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Java Applet
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Introductions Applet is java program that can be embedded into HTML pages Java applets runs on the java enables web browsers such as mozila and internet explorer. Applet is designed to run remotely on the client browser, so there are some restrictions on it. Applet can't access system resources on the local computer. Applets are used to make the web site more dynamic and entertaining.
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Advantages of Applet * Applets are cross platform and can run on Windows, Mac OS and Linux platform * Applets can work all the version of Java Plugin * Applets runs in a sandbox, so the user does not need to trust the code, so it can work without security approval * Applets are supported by most web browsers * Applets are cached in most web browsers, so will be quick to load when returning to a web page * User can also have full access to the machine if user allows
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Disadvantages of Java Applet
* Java plug-in is required to run applet * Java applet requires JVM so first time it takes significant startup time * If applet is not already cached in the machine, it will be downloaded from internet and will take time * Its difficult to desing and build good user interface in applets compared to HTML technology
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The Applet class To create an applet, you must import the Applet class
This class is in the java.applet package The Applet class contains code that works with a browser to create a display window Capitalization matters! applet and Applet are different names
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Importing the Applet class
Here is the directive that you need: import java.applet.Applet; import is a keyword java.applet is the name of the package A dot ( . ) separates the package from the class Applet is the name of the class There is a semicolon ( ; ) at the end
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Applet versus Application
Applets are small programs while applications are larger programs. Applets don't have the main method while in an application execution starts with the main method. Applets can run in our browser's window or in an appletviewer. To run the applet in an appletviewer will be an advantage for debugging. Applets are designed for the client site programming purpose while the applications don't have such type of criteria.
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The Life cycle of An Applet
init(): This method is called to initialized an applet start(): This method is called after the initialization of the applet. stop(): This method can be called multiple times in the life cycle of an Applet. destroy(): This method is called only once in the life cycle of the applet when applet is destroyed.
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The Life cycle of An Applet
import java.awt.*; import java.applet.*; class Myclass extends Applet{ public void init(){} Publicvoidstart(){} public void stop() {} public void destroy() {} public void paint(Graphics g) {}
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Applet Tag [<] APPLET [CODEBASE = codebaseURL] CODE = appletFile [ALT = alternateText] [NAME = appletInstanceName] WIDTH = pixels HEIGHT = pixels [ALIGN = alignment] [VSPACE = pixels] [HSPACE = pixels] > [< PARAM NAME = appletParameter1 VALUE = value >] [< PARAM NAME = appletParameter2 VALUE = value >] [alternateHTML] [</]APPLET[>]
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Creating First Applet Example
import java.applet.*; import java.awt.*; public class FirstApplet extends Applet{ public void paint(Graphics g){ g.drawString("Welcome in Java Applet.",40,20); }
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HTML code <HTML> <HEAD> </HEAD> <BODY>
<APPLET ALIGN="CENTER" CODE="FirstApplet.class" WIDTH="800" HEIGHT="500"></APPLET> </BODY> </HTML>
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Drawing Shapes Example in java
Graphics.drawLine() : to draw the line in the applet. drawLine(int X_from_coordinate, int Y_from_coordinate, int X_to_coordinate, int Y_to_coordinate); Graphics.drawString() : draws the given string as the parameter drawString(String string, int X_coordinate, int Y_coordinate); Graphics.drawOval() : draws the circle g.drawOval(int X_coordinate, int Y_coordinate, int Wdth, int height); Graphics.drawRect() : draws the rectangle. Here is the syntax of the drawRect() method : g.drawRect(int X_coordinate, int Y_coordinate, int Wdth, int height)
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Drawing Shapes Example using color in java
Graphics.setColor() : sets the color for the object by specified color. setColor(Color.color_name); Graphics.fillOval() : to fill the color inside the oval by specified color g.fillColor(Color.color_name); Graphics.fillRect() :to fill the color inside the rectangle by specified color g.fillRect(int X_coordinate, int Y_coordinate, int Wdth, int height)
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Passing Parameter in Java Applet
The param tag(<parma name="" value=""></param>) is used to pass the parameters to an applet To access values String strParameter = this.getParameter("Message");
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Event Listeners Example
To handle the events generated by these buttons you add action listeners e.g. object_name.addActionListener(this);. When the action event occurs, that object's actionPerformed method is invoked. actionPerformed(ActionEvent e)
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Display image paint(Graphics g) has used
MediaTracker is a utility class that tracks the status of a number of media objects img - image name type of Image. x - lower X - Coordinate type of int. y - lower Y - Coordinate type of int. x1 - upper X - Coordinate type of int. y1 - upper Y - Coordinate type of int.
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