JAVA APPLET PREPARED BY Mr. Jahanzaib Ahmed jahanzaibmca@gmail JAVA APPLET PREPARED BY Mr. Jahanzaib Ahmed jahanzaibmca@gmail.com Dept. Of Computer Application

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. Java applets do not need a ‘main’ method. they can run in a web browser environment An applet can be a fully functional Java application because it has the entire Java API at its disposal.

Differences between an applet and a standalone Java application 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.

Cont. 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.

Hierarchy of Applet Object Component Container Panel Applet JApplet

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.

Cont. 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.

cont. Applet is initialized. Applet is started. Applet is painted. Applet is stopped. Applet is destroyed. Applet is initialized Applet is started Applet is destroyed Applet is painted Applet is stopped

Step by Step Instructions for Creating Your First Applet in Net Beans 7.2 Choose File /  New Project. Under Categories, select Java.  Under Projects, select Java Class Library. Click Next. Under Project Name, enter the name of your application.  Change the Project Location to any folder on your computer. Click Finish. The IDE creates the project folder. Right-click the project node in the Projects window or  Files window and select New > Other. Under Categories, select Swing GUI Forms. Under File Types, select JApplet Form. Click Next.

Cont. Under Class Name, enter the name of your applet. Leave Package blank for now (default package). Click Finish. The IDE creates the applet in the specified package. The applet opens in the Source editor. IMPORTANT: Right click your applet's background panel (not on any component or embedded panel) and choose "Set Layout..." on the pop-up menu, then select "Absolute Layout". Add several components (click component, click on design panel; OR right-click on design panel and choose Add From Palette). Repeat step 7 as necessary (see below for instructions for specific components). To Run - Right click on Projects.../<appletname>.java  select Run File (or SHIFT-F6)

Simple example of Applet by html file: To execute the applet by html file, create an applet and compile it. After that create an html file and place the applet code in html file. Now click the html file. //First.java    import java.applet.Applet;   import java.awt.Graphics;   public class First extends Applet{   public void paint(Graphics g){   g.drawString("welcome",150,150);   }     }  

Cont. myapplet.html <html> <body> <applet code="First.class" width="300" height="300">   </applet>   </body>   </html>  

Example of displaying image in applet: In the below example, drawImage() method of Graphics class is used to display the image. The 4th argument of drawImage() method of is ImageObserver object. The Component class implements ImageObserver interface. So current class object would also be treated as ImageObserver because Applet class indirectly extends the Component class. import java.awt.*;   import java.applet.*;      public class DisplayImage extends Applet {      Image picture;     public void init() {       picture = getImage(getDocumentBase(),"sonoo.jpg");     }        public void paint(Graphics g) {       g.drawImage(picture, 30,30, this);     }         

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