ch14.htm

Java_by_Example,初级经典例子哦,珍藏版本

programmers at Sun created the <TT>Applet</TT> class, they didn't

think to add the methods needed to play Tic-Tac-Toe. You'll have

to add those methods yourself.

<H3><A NAME="ExampleAddingFieldsandMethods">

Example: Adding Fields and Methods</A></H3>

<P>

Take the <TT>MyClass</TT> class that you created earlier in this

chapter (shown in Listing 14.3). Suppose you want to create a

new class that has a new data field called <TT>myNewField</TT>,

as well as a constructor and methods for setting and retrieving

the value of this new data field. You might come up with something

like Listing 14.6.

<HR>

<BLOCKQUOTE>

<B>Listing 14.6&nbsp;&nbsp;MYSUBCLASS.JAVA: Creating a Subclass.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import MyClass;



class MySubClass extends MyClass

{

    private int myNewField;



    public MySubClass(int value)

    {

        super(value);

        myNewField = value;

    }



    public void SetNewField(int value)

    {

        myNewField = value;

    }



    public int GetNewField()

    {

        return myNewField;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The file containing the <TT>MySubClass</TT> class first imports

the <TT>MyClass</TT> file, because Java will need the information

contained in that file. In the constructor, the class first calls

<BLOCKQUOTE>

<PRE>

super(value);

</PRE>

</BLOCKQUOTE>

<P>

which ensures that the superclass (<TT>MyClass</TT>) is properly

initialized by calling its constructor. The keyword <TT>super</TT>

refers to the class's superclass. After calling the superclass's

constructor, the <TT>MySubClass</TT> constructor initializes the

new data field, <TT>myNewField</TT>. The new class also supplies

new methods for setting and getting the value of <TT>myNewField</TT>.

In short, <TT>MySubClass</TT> now has two data fields-<TT>myField</TT>,

which it inherited from <TT>MyClass</TT>, and the new <TT>myNewField</TT>-and

four methods-<TT>SetField()</TT> and <TT>GetField</TT>, which

it inherited from <TT>MyClass</TT>, and the new <TT>SetNewField()</TT>

and <TT>GetNewField()</TT> methods.

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

Example: Using a Subclass in a Program</FONT></A></H2>

<P>

Now that you have the <TT>MySubClass</TT> subclass, it might be

nice to see how it works in a real programming situation. Applet20,

which is shown in Listing 14.7, does the honors of putting <TT>MySubClass</TT>

to work. In most places, compared to Applet19, the applet merely

replaces occurrences of <TT>MyClass</TT> with <TT>MySubClass</TT>.

The <TT>paint()</TT> method has to work a bit harder, though,

calling all four of <TT>MySubClass</TT>'s methods to prove they

work.

<HR>

<BLOCKQUOTE>

Listing 14.7&nbsp;&nbsp;APPLET20.JAVA: Using a Subclass in a Program.

<BR>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;

import MySubClass;



public class Applet20 extends Applet

{

    MySubClass mySubObject;

    TextField textField1;

    TextField textField2;



    public void init()

    {

        mySubObject = new MySubClass(1);

        textField1 = new TextField(10);

        add(textField1);

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

        textField2 = new TextField(10);

        add(textField2);

        textField2.setText(&quot;2&quot;);

    }



    public void paint(Graphics g)

    {

        String s = textField1.getText();

        int value = Integer.parseInt(s);

        mySubObject.SetField(value);

        value = mySubObject.GetField();

        s = String.valueOf(value);

        g.drawString(&quot;The myField data field&quot;, 30, 80);

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

        g.drawString(s, 90, 125);



        s = textField2.getText();

        value = Integer.parseInt(s);

        mySubObject.SetNewField(value);

        value = mySubObject.GetNewField();

        s = String.valueOf(value);

        g.drawString(&quot;The myNewField data field&quot;, 30, 155);

        g.drawString(&quot;is now set to this value:&quot;, 40, 170);

        g.drawString(s, 90, 200);

    }



    public boolean action(Event event, Object arg)

    {

        repaint();

        return true;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

When you run Applet20 under Appletviewer, you see the window shown

in Figure 14.4. Use the first text box to enter values for the

original <TT>myField</TT> data field. Use the second text box

to enter values for <TT>myNewField</TT>. Whenever you press Enter,

the applet reads the values from the boxes and calls <TT>MySubClass</TT>'s

methods to set the new values and to retrieve the set values from

the object.

<P>

<A HREF="f14-4.gif"><B> Figure 14.4 : </B><I>This is Appletviewer running the Applet20 applet.



</I></A><P>

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

Overriding Methods of the Superclass</FONT></A></H2>

<P>

If you've been reading the pseudocode sections that follow many

of the listings in this book, you've seen the term &quot;overriding&quot;

many times. When you override a method, you are creating a new

version of a method that's part of the superclass. For example,

in many of the applets you've created, you've overridden methods

like <TT>init()</TT>, <TT>paint()</TT>, and <TT>action()</TT>.

