ch14.htm

Java_by_Example,初级经典例子哦,珍藏版本

be defined as <TT>public</TT>, methods that must be callable only

from the class and its derived classes should be defined as <TT>protected</TT>,

and methods that must be callable only from within the class should

be declared as <TT>private</TT>.

<P>

Suppose <TT>myField</TT> is defined as <TT>private</TT>, and you

now want to be able to set the value of <TT>myField</TT> from

outside the <TT>MyClass</TT> class. Because that data field is

defined as <TT>private</TT>, meaning it can be accessed only from

within the same class, you cannot access it directly by name.

To solve this problem, you can create a public method that can

set the value for you. You might also want to create a method

that returns the value of the field, as well, as shown in Listing

14.3.

<HR>

<BLOCKQUOTE>

<B>Listing 14.3&nbsp;&nbsp;LST14_3.TXT: Adding a Method to the

Class.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

class MyClass

{

    private int myField;



    public MyClass(int value)

    {

        myField = value;

    }



    public void SetField(int value)

    {

        myField = value;

    }



    public int GetField()

    {

        return myField;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

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

<BLOCKQUOTE>

Start defining the <TT>MyClass</TT> class.<BR>

    Declare the class's <TT>myField</TT> data field.<BR>

    Define the class's constructor.<BR>

        Initialize the data field.<BR>

    Start defining the <TT>SetField()</TT> method.<BR>

        Set the data field to the value passed to <TT>SetField().

<BR>

</TT>    Start Defining the <TT>GetField()</TT> method.<BR>

        Return the value of the <TT>myField</TT> data field.<BR>

</BLOCKQUOTE>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

According to the rules of strict object-oriented design, all class data fields should be declared as <TT>private</TT>. Some programmers would go so far as to say that you should not even provide access to data fields through public methods. However, you'll see these rules broken a lot, even by programmers hired by big companies like Microsoft, Borland, and Sun. As you become more familiar with object-oriented programming, you'll better understand why the rules were made and when it's appropriate to break them.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

Example: Using Classes in Applets</FONT></A></H2>

<P>

Up till now, you've been writing applets using the classes already

supplied as part of Java. Now, you'll see how to use your own

classes in applets. This will help you understand not only how

your own classes work, but also help you to understand why you

used Java's classes as you did. Follow the steps below to see

how all this class stuff works.

<OL>

<LI>Type Listing 14.3 and save it to your CLASSES folder under

the name <TT>MyClass.java</TT>. (If you don't want to type, you

can find the listing on this book's CD-ROM, in the CHAP14 folder.)

<LI>Start a DOS session by selecting Programs/MS-DOS Prompt from

the Start menu. The DOS window appears, as shown in Figure 14.1.

<BR>

<A HREF="f14-1.gif"><B> Figure 14.1 : </B><I>This is the DOS prompt window.



</I></A><P>

<LI>Type <TT>CD C:\CLASSES</TT> to switch to your CLASSES folder.

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

class. You'll then find the <TT>MyClass.class</TT> file in your

CLASSES folder.

<LI>Type Listing 14.4 and save it as <TT>Applet19.java</TT> in

your CLASSES folder.



<HR>

<BLOCKQUOTE>

<B>Listing 14.4&nbsp;&nbsp;Applet19.java: An Applet That Uses

the </B><I>MyClass</I><B> Class</B>.<BR>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;

import MyClass;



public class Applet19 extends Applet

{

    MyClass myObject;

    TextField textField1;



    public void init()

    {

        myObject = new MyClass(1);

        textField1 = new TextField(10);

        add(textField1);

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

    }



    public void paint(Graphics g)

    {

        String s = textField1.getText();

        int value = Integer.parseInt(s);

        myObject.SetField(value);

        value = myObject.GetField();

        s = String.valueOf(value);

        g.drawString(&quot;The data field of the object&quot;, 30, 80);

        g.drawString(&quot;is now set to this value:&quot;, 40, 95);

        g.drawString(s, 90, 125);

    }



    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 the <TT>awt </TT>package.<BR>

Tell Java that the program uses the <TT>applet</TT> package.<BR>

Tell Java that the program uses the <TT>MyClass</TT> class.<BR>

Derive the <TT>Applet19</TT> class from the <TT>Applet</TT> class.

<BR>

    Declare an object of the <TT>MyClass</TT> class.<BR>

    Declare a <TT>TextField</TT> object.<BR>

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

<BR>

        Create an object of the <TT>MyClass</TT> class.<BR>

        Create a <TT>TextField</TT> object.<BR>

        Add the <TT>TextField</TT> object to the applet.<BR>

        Set the <TT>TextField</TT> object's text.<BR>

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

<BR>

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

        Convert the text to an integer.<BR>

        Set <TT>myObject</TT>'s <TT>myField</TT> data field.<BR>

        Get the value of <TT>myField</TT> from <TT>myObject.<BR>

</TT>        Convert the value to a string.<BR>

        Draw the applet's display area.<BR>

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

<BR>

        Repaint the applet's display area.<BR>

        Tell Java the method executed okay.

</BLOCKQUOTE>



<LI>Type <TT>javac Applet19.java</TT> to compile the Applet19

applet. You'll then have the <TT>Applet19.class</TT> file in your

CLASSES folder.

<LI>Type Listing 14.5 and save the file as APPLET19.htmL.

