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Applets are small programs transferred through Internet, automatically installed and run as part of web-browser. Applets implements functionality of a client. Applet is a dynamic and interactive program that runs inside a Web page displayed by a Java-capable browser. We don’t have the concept of Constructors in Applets. Applets can be invoked either through browser or through Appletviewer utility provided by JDK.
1. Place the .class file in the directory containing the HTML document into which you want to insert the applet.
2. Copy the <applet>...</applet> tag from your applet implementation or examples to the clipboard.
3. In FrontPage select the "HTML" tab from the lower left hand corner.
4. Paste the <applet>...</applet> tag in an appropriate place between the <body> and </body> tags. You'll find a gray box with the aqua letter "J" in the "Normal" view indicating the the applet tag has been inserted.
5. To see the applet appearance select the "Preview" tab.
► public void init() : Initialization method called once by browser.
► public void start() : Method called after init() and contains code to start processing. If the user leaves the page and returns without killing the current browser session, the start () method is called without being preceded by init ().
► public void stop() : Stops all processing started by start (). Done if user moves off page.
► public void destroy() : Called if current browser session is being terminated. Frees all resources used by applet.
methods in the life cycle of an Applet:
► init() method - called when an applet is first loaded. This method is called only once in the entire cycle of an applet. This method usually intialize the variables to be used in the applet.
► start( ) method - called each time an applet is started.
► paint() method - called when the applet is minimized or refreshed. This method is used for drawing different strings, figures, and images on the applet window.
► stop( ) method - called when the browser moves off the applet’s page.
► destroy( ) method - called when the browser is finished with the applet.
When an applet begins, the AWT calls the following methods, in this sequence:
Following are the main differences:
Application: Stand Alone, doesn’t need web-browser.
Applet: Needs no explicit installation on local machine. Can be transferred through Internet on to the local machine and may run as part of web-browser.
Application: Execution starts with main() method. Doesn’t work if main is not there.
Applet: Execution starts with init() method.
Application: May or may not be a GUI.
Applet: Must run within a GUI (Using AWT). This is essential feature of applets.
We can pass parameters to an applet using <param> tag in the following way:
► <param name=”param1″ value=”value1″>
► <param name=”param2″ value=”value2″>
Access those parameters inside the applet is done by calling getParameter() method inside the applet. Note that getParameter() method returns String value corresponding to the parameter name.
Use the parseInt() method in the Integer Class, the Float(String) constructor or parseFloat() method in the Class Float, or the
Double(String) constructor or parseDoulbl() method in the class Double.
Name your applets inside the Applet tag and invoke AppletContext’s getApplet() method in your applet code to obtain references to the other applets on the page.
Ask the applet for its applet context and invoke showDocument() on that context object.
AppletContext context = getAppletContext();
targetURL = new URL(URLString);
catch (MalformedURLException e)
// Code for recover from the exception
context. showDocument (targetURL);
Use the getSize() method, which the Applet class inherits from the Component class in the Java.awt package. The getSize() method returns the size of the applet as a Dimension object, from which you extract separate width, height fields.
The following code snippet explains this:
Dimension dim = getSize();
int appletwidth = dim.width();
int appletheight = dim.height();
Applet class consists of a single class, the Applet class and three interfaces: AppletContext, AppletStub, and AudioClip.
The applet stub interface provides the means by which an applet and the browser communicate. Your code will not typically implement this interface.
The following are the Applet’s information methods:
getAppletInfo() method: Returns a string describing the applet, its author, copyright information, etc.
getParameterInfo( ) method: Returns an array of string describing the applet’s parameters.
Following are the steps involved in Applet development:
► Create/Edit a Java source file. This file must contain a class which extends Applet class.
► Compile your program using javac
► Execute the appletviewer, specifying the name of your applet’s source file or html file. In case the applet information is stored in html file then Applet can be invoked using java enabled web browser.
drawString( ) method is used to output a string to an applet. This method is included in the paint method of the Applet.
Whenever a screen needs redrawing (e.g., upon creation, resizing, validating) the update method is called. By default, the update method clears the screen and then calls the paint method, which normally contains all the drawing code.
The simplest method is to use the static variables of a shared class since there's only one instance of the class and hence only one copy of its static variables.
A slightly more reliable method relies on the fact that all the applets on a given page share the same AppletContext.
We obtain this applet context as follows:
AppletContext ac = getAppletContext();
AppletContext provides applets with methods such as getApplet(name), getApplets(),getAudioClip, getImage, showDocument and showStatus().
We can use the java.net.URLConnection and java.net.URL classes to open a standard HTTP connection and "tunnel" to the web server. The server then passes this information to the servlet in the normal way. Basically, the applet pretends to be a web browser, and the servlet doesn't know the difference. As far as the servlet is concerned, the applet is just another HTTP client
JComponent (except top-level containers)
The Canvas class of java.awt is used to provide custom drawing and event handling. It provides a general GUI component for drawing images and text on the screen. It does not support any drawing methods of its own, but provides access to a Graphics object through its paint() method. The paint() method is invoked upon the creation and update of a canvas so that the Graphics object associated with a Canvas object can be updated.
Java v1.02 only supports the "voice format" of the .au sound files. This is also know as "µ-law, 8/16-bit, mono, 8000hz sample rate"
The getParameter() method can be used within the init() method to access the parameter data.
It takes the parameter name as an argument.
public void init()
String val = getParameter("foreground-color");
The methods that control an applet’s on-screen appearance I.e. update and paint.
- The paint() method is called in situations the applet window being overwritten by another window or uncovered or the applet window being resized.
- The paint() is also called when the applet begins execution. The paint() method has one parameter of type Graphics which is needed to know the location where the applet is supposed to paint its output.
- The update() is called when a portion of its window be redrawn. It is defined by the AWT.
- However, the update() first fills an applet with the default background colour and then calls paint() due to which an instance of the default color appears each time update is called.
- Thus update() method should be overridden to avoid this situation.
We can use the java.net.URLConnection and java.net.URL classes to open a standard HTTP connection and "tunnel" to a Web server. The server then passes this information to the servlet. Basically, the applet pretends to be a Web browser, and the servlet doesn't know the difference.
As far as the servlet is concerned, the applet is just another HTTP client. Applets can communicate with servlets using GET or POST methods. The parameters can be passed between the applet and the servlet as name value pairs. Objects can also be passed between applet and servlet using object serialization. Objects are serialized to and from the inputstream and outputstream of the connection respectively.
Yes. Add a main() method to the applet.
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