ch6.htm

Java_by_Example,初级经典例子哦,珍藏版本

</applet>

</PRE>

</BLOCKQUOTE>

<HR>

<H3><A NAME="ExampleCreatingandRunningApplet">

Example: Creating and Running Applet1</A></H3>

<P>

In the next section, you'll see exactly how the Applet1 applet

works. But before you get too much farther, you need to know how

to create and run the many applets that you'll find in this book.

All the applets that follow use the same sort of procedure. This

procedure involves creating the applet's source code, compiling

the code, and then writing the HTML document that demonstrates

the applet. Follow the steps below to get the Applet1 applet up

on your screen.

<OL>

<LI>Create a folder called CLASSES on the root directory of your

hard drive.

<LI>Type Listing 6.1 and save it in the CLASSES folder as an ASCII

file. Name the file <TT>Applet1.java</TT>. (Use uppercase and

lowercase letters exactly as shown for the program's file name.)

Note that you can also copy the listing from this book's CD-ROM,

if you don't want to type it.

<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>If you haven't added the JAVA\BIN path to your AUTOEXEC.BAT

file, type <TT>PATH=JAVA\BIN</TT> at the MS-DOS prompt. This ensures

that the system will be able to find Java's executables.

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

<LI>Type <TT>javac Applet1.java</TT> to compile the Applet1 applet.

You should receive no error messages, as shown in Figure 6.3.

If you do receive error messages, carefully check your typing

of Listing 6.1. Also, make sure you typed the command line <TT>javac

Applet1.java</TT> properly. When the compiler finishes, you'll

have the file <TT>Applet1.class</TT> in your CLASSES directory.

This is the applet's byte-code file.<BR>

<A HREF="f6-3.gif"><B> Figure 6.3 : </B><I>The Applet1.class file must compile with no errors.



</I></A><P>

<LI>Type Listing 6.2 and save it as an ASCII file to your CLASSES

folder. Name the file APPLET1.htmL. (This time, the case of the

letters doesn't matter.)

<LI>At the MS-DOS prompt, type <TT>appletviewer applet1.html</TT>

(case doesn't matter). When you do, Appletviewer will load and

display the applet.

</OL>

<P>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Java insists that the name of a Java source-code file is the same name as the class contained in the file. If you try to use different names, even if the difference is only the case of a letter or two, the Java compiler will complain and refuse to compile the program.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="HowAppletWorks">

How Applet1 Works</A></H3>

<P>

By now, you have the source code for Applet1 properly typed and

compiled. You've even run the applet using Appletviewer. You know

that the call to the <TT>drawString()</TT> method prints a string

as graphical text in the applet's display area. But what are the

values between <TT>drawString()</TT>'s parentheses? And how does

the program know when to execute the call to <TT>drawString()</TT>?

<P>

First, look at the <TT>paint()</TT> method. Java calls <TT>paint()</TT>

whenever the applet's display area (or canvas, as it's often called)

needs to be redrawn. The <TT>paint()</TT> method always gets called

when the applet first appears on the screen, which is exactly

what's happening in Applet1. You run the applet, Java calls <TT>paint()</TT>

when the applet appears, and <TT>paint()</TT> calls <TT>drawString()</TT>,

which displays the text string &quot;Hello from Java.&quot;

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

The Java compiler is case-sensitive, meaning that it can differentiate between upper- and lowercase letters. For this reason, you have to be extra careful to type method names properly. For example, if you type <TT>Paint()</TT> instead of <TT>paint()</TT>, Java will not recognize the method and will not call it when the applet needs to be redrawn.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

What's that &quot;g&quot; followed by a period in front of the

call to <TT>drawString()</TT>? Remember that I said <TT>drawString()</TT>

is a method of the <TT>Graphics</TT> class. If you look at the

first line of the <TT>paint()</TT> method, you'll see <TT>Graphics

g</TT> in the parentheses. This means that Java is sending an

object of the <TT>Graphics</TT> class to the <TT>paint()</TT>

method and that object is called <TT>g</TT>. Whenever you need

to call an object's method, you must preface the method's name

with the object's name followed by a period. So, the line

<BLOCKQUOTE>

<PRE>

g.drawString(&quot;Hello from Java!&quot;, 60, 75);

</PRE>

</BLOCKQUOTE>

<P>

tells Java to call the <TT>g</TT> object's <TT>drawString()</TT>

method. The values in the parentheses are called arguments, which

are values that you need to send to the method. The arguments

in the above call to <TT>drawString()</TT> tell Java to draw the

text &quot;Hello from Java!&quot; at column 60 and row 75 of the

display area. (The position is measured in pixels, not characters.

A pixel is the smallest dot that can be displayed on the screen.)

To display the text at a different location, just change the second

and third arguments. For example, Figure 6.4 shows the text positioned

at 25,25.

