Applet Fundamentals

Applet Fundamentals

All of the preceding examples in this book have been Java console-based applications. However, these types of applications constitute only one class of Java programs. Another type of program is the applet. As mentioned in Chapter 1, applets are small applications that are accessed on an Internet server, transported over the Internet, automatically installed, and run as part of a web document. After an applet arrives on the client, it has limited access to resources so that it can produce a graphical user interface and run various computations without introducing the risk of viruses or breaching data integrity.

Many of the issues connected with the creation and use of applets are found in Part II, when the applet package is examined, and also when Swing is described in Part III. However, the fundamentals connected to the creation of an applet are presented here, because applets are not structured in the same way as the programs that have been used thus far. As you will see, applets differ from console-based applications in several key areas.

Let’s begin with the simple applet shown here:

import java.awt.*; import java.applet.*;

public class SimpleApplet extends Applet {

public void paint(Graphics g) {

g.drawString("A Simple Applet", 20, 20);

}

}

This applet begins with two import statements. The first imports the Abstract Window Toolkit (AWT) classes. Applets interact with the user through a GUI framework, not through the console-based I/O classes. One of these frameworks is the AWT, and that is the framework used here to introduce applet programming. The AWT contains very basic support for a window-based, graphical user interface. As you might expect, the AWT is quite large, and a detailed discussion of it is found in Part II of this book. Fortunately, this simple applet makes very limited use of the AWT. (Another commonly used GUI for applets is Swing, but this approach is described later in this book.) The second import statement imports the applet package, which contains the class Applet. Every AWT-based applet that you create must be a subclass (either directly or indirectly) of Applet.

The next line in the program declares the class SimpleApplet. This class must be declared as public, because it will be accessed by code that is outside the program.

Inside SimpleApplet, paint( ) is declared. This method is defined by the AWT and must be overridden by the applet. paint( ) is called each time that the applet must redisplay its output. This situation can occur for several reasons. For example, the window in which the applet is running can be overwritten by another window and 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 paint( ) method has one 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 paint( ) is a call to drawString( ), which is a member of the Graphics class. This method outputs a string beginning at the specified X,Y location. It has the following general form:

void drawString(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 corner is location 0,0. The call to drawString( ) 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 SimpleApplet, compile in the same way that you have been compiling programs. However, running SimpleApplet 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.

        Using an applet viewer, such as the standard tool, appletviewer. 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.

One way to execute an applet in a web browser is to write a short HTML text file that contains a tag that loads the applet. At the time of this writing, Oracle recommends using the APPLET tag for this purpose. (The OBJECT tag can also be used. See Chapter 23 for further information regarding applet deployment strategies.) Using APPLET, here is the HTML file that executes SimpleApplet:

<applet code="SimpleApplet" width=200 height=60> </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 use it to execute the applet.

To execute SimpleApplet with an applet viewer, you may also execute the HTML file shown earlier. For example, if the preceding HTML file is called RunApp.html, then the following command line will run SimpleApplet:

C:\>appletviewer RunApp.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 SimpleApplet source file looks like this:

import java.awt.*;

import java.applet.*; /*

<applet code="SimpleApplet" width=200 height=60> </applet>

*/

public class SimpleApplet extends Applet {

public void paint(Graphics g) {

g.drawString("A Simple Applet", 20, 20);

}

}

With this approach, you can quickly iterate through applet development by using these three steps:

            Edit a Java source file.

            Compile your program.

            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 SimpleApplet, as displayed by the applet viewer, is shown in the following illustration. Of course, the precise appearance of the applet viewer frame may differ based on your environment. For this reason, the screen captures in this book reflect a number of different environments.

While the subject of applets is more fully discussed later in this book, here are the key points that you should remember 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 Java’s stream I/O classes. Instead, applets use the interface provided by a GUI framework.