All of the preceding examples in this book have been Java applications. However, applications constitute only one class of Java programs. The other type of program is the applet. As mentioned in Chapter 1, applets arc small applications that are accessed on an Internet server, transported over the Internet, automatically installed, and run as part of n Web document. After an applet arrives on the client, it has limited access to resources, so that it call produce an arbitrary multimedia user interface CWD run complex computations without introducing the risk of viruses or breaching data integrity.
Many of the issues connected with the creation and use of applets arc found in Part IT, when the applet package is examined. However, the fundamentals connected to the creation of an applet are presented here, because applets arc not structured in the same way as the programs that have been used thus far. As you will see, applets differ from applications in several key areas. Let's begin with the simple apple shown here:
This applet begins with two import statements. The first imports the Abstract Window Toolkit (AWT) classes. Applets interact with the user through the AWI, not through the console-based I/O classes. The AWT contains support for a window-based, graphical interface. As you might expect, the AWT is quite large and sophisticated, and a complete discussion of ft consumes several chapters in Part 11 of this book. Fortunately, this simple applet makes very limited use of the AWT. The second import statement imports the applet package, which contains the class Applet. Every applet that you create must be a subclass of Applet.
I The next line in the program declares the class Simple Applet. class must by declared as public, because it will be accessed by code that is outside the program, Simple Applet, paint > ,is declared. This method is defined the AWT and must be overridden by the applet. paint( ) is called each time that the applet re display its output. This situation can occur for several reasons. For example, the window in which the applet is running can be oven by another window ell"! then uncovered. Or, the applet window. can be minimized and then restored. paint ) is also called when the applet begins execution, Whatever the cause, whenever the applet must redraw its output, paint( ) is called. The palatal method has Ola( parameter of type Graphics. This parameter contains the graphics context, which describes the graphics environment in which the applet is running. This context is used whenever output to the applet is required. ' Inside pain,t( ) is a call to stringing( ), which is n member of the Graphic.; class, This method outputs i\ string beginning at the specified X,Y location. It has Jill following general form: void draw String(String message, int x, int y) Here, message is the string to be output beginning 'at x,y. In a Java window, the upper-left comer is location 0,0. The call to draw String( ) in the applet causes the message" A Simple Applet" to be displayed beginning at location 20,20. Notice that the applet does not have a main( ) method. Unlike Java programs; 'applets do not begin execution at main( ). In fact, most' applets don't even have a main( ) method. Instead, an applet begins execution when the name of its class is passed to an applet viewer or to a.•network browser. . After you enter the source code for Simple Applet, compile in the same way that you have been compiling programs. However, running Simple Applet involves a . different process. In fact, there are two ways in which you can run an applet: • Executing the applet within a Java-compatible Web browser, such as Netscape .Using an applet viewer, such as the standard JDK tool, applet viewer, An applet viewer executes your applet in a window. This is generally the fastest and . easiest way to test your applet. Each of these methods is described next.. ' To execute-an applet in a Web browser, you need to write a short HTML text file . that contains the appropriate APPLET tag. Here is the HTML file that executes . Sill\applet
The width and height statements specify the dimensions of the display area used ' by the applet. (The APPLET tag contains several other options that are examined more closely in Part II.) After you create this file, you can execute your browser and then load this file, which & looses Simple Applet to be executed. . . To execute Simple.Applet with an applet viewer, you may also execute the HTML file shown earlier. For example, if the preceding HTML file'is called Run App.html, then the following command line will run Simple Applet: C:\>appletviewer Run App.html
However, a more convenient method exists that you can use to speed up testing. Simply include a comment at the head of your Java source code file that contains the APPLET tag. By doing so, your code is documented with a prototype of the necessary HTML statements, and you can test your compiled applet merely by starting the applet viewer.with your Java source code file. If you use this method, the Simple Applet
source file looks like this: In general, you ca~ quickly iterate through applet development by using these three steps:
1. Edit a Java source file.
2. Compile your program.
3. Execute the applet viewer, specifying the name of your applet's source file.
The applet viewer will encounter the APPLET tag within the comment and execute your applet. The window produced by. Simple Applet, as displayed by the applet viewer, is shown in the following illustration While the subject of applets is more fully discussed later in this book, here are the. key points that you should member now: Applets do not need a main) method. Applets must be run under an applet viewer or a java-compatible browser,User I/O is not accomplished with lava's stream I/O classes. Instead, applets use the. interface provided by the AWT.