<P>

<A HREF="f6-4.gif"><B> Figure 6.4 : </B><I>Here, Applet1 displays the text at position 25,25.



</I></A><P>

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

Getting Input from the User</FONT></A></H2>

<P>

Again, because you are now programming in a graphical environment,

getting input from the user isn't as easy as just calling an input

command. You must first create an area of the screen in which

the user can type and edit his response to your request. There

are several ways to do this, but one of the easiest is to add

a control of the <TT>TextField</TT> class in your applet. A <TT>TextField</TT>

control is much like the edit boxes you see when using Windows.

You might, for example, see a number of edit controls in a dialog

box. Listing 4.3 shows how to include a <TT>TextField</TT> control

in your applet. Figure 6.5 shows the Applet2 applet in action.

<P>

<A HREF="f6-5.gif"><B> Figure 6.5 : </B><I>The Applet2 applet displays an area in which you can type.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 6.3&nbsp;&nbsp;Applet2.java: Getting Input from the

User.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet2 extends Applet

{

    TextField textField;



    public void init()

    {

        textField = new TextField(20);

        add(textField);

    }

}

</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>Applet2</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.

</BLOCKQUOTE>

<P>

To run the Applet2 applet yourself, you'll need to type and save

the source code, naming it <TT>Applet2.java</TT>. (You can copy

the source code from the CD-ROM, if you like, and thus save on

typing. Of course, you won't learn as much that way.) Then compile

the source code (type <TT>javac Applet2.java</TT> at the MS-DOS

prompt), which gives you the <TT>Applet2.class</TT> file. Next,

create a new HTML document from the one shown in Listing 6.2,

changing all instances of Applet1 to Applet2. You can then run

Applet2 by typing <TT>appletviewer applet2.html</TT> at the DOS

prompt.

<H3><A NAME="HowAppletWorks1">

How Applet2 Works</A></H3>

<P>

Applet2 looks quite a bit different from Applet1. First, it declares

a data field named <TT>textField</TT> as an object of the <TT>TextField</TT>

class, which represents a control very much like a standard Windows

edit box. The program declares the control like this:

<BLOCKQUOTE>

<PRE>

TextField textField;

</PRE>

</BLOCKQUOTE>

<P>

Notice that the <TT>textField</TT> object is declared just as

you would declare any other kind of data object such as an integer

or a floating-point value. Next, notice that, although the name

of the class and the name of the object are spelled identically,

the object's name starts with a lowercase letter. This makes all

the difference in the world to Java, which is a case-sensitive

language.

<P>

Another difference between Applet1 and Applet2 is that Applet2

has an <TT>init()</TT> method instead of <TT>paint()</TT>. The

<TT>Init()</TT> method is another of those methods that Java calls

automatically-in this case, as soon as you run the Applet2 applet.

Because Java calls <TT>init()</TT> almost immediately, it's a

great place to initialize the applet. (Guess that's why they called

it <TT>init()</TT>, huh?)

<P>

In Applet2, when <TT>init()</TT> gets called, <TT>textField</TT>

has already been declared as an object of the <TT>TextField</TT>

class, but <TT>textField</TT> hasn't yet been assigned a value.

The first line of code in <TT>init()</TT> creates the <TT>TextField</TT>

object and assigns it to <TT>textField</TT>, like this:

<BLOCKQUOTE>

<PRE>

textField = new TextField(20);

</PRE>

</BLOCKQUOTE>

<P>

After Java executes this line, <TT>textField</TT> refers to an

actual <TT>TextField</TT> object. The value in the parentheses

is the width of the <TT>TextField</TT> control; the larger this

number, the wider the control. Keep in mind that a <TT>textField</TT>

control can hold more text than its width allows. If you type

past the end of the control's text box, the text scrolls horizontally.

<P>

The next step is to add the object to the applet's display area,

like this:

<BLOCKQUOTE>

<PRE>

add(textField);

</PRE>

</BLOCKQUOTE>

<P>

The <TT>add()</TT> method's single argument is the control you

want to add to the applet.

<P>

After creating the control and adding it to the applet, the control

will automatically appear when Java draws the applet. You don't

need a <TT>paint()</TT> method to draw a control, because the

control can take care of itself.

<P>

Once the <TT>TextField</TT> control is on the screen, you can

type in it. First, click the control to give it the focus, then

type and edit to your heart's content.

<H3><A NAME="ExampleRetrievingtextfromaITextFieldIcontrol">

Example: Retrieving text from a <I>TextField</I> control</A></H3>

<P>

In the Applet2 applet, you can type all you like in the <TT>TextField</TT>

control, but how do you get the text out of the control and actually

do something with it? Follow the steps below to create Applet3,

a new version of Applet2 that can retrieve and display any text

entered into the <TT>TextField</TT> control.

<OL>