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SHAPE ="RECT" HREF ="/reference/author.html" COORDS ="361,0,440,29"><AREA SHAPE ="RECT" HREF ="/reference/basic.html" COORDS ="254,0,360,29"><AREA SHAPE ="RECT" HREF ="/reference/help.html" COORDS ="183,0,251,29"><AREA SHAPE ="RECT" HREF ="/reference/cool.html" COORDS ="100,0,181,29"><AREA SHAPE ="RECT" HREF ="/reference/mybook.html" COORDS ="0,0,100,29"></MAP><MAP NAME ="header_r1.gif.map"><AREA SHAPE ="RECT" HREF ="/reference/personalize.html" COORDS ="339,0,439,29"></MAP><p><font face="Arial,Helvetica" size="-1" color="#006666"><A HREF="/reference/r_library.html"><B>All Categories</A> :</B><b><A HREF="/reference/r_java.html">Java</A></b></font><p><META><H1><FONT SIZE=6 COLOR=#Ff0000>Chapter 27</FONT></H1><H1><FONT SIZE=6 COLOR=#Ff0000>Images and Sounds</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#ImageTypes">Image Types</A><LI><A HREF="#LoadingandDisplayinganImage">Loading and Displaying an Image</A><UL><LI><A HREF="#ExampleUsingtheIgetDocumentBaseIMethod">Example: Using the <I>getDocumentBase()</I> Method</A><LI><A HREF="#ExampleUsingtheIgetCodeBaseIMethod">Example: Using the <I>getCodeBase()</I> Method</A><LI><A HREF="#LoadinganImage">Loading an Image</A><LI><A HREF="#DisplayinganImage">Displaying an Image</A><LI><A HREF="#ExampleDisplayinganImageinanApplet">Example: Displaying an Image in an Applet</A></UL><LI><A HREF="#PlayingaSound">Playing a Sound</A><UL><LI><A HREF="#ExampleUsingtheIplayIMethod">Example: Using the <I>play()</I> Method</A><LI><A HREF="#ExamplePlayingaSoundinanApplet">Example: Playing a Sound in an Applet</A><LI><A HREF="#ControllingSounds">Controlling Sounds</A><LI><A HREF="#ExampleUsinganAudioClipinanApplet">Example: Using an AudioClip in an Applet</A></UL><LI><A HREF="#Summary">Summary</A><LI><A HREF="#ReviewQuestions">Review Questions</A><LI><A HREF="#ReviewExercises">Review Exercises</A></UL><HR><P>If you've seen a lot of the applets that are floating around,you've undoubtedly noticed that many of them feature vivid graphicsand even sound effects. When programming in a language such aC++, displaying graphics and playing sounds can be infamouslydifficult, thanks to the fact that these languages provide nodirect support for handling these types of files. Even the WindowsAPI, as immense as it is, provides little help when it comes tohandling these graphical and aural chores. Java, on the otherhand, was designed to make creating applets as easy as possible.For that reason, Java's classes handle almost all the difficultiesassociated with displaying images (commonly called bitmaps) andplaying sounds. In this chapter, you use Java's power to add imagesand sounds to your applets.<H2><A NAME="ImageTypes"><FONT SIZE=5 COLOR=#Ff0000>Image Types</FONT></A></H2><P>In the world of computers, there are many types of images, eachof which is associated with a specific file format. These imagetypes are usually identified by their file extensions, which includePCX, BMP, GIF, JPEG (or JPG), TIFF (or TIF), TGA, and more. Eachof these file types was created by third-party software companiesfor use with their products, but many became popular enough togrow into standards. The PCX graphics file type, for example,began as the format for PC Paintbrush files, whereas BMP filesare usually associated with the Windows graphical interface.<P>If you were writing your Internet applications using a more conventionallanguage like C++, you could choose to support whatever imagetype was most convenient for your use. This is because you'd haveto write all the file-loading code from scratch, anyway. Java,on the other hand, comes complete with classes that are capableof loading image files for you. This convenience comes with asmall price, however, since Java can load only GIF and JPEG imagefile formats. In this book, you'll use GIF files, which are morecommon, although JPEG files are rapidly gaining a reputation,especially for high-resolution, true-color images.<H2><A NAME="LoadingandDisplayinganImage"><FONT SIZE=5 COLOR=#Ff0000>Loading and Displaying an Image</FONT></A></H2><P>The first step in displaying an image in your applet is to loadthe image from disk. To do this, you must create an object ofJava's <TT>Image</TT> class. This is fairly easy to do; however,in order to do so, you need to create an <TT>URL</TT> object thatholds the location of the graphics file. You could just type theimage's URL directly into your Java source code. If you do this,however, you have to change and recompile the applet wheneveryou move the graphics file to a different directory on your disk.A better way to create the image's <TT>URL</TT> object is to calleither the <TT>getDocumentBase()</TT> or <TT>getCodeBase()</TT>method. The former returns the URL of the directory from whichthe current HTML file was loaded, whereas the latter returns theURL of the directory from which the applet was run.<H3><A NAME="ExampleUsingtheIgetDocumentBaseIMethod">Example: Using the <I>getDocumentBase()</I> Method</A></H3><P>As I said previously, the <TT>getDocumentBase()</TT> method returnsthe URL of the directory from which the HTML document was loaded.If you're storing your images in the same directory (or a subdirectoryof that directory) as your HTML files, you'd want to use thismethod to obtain an URL for an image.