An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.
There are some important differences between an applet and a standalone Java application, including the following:
An applet is a Java class that extends the java.applet.Applet class.
A main() method is not invoked on an applet, and an applet class will not define main().
Applets are designed to be embedded within an HTML page.
When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user's machine.
A JVM is required to view an applet. The JVM can be either a plug-in of the Web browser or a separate runtime environment.
The JVM on the user's machine creates an instance of the applet class and invokes various methods during the applet's lifetime.
Applets have strict security rules that are enforced by the Web browser. The security of an applet is often referred to as sandbox security, comparing the applet to a child playing in a sandbox with various rules that must be followed.
Other classes that the applet needs can be downloaded in a single Java Archive (JAR) file.
Life Cycle of an Applet:
Four methods in the Applet class give you the framework on which you build any serious applet:
init: This method is intended for whatever initialization is needed for your applet. It is called after the param tags inside the applet tag have been processed.
start: This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages.
stop: This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet.
destroy: This method is only called when the browser shuts down normally. Because applets are meant to live on an HTML page, you should not normally leave resources behind after a user leaves the page that contains the applet.
paint: Invoked immediately after the start() method, and also any time the applet needs to repaint itself in the browser. The paint() method is actually inherited from the java.awt.
A "Hello, World" Applet:
The following is a simple applet named HelloWorldApplet.java:
import java.applet.*;
import java.awt.*;
public class HelloWorldApplet extends Applet
{
public void paint (Graphics g)
{
g.drawString ("Hello World", 25, 50);
}
}
These import statements bring the classes into the scope of our applet class:
java.applet.Applet.
java.awt.Graphics.
Without those import statements, the Java compiler would not recognize the classes Applet and Graphics, which the applet class refers to.
The Applet CLASS:
Every applet is an extension of the java.applet.Applet class. The base Applet class provides methods that a derived Applet class may call to obtain information and services from the browser context.
These include methods that do the following:
Get applet parameters
Get the network location of the HTML file that contains the applet
Get the network location of the applet class directory
Print a status message in the browser
Fetch an image
Fetch an audio clip
Play an audio clip
Resize the applet
Additionally, the Applet class provides an interface by which the viewer or browser obtains information about the applet and controls the applet's execution. The viewer may:
request information about the author, version and copyright of the applet
request a description of the parameters the applet recognizes
initialize the applet
destroy the applet
start the applet's execution
stop the applet's execution
The Applet class provides default implementations of each of these methods. Those implementations may be overridden as necessary.
The "Hello, World" applet is complete as it stands. The only method overridden is the paint method.
Invoking an Applet:
An applet may be invoked by embedding directives in an HTML file and viewing the file through an applet viewer or Java-enabled browser.
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