All of these methods are defined in some general way in the <TT>Applet</TT>

superclass. When you derive a new class from applet, you can override

these methods to perform the tasks you want them to perform, rather

than the general tasks assigned to them by the <TT>Applet</TT>

class.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

In general object-oriented programming discussions, the term &quot;derive&quot; means exactly the same thing as &quot;subclass&quot; (when the latter is used as a verb). Ditto for the terms &quot;base class&quot; and &quot;superclass,&quot; which are the same thing. In other words, when you derive a new class from a base class, you are subclassing a new class from a superclass.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

For example, you know that, when Java starts up an applet, it

calls the applet's <TT>init()</TT> method. Here's how the <TT>Applet</TT>

class defines <TT>init()</TT>:

<BLOCKQUOTE>

<PRE>

public void init()

{

}

</PRE>

</BLOCKQUOTE>

<P>

No, your eyes aren't fooling you. In the <TT>Applet</TT> class,

the <TT>init()</TT> method does nothing at all. It's only there

so you can override it in your own class. Here's how it all works:

When you derive your applet class from <TT>Applet</TT>, your applet

class inherits all of <TT>Applet</TT>'s data fields and methods.

If you don't override a method, Java calls the original version

as necessary. In other words, if you don't override <TT>init()</TT>

in your applet class, when Java starts your applet, it calls the

<TT>Applet</TT> class's version of <TT>init()</TT>, which does

nothing. However, if you override <TT>init()</TT> in your class,

Java is smart enough to call the new version rather than the original

do-nothing version. Cool, eh?

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

The <I>this</I> Keyword</FONT></A></H2>

<P>

There may be times when you need to explicitly refer to an object

from within the object's methods. For example, you might need

to pass a reference to the object as an argument in a method call.

When you need to refer to the object explicitly, use the <TT>this</TT>

keyword. In many cases, the <TT>this</TT> keyword is implicit

in the method call or variable reference. For example, inside

an object that has the data field <TT>dataField</TT>, the line

<BLOCKQUOTE>

<PRE>

dataField = 1;

</PRE>

</BLOCKQUOTE>

<P>

is the same as

<BLOCKQUOTE>

<PRE>

this.dataField = 1;

</PRE>

</BLOCKQUOTE>

<P>

In the first case, the <TT>this</TT> keyword is implicit, whereas

in the second case, you've included it explicitly. If you needed

to pass a reference to the object as an argument, you might write

something like this:

<BLOCKQUOTE>

<PRE>

SomeMethod(this);

</PRE>

</BLOCKQUOTE>

<P>

Of course, the <TT>SomeMethod()</TT> method would have been written

to accept an object of <TT>this</TT>'s type as its single argument.

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

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

<P>

If you've never done any object-oriented programming, it might

take you a while to get used to using classes. Classes are the

single biggest hurdle to jump when making the transition from

normal procedural programming to object-oriented programming (OOP).

Just think of classes as a way to provide another level of abstraction

to your programs. In your non-OOP programs, you had program elements

called programs, files, and procedures, listed in the order of

their level of abstraction. Now, you can add classes to the end

of the list, right between files and procedures.

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

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

<OL>

<LI>What is a class?

<LI>How do classes help you to organize your programs?

<LI>What are the three parts of a simple, empty class?

<LI>What two elements do you add to complete the class?

<LI>How do you create an object from a class?

<LI>How do you use a class that's defined in a different file

than the file that accesses the class?

<LI>What is inheritance and how does it help you create new classes

quickly?

<LI>What is a subclass and a superclass?

<LI>How do you create a subclass?

<LI>How do you override a method inherited from a superclass?

</OL>

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

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

<OL>

<LI>Write a basic, empty class called <TT>TestClass</TT>.

<LI>Add to <TT>TestClass</TT> a string data field called <TT>data1</TT>.

This data field should be private to the class.

<LI>Add to <TT>TestClass</TT> a constructor that accepts a starting

value for <TT>data1</TT> as its single argument, and public methods

for setting and retrieving the value of <TT>data1</TT>. Call these

methods <TT>SetData()</TT> and <TT>GetData()</TT>.

<LI>Compile the finished class.

<LI>Write a subclass called <TT>TestSubClass</TT> that is derived

from <TT>TestClass</TT> and that adds an integer data field called

<TT>data2</TT> (declared as <TT>private</TT>) and a public method

called <TT>CreateDataString()</TT> that creates a string object

from <TT>data1</TT> and <TT>data2</TT>. That is, if <TT>data1</TT>

is equal to <TT>Java is cool!</TT> and <TT>data2</TT> is equal

to 15, the <TT>CreateDataString()</TT> method should return <TT>Java

is cool! 15</TT> as a single string object. Also, create public

methods called <TT>SetData2()</TT> and <TT>GetData2()</TT> for

setting and retrieving the value of <TT>data2</TT>, as well as

a constructor that accepts arguments for the starting values of

<TT>data1</TT> and <TT>data2</TT>.

<LI>Modify Applet20 so that it creates an object of the <TT>TestSubClass</TT>

class. You should provide text boxes for enabling the user to

set <TT>data1</TT> and <TT>data2</TT>, as well as write <TT>paint()</TT>

so that it displays the string returned by <TT>CreateDataString()</TT>.

Figure 14.5 shows what the ClassApplet applet should look like.

(You can find the solutions for these exercises in the CHAP14

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

<A HREF="f14-5.gif"><B> Figure 14.5 : </B><I>This is ClassApplet running under Appletviewer.



</I></A><P>

</OL>

<HR>











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