</OL>

<HR>

<BLOCKQUOTE>

<B>Listing 14.5&nbsp;&nbsp;APPLET19.htmL: The HTML Document for

Applet19.<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;Applet19.class&quot;

    width=200

    height=200

    name=&quot;Applet19&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

You're now ready to run the Applet19 applet, which uses the <TT>MyClass</TT>

class. To run the program, type <TT>APPLETVIEWER APPLET19.htmL</TT>.

When you do, you see the window shown in Figure 14.2.

<P>

<A HREF="f14-2.gif"><B> Figure 14.2 : </B><I>This is Applet19 running under Appletviewer.



</I></A><P>

<P>

The applet's display area shows the current setting of <TT>MyClass</TT>'s

data field, <TT>myField</TT>. You can change this setting by typing

a new value into the text box and pressing Enter.

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

Understanding the Applet</FONT></A></H2>

<P>

By now, you should have a good idea of how classes work, at least

generally. Still, you'll now examine parts of Applet19's source

code. First, near the top of the source code is this line:

<BLOCKQUOTE>

<PRE>

import MyClass;

</PRE>

</BLOCKQUOTE>

<P>

Because the <TT>MyClass</TT> class is located in a different file

than Applet19, you need to tell Java where to find it. The above

line tells Java that it can find everything it needs to know about

<TT>MyClass</TT> in the <TT>MyClass.class</TT> file, which you

created when you compiled <TT>MyClass.java</TT>.

<P>

The next line of interest is this one, which is located at the

top of Applet19's definition:

<BLOCKQUOTE>

<PRE>

MyClass myObject;

</PRE>

</BLOCKQUOTE>

<P>

This line tells Java that you'll be creating an object of the

<TT>MyClass</TT> class and that the object will be called <TT>myObject</TT>.

<P>

At this point, you don't have the object created yet. You have

to create the object, which Applet19 does in its <TT>init()</TT>

method, using the <TT>new</TT> operator, like this:

<BLOCKQUOTE>

<PRE>

myObject = new MyClass(1);

</PRE>

</BLOCKQUOTE>

<P>

After the above line executes, Java has created the object, which

means you can now call the object's public methods to manipulate

the object as appropriate. Objects of the <TT>MyClass</TT> class

have only two public methods (not counting the constructor). Applet19

calls these methods in its <TT>paint()</TT> method, like this:

<BLOCKQUOTE>

<PRE>

myObject.SetField(value);

value = myObject.GetField();

</PRE>

</BLOCKQUOTE>

<P>

In the first line, Applet19 calls the object's <TT>SetField()</TT>

method, whose single argument is the value to which to set the

<TT>myField</TT> data field. Due to its private access, this is

the only way to set the value of <TT>myField</TT> outside of the

class. If you tried to access <TT>myField</TT> with the line

<BLOCKQUOTE>

<PRE>

myObject.myField = value;

</PRE>

</BLOCKQUOTE>

<P>

you wouldn't even be able to compile the file. The compiler will

generate an error telling you that you cannot access <TT>myField</TT>

in this way (Figure 14.3).

<P>

<A HREF="f14-3.gif"><B> Figure 14.3 : </B><I>The compiler complains when you ignore rules of access.



</I></A><P>

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

Using Inheritance</FONT></A></H2>

<P>

You may not know it, but throughout the second part of this book,

you've been using inheritance to create your applets. Specifically,

you've been deriving your applet classes (i.e, Applet10, Applet11,

and so on) from Java's <TT>Applet</TT> superclass. By using inheritance

in this way, you can take an existing class and create a new similar

class that does things a little differently. The new class will

have all the characteristics of its superclass, but will also

have any new characteristics that you choose to add.

<H3><A NAME="CreatingaSubclass">

Creating a Subclass</A></H3>

<P>

You derive a new class from a superclass by using the <TT>extends</TT>

keyword, an apt name for a keyword because by deriving a new class

you usually extend the abilities of the original class. For example,

when you create a new applet, you start the applet's class with

a line that looks something like this:

<BLOCKQUOTE>

<PRE>

public class MyApplet extends Applet

</PRE>

</BLOCKQUOTE>

<P>

First, Java insists that all applet classes be declared as <TT>public</TT>.

Next in the line, you can see the <TT>class</TT> keyword followed

by the name of the class. Finally comes the <TT>extends</TT> keyword

followed by the name of the superclass. In English, the above

line means that the public class <TT>MyApplet</TT> is derived

from (is a subclass of) the existing <TT>Applet</TT> class. As

you've probably already figured out, <TT>Applet</TT> is a class

that the Java makers created for you. This class contains all

the basic functionality you need to create an applet. You need

only extend (there's that word again) the specifics of the class

in order to create your own applet class. Again, you've been doing

that in every applet you've written so far.

<H3><A NAME="AddingFieldsandMethodstotheSubclass">

Adding Fields and Methods to the Subclass</A></H3>

<P>

One thing you can do when you create a subclass is to add your

own data fields and methods. For example, when you derive your

own applet class from Java's <TT>Applet</TT> class, although your

class inherits tons of data fields and methods from the superclass,

you'll undoubtedly need fields and methods not supplied in the

superclass. Maybe your new applet is designed to play Tic-Tac-Toe.

(Hmmmm. Where have I seen that before?) Obviously, when the fine