<P>Suppose you have your HTML documents in a directory called PUBLICand the image you want, called IMAGE.gif, is stored in a subdirectoryof PUBLIC called IMAGES. A call to <TT>getDocumentBase()</TT>will get you the appropriate base URL. That call looks like this:<BLOCKQUOTE><PRE>URL url = getDocumentBase();</PRE></BLOCKQUOTE><P>As you'll soon see, once you have the URL, you can load the fileby using the URL along with the relative location of the image,which in this case would be IMAGES/IMAGE.gif. The full URL tothe file would then be FILE:/C:/PUBLIC/IMAGES/IMAGE.gif. If youdecided to move your public files to a directory called MYHOMEPAGE,the call to <TT>getDocumentBase()</TT> will give you the URL forthat new directory, without your having to change the applet'ssource code. This new URL, once you included the relative locationof the image file, would be FILE:/C:/MYHOMEPAGE/IMAGES/IMAGE.gif.<H3><A NAME="ExampleUsingtheIgetCodeBaseIMethod">Example: Using the <I>getCodeBase()</I> Method</A></H3><P>The <TT>getCodeBase()</TT> method works similarly to <TT>getDocumentBase()</TT>,except that it returns the URL of the directory from which theapplet was loaded. If you're storing your images in the same directory(or a subdirectory of that directory) as your CLASS files, you'dwant to call <TT>getCodeBase()</TT> to obtain an URL for an image.<P>Suppose you have your CLASS files in a directory called CLASSESand the image you want (still called IMAGE.gif) is stored in asubdirectory of CLASSES called IMAGES. A call to <TT>getCodeBase()</TT>will get you the base URL you need to load the image. That calllooks like this:<BLOCKQUOTE><PRE>URL url = getCodeBase();</PRE></BLOCKQUOTE><P>Again, once you have the URL, you can load the file by using theURL along with the relative location of the image, which wouldstill be IMAGES/IMAGE.gif. The full URL to the file would thenbe FILE:/C:/CLASSES/IMAGES/IMAGE.gif.<H3><A NAME="LoadinganImage">Loading an Image</A></H3><P>Once you have the image's base URL, you're ready to load the imageand create the <TT>Image</TT> object. You can complete both ofthese tasks at the same time, by calling your applet's <TT>getImage()</TT>method, like this:<BLOCKQUOTE><PRE>Image image = getImage(baseURL, relLocation);</PRE></BLOCKQUOTE><P>The <TT>getImage()</TT> method's two arguments are the URL returnedby your call to <TT>getCodeBase()</TT> or <TT>getDocumentBase()</TT>and the relative location of the image. For example, assumingthat you've stored your CLASS files in the directory C:\CLASSESand your images in the directory C:\CLASSES\IMAGES, you'd havea code that looks something like this:<BLOCKQUOTE><PRE>URL codeBase = getCodeBase();Image myImage = getImage(codeBase, "images/myimage.gif");</PRE></BLOCKQUOTE><P>After Java has executed the above lines, your image is loadedinto the computer's memory and ready to display.<H3><A NAME="DisplayinganImage">Displaying an Image</A></H3><P>Displaying the image is a simple matter of calling the <TT>Graphics</TT>object's <TT>drawImage()</TT> method, like this:<BLOCKQUOTE><PRE>g.drawImage(myImage, x, y, width, height, this);</PRE></BLOCKQUOTE><P>This method's arguments are the image object to display, the Xand Y coordinates at which to display the image, the width andheight of the image, and the applet's <TT>this</TT> reference.<P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD><B>TIP</B></TD></TR><TR VALIGN=TOP><TD><BLOCKQUOTE>When you want to display an image with its normal width and height, you can call a simpler version of the <TT>drawImage()</TT> method, which leaves out the width and height arguments, like this: <TT>drawImage(image, x, y, this)</TT>. This version of the method actually draws the image faster because it doesn't have to worry about reducing or expanding the image to the given width and height. It just blasts it on to the screen exactly as the image normally appears.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><P>You may be wondering where you can get the width and the heightof the image. As it turns out (no doubt thanks to careful considerationby Java's programmers over hundreds of cups of coffee), the <TT>Image</TT>class has two methods, <TT>getWidth()</TT> and <TT>getHeight()</TT>,that return the width and height of the image. The complete codefor displaying the image, then, might look like this:<BLOCKQUOTE><PRE>int width = image.getWidth(this);int height = image.getHeight(this);g.drawImage(image, x, y, width, height, this);</PRE></BLOCKQUOTE><P>As you can see, the <TT>getWidth()</TT> and <TT>getHeight()</TT>methods require a single argument, which is the applet's <TT>this</TT>reference.<H3><A NAME="ExampleDisplayinganImageinanApplet">Example: Displaying an Image in an Applet</A></H3><P>You're now ready to write an applet that can display images. Listing27.1 is the Java source code for an applet called ImageApplet⌨️ 快捷键说明
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