Summary Applets

Java Applet

Applet is a special type of program that is embedded in the webpage to generate the dynamic content. It runs
inside the browser and works at client side.

Advantage of Applet

There are many advantages of applet. They are as follows:

 It works at client side so less response time.
 Secured
 It can be executed by browsers running under many plateforms, including Linux, Windows, Mac Os
etc.

Drawback of Applet

 Plugin is required at client browser to execute applet.

Hierarchy of Applet




As displayed in the above diagram, Applet class extends Panel. Panel class extends Container which is the
subclass of Component.

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

Karandeep Singh
Assistant Professor
Department of Computer Engineering
Punjabi University, Patiala Page 1

, 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 gives 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.




Lifecycle methods for Applet:

The java.applet.Applet class 4 life cycle methods and java.awt.Component class provides 1 life cycle
methods for an applet.

java.applet.Applet class

For creating any applet java.applet.Applet class must be inherited. It provides 4 life cycle methods of applet.

1. public void init(): is used to initialized the Applet. It is invoked only once.
2. public void start(): is invoked after the init() method or browser is maximized. It is used to start the
Applet.
3. public void stop(): is used to stop the Applet. It is invoked when Applet is stop or browser is
minimized.
4. public void destroy(): is used to destroy the Applet. It is invoked only once.

Karandeep Singh
Assistant Professor
Department of Computer Engineering
Punjabi University, Patiala Page 2