ch29.htm

Java_by_Example,初级经典例子哦,珍藏版本

to declare a set of behaviors that can be inherited by one or

more classes. So, they came up with interfaces.

<H3><A NAME="TheBasicInterface">

The Basic Interface</A></H3>

<P>

An interface is very much like a class-with one important difference.

None of the methods declared in an interface are implemented in

the interface itself. Instead, these methods must be implemented

in any class that uses the interface. In short, interfaces describe

behaviors but do not detail how those behaviors will be carried

out.

<P>

Interfaces are so much like classes, in fact, that they are declared

in almost exactly the same way. You just replace the <TT>class</TT>

keyword with the <TT>interface</TT> keyword, like this:

<BLOCKQUOTE>

<PRE>

interface MyInterface

{

}

</PRE>

</BLOCKQUOTE>

<P>

The preceding example is a complete interface, meaning that it

can be compiled, after which other Java programs can reference

it. You compile an interface exactly the same way you compile

a class. First, you save the interface's source code in a file

with the .java extension. Then you use the Java compiler, javac,

to compile the source code into byte-code form. Just like a normal

class, the byte-code file will have the .class extension.

<P>

Missing from the preceding sample interface are the methods that

describe the interface's behaviors. In the sections that follow,

you'll learn how to add this important element to an interface.

<H3><A NAME="ExampleCreatinganInterface">

Example: Creating an Interface</A></H3>

<P>

Suppose that instead of creating a full-fledged class out of your

<TT>Display</TT> package's <TT>DisplayClass</TT> class, you want

to make it an interface. (Yes, just like classes, you can make

interfaces part of a package.) Listing 29.4 shows how the source

code would be modified.

<HR>

<BLOCKQUOTE>

<B>Listing 29.4&nbsp;&nbsp;DisplayInterface.java: Creating an

Interface.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

package MyPackages.Display;



public interface DisplayInterface

{

    public String GetDisplayText();

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The first line in Listing 29.4 is exactly like the first line

in Listing 29.1, which shows the original class. The first line

specifies that this interface is to be part of the <TT>MyPackages.Display</TT>

package. The second line declares the interface. The only difference

here is the use of the <TT>interface</TT> keyword in place of

<TT>class</TT> and the new name, <TT>DisplayInterface</TT>.

<P>

Now, the real difference between a class and an interface becomes

evident. Although Listing 29.4 declares the <TT>GetDisplayText()</TT>

method, it doesn't actually implement the method. That is, you

can see from the interface that <TT>GetDisplayText()</TT> is supposed

to return a <TT>String</TT> object, but there isn't a clue as

to how that <TT>String</TT> object is created or what it contains.

Those details are left up to any class that decides to implement

the interface.

<P>

But before you worry about implementing the interface in a class,

you have to compile the interface source code. To do that, follow

these steps:

<OL>

<LI>Type in Listing 29.3 and save it in the DISPLAY folder, under

the name DisplayInterface.java. (If you don't want to type, you

can copy the file from this book's CD-ROM.)

<LI>Select the MS-DOS Prompt command from the Start menu's Program

group and switch to the CLASSES\MYPACKAGES\DISPLAY folder.

<LI>At the MS-DOS prompt, type <TT>javac DisplayInterface.java</TT>.

Java's compiler then compiles the DisplayInterface.java file,

creating the DisplayInterface.class file in your DISPLAY folder.

</OL>

<H3><A NAME="ImplementinganInterface">

Implementing an Interface</A></H3>

<P>

Now that you have your new interface compiled, you can implement

it in a class. This means not only telling Java that you'll be

using the interface, but also implementing the interface within

the new class. That is, every method that is listed in the interface's

declaration must be defined in the class's source code. Listing

29.5 is a new applet, called InterfaceApplet, that shows how to

implement the <TT>DisplayInterface</TT> interface. When you compile

and run this applet, you see exactly the same display as that

produced by the original version, PackageApplet. Notice how the

listing uses the <TT>implements</TT> keyword to tell Java that

the applet will be implementing the <TT>DisplayInterface</TT>

interface.

<HR>

<BLOCKQUOTE>

<B>Listing 29.5&nbsp;&nbsp;InterfaceApplet.java: An Applet That

Implements the </B><I>DisplayInterface</I><B> Interface.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;

import MyPackages.Display.DisplayInterface;



public class InterfaceApplet extends Applet

    implements DisplayInterface

{

    public void paint(Graphics g)

    {

        Font font = new Font(&quot;TimesRoman&quot;, Font.PLAIN, 24);

        g.setFont(font);



        String s = GetDisplayText();



        g.drawString(s, 60, 80);

    }



    public String GetDisplayText()

    {

        return &quot;Display Text&quot;;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

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

<BLOCKQUOTE>

Tell Java that the applet uses the classes in the <TT>awt</TT>

package.<BR>

Tell Java that the applet uses the classes in the <TT>applet</TT>

package.<BR>

Tell Java that the applet uses the <TT>Display</TT> package's

<TT>DisplayInterface</TT>.<BR>

Derive <TT>InterfaceApplet</TT> from <TT>Applet</TT> and implement

<TT>DisplayInterface</TT>.<BR>

    Override the <TT>paint()</TT> method.<BR>

        Create and set the <TT>Graphics</TT> object's font.<BR>

        Get the display string.<BR>

        Draw the display string.<BR>

    Define the interface's <TT>GetDisplayText()</TT> method.<BR>

        Return the display string.<BR>

</BLOCKQUOTE>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

A class can implement as many interfaces as it needs to. To implement multiple interfaces, just list the interfaces after the <TT>implements</TT> keyword, separating each interface name from the others with a comma. Remember, however, that you must implement in your new class all the methods declared in all the interfaces you implement.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

There's quite a lot to learn about interfaces. However, due to the skill level of this book and the fact that you probably won't have to worry about interfaces until you're an experienced Java programmer, only the basics have been presented here. Chances are good that you won't have to create interfaces at all. It's just good to know they're there in case you need them.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

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

<P>

Packages are a great way to create and organize class libraries.

The more Java code you write, the more you'll appreciate the ability

to create your own packages. Although you won't need to use interfaces

for some time to come (if at all), interfaces are a unique element

of the Java programming language. Although other object-oriented

languages such as C++ allow multiple inheritance, only Java supports

the idea of interfaces, which can be powerful tools when they're

needed.

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

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

<OL>

<LI>What's a package?

<LI>How do you tell Java that a source-code file uses a particular

package?

<LI>How do you add a class or interface to a package?

<LI>How do you tell Java that the class you're creating implements

a particular interface?

<LI>What's the biggest difference between an interface and a class?

<LI>How are interfaces similar to classes?

<LI>How does the complete name of a package relate to the package's

storage on your disk?

</OL>

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

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

<OL>

<LI>Add a new class to the Display package. This class should

return an integer representing the size of the font to use when

drawing the display text.

<LI>Change the class you wrote in exercise 1 to an interface.

Write an applet that implements the new interface.

</OL>

<HR>











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