Java Applet Basics, Applet Complete tutorial

All applets are subclasses of Applet .Thus,all applets must import java.applet .Applets mus also import java.awt.Recall that awt stands for the Abstract window too kit.Since all applets run in a window ,It is necessary to iclude support for that window .Applets are not executed by the consile based java run time interpreter.Rather they are executed by either a Web browser or applet viewer .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.
  

Advantages

A Java applet can have any or all of the following advantages:

• It is simple to make it work on Linux, Microsoft Windows and Mac OS X i.e. to make it cross platform. Applets are supported by most web browsers.

• The same applet can work on "all" installed versions of Java at the same time, rather than just the latest plug-in version only. However, if an applet requires a later version of the Java Runtime Environment (JRE) the client will be forced to wait during the large download.

• Most web browsers cache applets so will be quick to load when returning to a web page. Applets also improve with use: after a first applet is run, the JVM is already running and starts quickly (the JVM will need to restart each time the browser starts afresh). It should be noted that JRE versions 1.5 and greater stop the JVM and restart it when the browser navigates from one HTML page containing an applet to another containing an applet.

• It can move the work from the server to the client, making a web solution more scalable with the number of users/clients.

• If a standalone program (like Google Earth) talks to a web server, that server normally needs to support all prior versions for users which have not kept their client software updated. In contrast, a properly configured browser loads (and caches) the latest applet version, so there is no need to support legacy versions.

• The applet naturally supports the changing user state, such as figure positions on the chessboard.

• Developers can develop and debug an applet direct simply by creating a main routine (either in the applet's class or in a separate class) and calling init() and start() on the applet, thus allowing for development in their favorite Java SE development environment. All one has to do after that is re-test the applet in the AppletViewer program or a web browser to ensure it conforms to security restrictions.

• An untrusted applet has no access to the local machine and can only access the server it came from. This makes such an applet much safer to run than a standalone executable that it could replace. However, a signed applet can have full access to the machine it is running on if the user agrees.

• Java applets are fast - and can even have similar performance to native installed software.

Disadvantages

A Java applet may have any of the following disadvantages:

• It requires the Java plug-in.
• Some browsers, notably mobile browsers running Apple iOS or Android do not run Java applets at all.
• Some organizations only allow software installed by the administrators. As a result, some users can only view applets that are important enough to justify contacting the administrator to request installation of the Java plug-in.
• As with any client-side scripting, security restrictions may make it difficult or even impossible for an untrusted applet to achieve the desired goals. However, simply editing the java.policy file in the JAVA JRE installation, one can grant access to the local filesystem or system clipboard for example, or to other network sources other than the network source that served the applet to the browser.
• Some applets require a specific JRE. This is discouraged.
• If an applet requires a newer JRE than available on the system, or a specific JRE, the user running it the first time will need to wait for the large JRE download to complete.
• Java automatic installation or update may fail if a proxy server is used to access the web. This makes applets with specific requirements impossible to run unless Java is manually updated. The Java automatic updater that is part of a Java installation also may be complex to configure if it must work through a proxy.
• Unlike the older applet tag, the object tag needs workarounds to write a cross-browser HTML document.
• There is no standard to make the content of applets available to screen readers. Therefore, applets can harm the accessibility of a web site to users with special needs.

/*<applet code="Myapplet" width=200 height=200></applet>*/

This comment contains a n applet tag that will run an applet called Myapplet in a window that is 200 pixel wide and 200 pixel hight .Since th inclusion of an applet of an APPLET command makes applets easier.