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PART 24
Java Applets
24.1 What is an Applet?
An applet is a Java program that runs in a Web browser. An applet can be a fully
functional Java application because it has the entire Java API. 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 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.
24.2 Advantages of Applets
There are many advantages of applet. They are as follows:
It works at client side so less response time.
Secured
Automatically integrated with HTML; hence, resolved all installation issues.
It can be executed by browsers running under many platforms, including Linux,
Windows, Mac Os etc.
Can provide dynamic, graphics capabilities and visualizations
24.3 Drawbacks of Applets
Plugin is required at client browser to execute applet.
Applets can’t run any local executable programs
Applets can’t read/write to local computer’s file system
Applets can’t find any information about the local computer
All java created pop-up windows carry a warning message
Performance directly depend on client’s machine
24.4 Life Cycle of an Applet:
Four methods in the Applet class give you the framework on which you build any
serious applet:
init: This method is intended for whatever
initialization is needed for your applet.
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.
paint: Invoked immediately after the start()
method, and also any time the applet
needs to repaint itself in the browser.
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.
"Hello, World" Applet:
The following is a simple applet named HelloWorldApplet.java:
import java.applet.*;
import java.awt.*;
public class HelloWorldApplet extends Applet
{
public void paint (Graphics g)
{
setBackground(Color.GREEN);
g.drawString ("Hello World", 25, 50);
}
}
Applet CLASS:
Every applet is an extension of the java.applet.Applet class. The base Applet
class provides methods that a derived Applet class may call to obtain information and
services from the browser context.
24.5 Invoking an Applet:
An applet may be invoked by embedding directives in an HTML file and viewing the
file through an applet viewer or Java-enabled browser.
The <applet> tag is the basis for embedding an applet in an HTML file. Below is an
example that invokes the "Hello, World" applet:
<html>
<title> Hello World Applet</title>
<applet code="HelloWorldApplet.class" width="320" height="120">
</applet>
</html>
The code attribute of the <applet> tag is required. It specifies the Applet class to run.
Width and height are also required to specify the initial size of the panel in which an
applet runs. The applet directive must be closed with a </applet> tag.
Enabling Local Applets to run in the Browsers
You have to install 32 bit JRE to run your applets within the Web Browsers.
64 bit java versions does not support Browser Plug-in Integration.
Use the following settings:
24.6 Specifying Applet Parameters:
The following example demonstrates how to make an applet respond to setup
parameters specified in the document.
The Applet.getParameter() method fetches a parameter given the parameter's name
(the value of a parameter is always a string).
import java.applet.*;
import java.awt.*;
public class appletParameter extends Applet {
private String strDefault = "Hello! Java Applet.";
public void paint(Graphics g) {
String strParameter = getParameter("Message");
if (strParameter == null)
strParameter = strDefault;
setBackground(Color.GREEN);
g.drawString(strParameter, 50, 25);
}
}
The following is an example of an HTML file with an appletParameter.class
embedded in it. The HTML file specifies both parameters to the applet by means of
the <param> tag.
<html>
<title>Applet Test</title>
<h1> This is the applet: </h1>
<applet code="appletParameter.class" width="800" height="100">
<param name="message" value="This is HTML parameter Text.">
</applet>
</html>
Note: Parameter names are not case sensitive.
24.7 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 void drawArc(int x, int y, int width, int height, int
startAngle, int arcAngle): is used draw a circular or elliptical arc.
8. 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.
9. public abstract void setColor(Color c): is used to set the graphics current
color to the specified color.
10. public abstract void setFont(Font font): is used to set the graphics current
font to the specified font.
Example of Graphics in applet:
import java.applet.Applet;
import java.awt.*;
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.green);
g.fillOval(170, 200, 30, 30);
g.drawArc(90, 150, 30, 30, 30, 270);
g.fillArc(270, 150, 30, 30, 0, 180);
}
}
24.8 Using SWING GUI in Applets: JApplet
JApplet — a class that enables applets to use Swing components. JApplet is a
subclass of java.applet.Applet.
Any applet that contains Swing components must be implemented with a
subclass of JApplet. If you're looking to use Swing components inside of your applet,
JApplet would give you that functionality.
WindowBuilder plugin could bu used to create JApplet Application:
Here is a sample Hello World JApplet Example:
import
import
import
import
import
javax.swing.JApplet;
javax.swing.JButton;
javax.swing.JTextField;
java.awt.event.ActionListener;
java.awt.event.ActionEvent;
public class swapplet extends JApplet {
private final JButton btnNewButton = new JButton("Say Hello");
private final JTextField textField = new JTextField();
public swapplet() {
textField.setBounds(42, 52, 119, 20);
getContentPane().setLayout(null);
btnNewButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
textField.setText("Hello Applet!");
}
});
btnNewButton.setBounds(220, 51, 114, 23);
getContentPane().add(btnNewButton);
getContentPane().add(textField);
}
}
PART 25
Other Network Applications
25.1 Sending E-mail
To send an email using your Java Application is simple enough but to start with you
should have JavaMail API installed on your machine.
You can download latest version of JavaMail from Java's standard website.
http://www.oracle.com/technetwork/java/javamail/index-138643.html
Download and unzip files, you will find a number of jar files for both the applications.
You need to add mail.jar files in your CLASSPATH (In Eclipse drag and drop into src
folder, right click mail.jar and select Build Path-> Add to Build Path)
Here is an example to send a simple email via GOOGLE GMAIL SMTP SERVER.
Here it is assumed that your localhost is connected to the internet and capable
enough to send an email.
import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;
public class SendEmail {
public static void main(String[] args) {
// Sender's email ID needs to be mentioned
String from = "[email protected]";
String pass = "123456";
// Recipient's email ID needs to be mentioned.
String to = "[email protected]";
String host = "smtp.gmail.com";
// Get system properties
Properties properties = System.getProperties();
// Setup mail server
properties.put("mail.smtp.starttls.enable", "true");
properties.put("mail.smtp.host", host);
properties.put("mail.smtp.user", from);
properties.put("mail.smtp.password", pass);
properties.put("mail.smtp.port", "587");
properties.put("mail.smtp.auth", "true");
// Get the default Session object.
Session session = Session.getDefaultInstance(properties);
try {
// Create a default MimeMessage object.
MimeMessage message = new MimeMessage(session);
// Set From: header field of the header.
message.setFrom(new InternetAddress(from));
// Set To: header field of the header.
message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));
// Set Subject: header field
message.setSubject("This is the Subject Line!");
// Now set the actual message
message.setText("This is actual message");
// Send message
Transport transport = session.getTransport("smtp");
transport.connect(host, from, pass);
transport.sendMessage(message, message.getAllRecipients());
transport.close();
System.out.println("Sent message successfully....");
} catch (MessagingException mex) {
mex.printStackTrace();
}
}
}
