JAVA COURSE- Java Applets

JAVA APPLETS
What is a Java Applet? Java Applets are small programs designed to run in a web browser. Typically, they are only a few kilobytes in length and can run in multiple operating systems (Mac OS, Windows, Unix, etc.). This makes them ideal for use on the Internet. What are Java Applets used for? Applets are usually used to add small interactive components to web pages (buttons, scrolling text, or a stock ticker) but can also be used to make larger programs like a game or word processor.
What do I need to make an Applet work?
You need the Java .class file... The Java applet usually comes with a .class file (i.e. "joybutton.class" or "button.class"). This file is the Java program and must be on your web site for the applet to work. The .class file comes with the Applet program already made and ready for use.	You need the Applet's HTML code in your web page… Just having the .class file on your site isn't enough, you need to put HTML code in the page which will display the applet. This HTML code is what the Applet program creates. It tells the .class file how to customize the applet - what colors and images to use, what size it is, etc.
Java .class Files A Java .class file is the Java program. An Applet will not work without a .class file in one form or another. It needs to be on your web site for the Applet to work. Important things to know about .class files The .class file needs to be on the same server as the HTML page with the applet. They need to be transferred to a web site in binary mode. If your FTP program is not in binary mode the .class file may become corrupted. An unlimited number of applets can run from one .class file.
Other extensions for .class files Java .class files can have other extensions when they are compressed. Compressed .class files can have the extension .zip or .jar. Since it is compressed it will decrease the time it takes to load an Applet on the Internet. Note: Older browsers do not support .jar or .zip Java files. If you use one of these files you will need to put both .class files (compress and uncompress) on your web site. Older browsers will load the .class files while newer ones load the .jar or .zip file.
The following is a list of common problems and solutions:
My browser says I have a "java.language.null.pointer.exception" You may be viewing an applet that contains images from your hard drive with Netscape 4. There is a bug in Netscape that can cause this error ONLY when an applet with images is viewed from a local hard drive. The only way I know to fix this is to change the way you open the html file in Netscape. Try this… Open Netscape, then drag and drop your HTML file over the Netscape window. This will open the HTML file and hopefully everything will work. NOTE: I have never seen this problem when an applet is viewed from the internet so it should NOT affect visitors to your site. Try emptying the browser’s cache and history folders. If you are using Netscape 4 you can empty these folders from the “Preferences” menu in Netscape. If you are using Internet Explorer the cache is called the "Temporary Internet Files" folder and can be deleted from the options menu in Internet Explorer. You should also empty the history folder from the browsers options menu. Go to the page that doesn't work. If you are using Internet Explorer 4 hold down control while pressing the reload button. If you are using Netscape 4 hold down shift while pressing the reload button. You started the path to an image or sound or HTML file with the letter of a hard drive ("c:\" etc.) remove the "c:\" . All I see is a gray box in my browser The .class file is not in the proper directory or is named incorrectly... it must be in the same directory as the HTML file and it must be named joybutton.class not JoyButton.class or joyButton.cla. The file names are case sensitive. My browser says I have a "java.lang.ClassFormatError" The .class file has been corrupted. This happens when the files are transferred to the internet with your ftp program in ASCII mode instead of binary mode. Look in your ftp program for a way to switch to binary mode and re-transfer the files. My browser says "Applet can't start: class joybutton not found" The .class file is not in the proper directory or is named incorrectly... it must be in the same directory as the HTML file and it must be named button.class not Button.class or Button.cla. The file names are case sensitive. My images do not show up The names of images are case sensitive.... make sure the names are exactly the same on both the HTML page and server with your page. You have an incorrect path to the images. Something is wrong with the image file. Try re-saving it with a good draws program or changing the extension. It opens a new browser window instead of showing the page in the correct frame You probably have entered the html page name in the "Target Frame" text box instead of the frame name specified in your frameset. Don't use frame names with the "-" character. by John Doerfler
Java programs for the Web Java’s level of interactivity is possible because of how Web browsers that can interpret Java programs for the Web (called applets) operate. In a non-Java-enabled Web browser, information content is defined in terms of Multipurpose Internet Mail Extensions (MIME) specifications, which define a variety of multimedia document formats. This content is specified so that it can be displayed in the browser or in a helper application (such as images, sound, and video). The result is that the user chooses and then observes content. A Java-enabled browser also downloads content defined by MIME specifications and displays it. When a Java-enabled browser downloads a Web page containing a Java applet, indicated by the hypertext tag (<APPLET>), the browser knows that a Java program (applet) is associated with that tag. The Java-enabled browser then can download that applet from the information provider’s Web server. The applet is in a special format called bytecodes. The Java-enabled browser can interpret these bytecodes and run them as an executable program on the user’s computer host. The result is that the user downloads and runs an executable program-not just content displayed in the Web browser. Java also uses this same scheme to support programmer-defined protocols and special document formats.
Java’s technical scheme: The user sends a request for an HTML document to the information provider’s Web server. The Web server returns an HTML document to the user’s browser. The document contains the <APPLET> tag that identifies the applet. The bytecode corresponding to that applet is transferred to the user’s host. This bytecode was created previously by the Java compiler using the Java source code file for that applet. The Java runtime system on the user’s host interprets the bytecodes and provides display. The user then can use the applet with no further downloading from the provider’s Web server. This is because the bytecode contains all the information necessary to run the applet.