Listing 6.5. Figure

6.7 shows Appletviewer running Applet4.

<P>

<A HREF="f6-7.gif"><B> Figure 6.7 : </B><I>The Applet4 applet converts and displays an integer value.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 6.5&nbsp;&nbsp;Applet4.java: The Source Code for Applet4.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet4 extends Applet

{

    public void paint(Graphics g)

    {

        int x = 10;

        String s = String.valueOf(x);

        g.drawString(s, 40, 50);

    }

}

</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>Applet4</TT> class from Java's <TT>Applet</TT>

class.<BR>

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

<BR>

        Declare the integer x and set its value to 10.<BR>

        Convert x to a string.<BR>

        Display the string that represents x's value.

</BLOCKQUOTE>

<P>

As you can see in Listing 6.5, the <TT>String</TT> class has a

method called <TT>valueOf()</TT> that can convert numerical values

to strings. The method's single argument is the value you want

to convert, which can be any of Java's numerical data types.

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

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

<P>

All computer programs must deal with some form of I/O. At the

very least, a program must be able to display information to the

user, as well as retrieve data from the user. Java has several

ways to handle this basic form of I/O, a couple of which you learned

in this chapter. As you dig deeper into the art of writing Java

applets, you'll see other ways to perform I/O.

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

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

<OL>

<LI>What is graphical text?

<LI>What's the difference between proportional and non-proportional

fonts?

<LI>What are arguments?

<LI>Describe the three arguments used with the <TT>drawString()</TT>

method.

<LI>What does the <TT>paint()</TT> method do?

<LI>Describe one way to get user input in your applets.

<LI>Describe how the <TT>init()</TT> method works.

<LI>Describe how the <TT>action()</TT> method works.

<LI>How can you convert a numerical value to a text string?

</OL>

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

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

<OL>

<LI>Modify Applet1 so that it displays the text string &quot;Out

to lunch!&quot; at position 30, 50 in the applet's display area.

<LI>Modify Applet2 so that it displays a <TT>TextField</TT> control

half as wide as the one displayed in the original program.

<LI>Write an applet that displays the days of the week in a column

down the center of the applet's display area.

<LI>Using Applet3 as a model, write a new applet called NameApplet

that asks the user for her name and age and then prints the answers

in the applet's display area. The finished applet should look

like Figure 6.8. (This one's tough, so, if you get stumped, check

out the file NameApplet.java in the CHAP06 folder of this book's

CD-ROM. First try to solve the problem yourself, though.)<BR>

<A HREF="f6-8.gif"><B> Figure 6.8 : </B><I>This is what the NameApplet applet should look like when it is run with Appletviewer.



</I></A><P>

</OL>

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