Applet in Java

Applet in Java

·         Applets are small Java applications that can be accessed on an Internet server, transported over Internet, and can be automatically installed and run as a part of a web document.

·         After a user receives an applet, the applet can produce a graphical user interface. It has limited access to resources so that it can run complex computations without introducing the risk of viruses or breaching data integrity.

·         Any applet in Java is a class that extends the java.applet.Applet class.

·         An Applet class does not have any main() method. It is viewed using JVM. The JVM can use either a plug-in of the Web browser or a separate runtime environment to run an applet application.

·         JVM creates an instance of the applet class and invokes init() method to initialize an Applet.

 

A Simple Applet

import java.awt.*;

import java.applet.*;

/*

<applet code="myApp.class" width="300" height="300"> 

</applet>

*/

public class myApp extends Applet

{

 public void paint(Graphics g)

 {

          g.drawString("Piyush Patel, Rajkot", 20, 20);

 }

}

Compile: javac Simple.java

Run   : appletviewer Simple.java

Every Applet application must import two packages - java.awt and java.applet.

java.awt.* imports the Abstract Window Toolkit (AWT) classes. Applets interact with the user (either directly or indirectly) through the AWT. The AWT contains support for a window-based, graphical user interface. java.applet.* imports the applet package, which contains the class Applet. Every applet that you create must be a subclass of Applet class.

The class in the program must be declared as public, because it will be accessed by code that is outside the program. Every Applet application must declare a paint() method. This method is defined by AWT class and must be overridden by the applet. The paint() method is called each time when an applet needs to redisplay its output. Another important thing to notice about applet application is that, execution of an applet does not begin at main() method. In fact an applet application does not have any main() method.

Advantages of Applets

1.     It takes very less response time as it works on the client side.

2.     It can be run on any browser which has JVM running in it.

How to run an Applet Program?

An Applet program is compiled in the same way as you have been compiling your console programs. However there are two ways to run an applet.

·         Executing the Applet within Java-compatible web browser.

·         Using an Applet viewer, such as the standard tool, applet viewer. An applet viewer executes your applet in a window

For executing an Applet in a web browser, create short HTML file in the same directory. Inside body tag of the file, include the following code. (applet tag loads the Applet class)

< applet code = "MyApplet" width=400 height=400 >

< /applet >

Applet Life Cycle

1.     init(): The applet's voyage starts here. In this method, the applet object is created by the browser. Because this method is called before all the other methods, programmer can utilize this method to instantiate objects, initialize variables, setting background and foreground colors in GUI etc.; the place of a constructor in an application. It is equivalent to born state of a thread.

2.     start(): In init() method, even though applet object is created, it is in inactive state. An inactive applet is not eligible for microprocessor time even though the microprocessor is idle. To make the applet active, the init() method calls start() method. In start() method, applet becomes active and thereby eligible for processor time.

3.     paint(): This method takes a java.awt.Graphics object as parameter. This class includes many methods of drawing necessary to draw on the applet window. This is the place where the programmer can write his code of what he expects from applet like animation etc. This is equivalent to runnable state of thread.

4.     stop(): In this method the applet becomes temporarily inactive. An applet can come any number of times into this method in its life cycle and can go back to the active state (paint() method) whenever would like. It is the best place to have cleanup code. It is equivalent to the blocked state of the thread.

5.     destroy(): This method is called just before an applet object is garbage collected. This is the end of the life cycle of applet. It is the best place to have cleanup code. It is equivalent to the dead state of the thread.

Displaying Graphics in Applet

java.awt.Graphics class provides many methods for graphics programming.

Commonly used methods of Graphics class:

1.     public abstract void drawString(String str, int x, int y): is used to draw the specified string.

2.     public void drawRect(int x, int y, int width, int height): draws a rectangle with the specified width and height.

3.     public abstract void fillRect(int x, int y, int width, int height): is used to fill rectangle with the default color and specified width and height.

4.     public abstract void drawOval(int x, int y, int width, int height): is used to draw oval with the specified width and height.

5.     public abstract void fillOval(int x, int y, int width, int height): is used to fill oval with the default color and specified width and height.

6.     public abstract void drawLine(int x1, int y1, int x2, int y2): is used to draw line between the points(x1, y1) and (x2, y2).

7.     public abstract boolean drawImage(Image img, int x, int y, ImageObserver observer): is used draw the specified image.

8.     public abstract void drawArc(int x, int y, int width, int height, int startAngle, int arcAngle): is used draw a circular or elliptical arc.

9.     public abstract void fillArc(int x, int y, int width, int height, int startAngle, int arcAngle): is used to fill a circular or elliptical arc.

10.            public abstract void setColor(Color c): is used to set the graphics current color to the specified color.

11.            public abstract void setFont(Font font): is used to set the graphics current font to the specified font.

 

Example of Graphics in applet:

GraphicsDemo.java

import java.applet.Applet;  

import java.awt.*;  

/*

<applet code="GraphicsDemo.class" width="300" height="300">  

</applet>

*/

public class GraphicsDemo extends Applet

{  

  

public void paint(Graphics g){  

g.setColor(Color.red);  

g.drawString("Welcome",50, 50);  

g.drawLine(20,30,20,300);  

g.drawRect(70,100,30,30);  

g.fillRect(170,100,30,30);  

g.drawOval(70,200,30,30);  

  

g.setColor(Color.pink);  

g.fillOval(170,200,30,30);  

g.drawArc(90,150,30,30,30,270);  

g.fillArc(270,150,30,30,0,180);  

}  

}

Parameter in Applet

We can get any information from the HTML file as a parameter. For this purpose, Applet class provides a method named getParameter(). Syntax:

public String getParameter(String parameterName)  

Example of using parameter in Applet:

import java.applet.Applet;  

import java.awt.Graphics;  

  

public class UseParam extends Applet{  

  

public void paint(Graphics g){  

String str=getParameter("msg");  

g.drawString(str,50, 50);  

}  

  

}  

myapplet.html

<html>  

<body>  

<applet code="UseParam.class" width="300" height="300">  

<param name="msg" value="Welcome to applet">  

</applet>  

</body>  

</html>