APPLETS

We’ll look at java applets in this blog, which are utilized in interactive online applications.
One sort of Java program is a console-based application, and another type of Java program is an applet. An applet is a type of Java program that runs in a browser that supports Java. On the client side, the JVM runs the applet. Applets are online apps that are interactive.

Syntax for Applet tag:

<APPLET [CODEBASE = codebaseURL] CODE= appletfile [ALT=alternate 
text] [NAME=applet-instance-name] 
 WIDTH =pixels HEIGHT= pixels [ALIGN=alignment] 
[VSPACE=pixels] [HSPACE= pixels]>
[<PARAM NAME=attr-name VALUE=attr-value>]
[<PARAM NAME=attr-name VALUE=attr-value>]
...
</APPLET>

CODEBASE: Its not compulsory or mandatory attribute that is used to specify the base URL of Applet code, which is the applet executable class file.

CODE: This attribute gives the name of the file which is having the applet complied class file.

ALT: It is also an optional attribute which is used to give a short messages that can displayed in the browser when it can understand its a applet tag but cannot run it.

NAME: This attribute is used to specify name for an applet instance.

WIDTH & HEIGHT: Used to specify the size of applet display area.

ALIGN: specifies the alignment of the applet(position of the applet).

VSPACE & HSPACE: VSPACE(Vertical Spaces) specifies the space in pixels, above and below the applet. HSPACE(Horizontal Spaces) specifies the space in pixels, on each side of the applet.

PARAM NAME AND VALUE: this tag allows you to specify applet specific arguments in the HTML page

Applets are of 2 types

1. AWT Applets
2. Swing Applets

Sample Applet Program:

import java.awt.*;
import java.applet.*;
/*<applet code="simpleApplet" width=250 height=200
*</applet>
*/
public class simpleApplet extends Applet
{
public void paint(Graphics g)
{
g.drawString("Hello World", 50, 50);
}
}

Output:

eclipse output done by author

Note: Applets will not have main() method and they must be under applet viewer or a java enabled web browser.

Life cycle of an Applet:

1. Applet is initialized
2. Applet is started
3. Applet is Painted
4. Applet is stopped
5. Applet is destroyed

Methods in Applet life cycle are

java.applet.Applet class:

1. public void init(): used to initialize the applet and is invoked only once.
2. public void start(): used to start an applet and is invoked after the init method.
3. public void stop(): used to stop the applet and is invoked when applet is stopped or browser is minimized.
4. public void destroy(): used to destroy the applet and is invoked only once.

java.awt.Component class:

1. public void paint(Graphic g): used to paint the applet and provides graphic class objects can be used for drawing some shapes like rectangle, oval..

Sample Code by using all methods:

import java.awt.*;
import java.applet.*;
/*
<applet code="SampleCode" width=300 height=100>
</applet>
*/
public class SampleCode extends Applet {
    public void init() {
    // initialization
    }
    public void start() {
    // start or resume execution
    }
    public void stop() {
    // suspends execution
    }
    public void destroy() {
    // perform shutdown activities
    }
    public void paint(Graphics g) {
    // redisplay contents of window
    } 
}

Advantages of Applets:

• It is secure and takes less time to execute.
• Used to create dynamic web pages.
• Provides interactive features to web pages.
• Writing and displaying with the opening of browser, graphics and animations is easier than applications.

Disadvantages/ Restrictions of Applets:

• It can’t read some system properties.
• Some Browsers like Google Chrome, Safari, Opera will not support Java Applets.
• Applets will not access client side data like files, OS.

AWT Applets:

• AWT supports window based, graphical interface.
• AWT class can be accessed from java.awt package it is one of the largest package in java packages.
• Some AWT Class are:
• Button (creates a push button control)
• Canvas( create a empty window)
• checkbox( create a check box control)
• Choice( creates a pop-up list)
• Dialog(creates a dialog window)
• Event (Encapsulates the event)
• Frame(creates a standard window that has a title bar, resize corners and a menu bar )
• Graphics( encapuslating the graphics context)
• Window( creates a window with no frame, no menu bar, and no title)
• Label ( creates a label that displays a string)
• GridLayout( it displays components in a 2-dimentional grid.)
• TextField( creates a single-line edit control)
• TextArea( creates a multi-line edit control)
• Panel( the simplest concrete subclass of container)

Example Program:

import java.awt.*;  
public class AwtDemo extends Frame {  
    AwtDemo (){  
        Label firstName = new Label("First Name");  
        firstName.setBounds(20, 50, 80, 20);  
        Label lastName = new Label("Last Name");  
        lastName.setBounds(20, 80, 80, 20);  
        Label dob = new Label("Date of Birth");  
        dob.setBounds(20, 110, 80, 20);  
        TextField firstNameTF = new TextField();  
        firstNameTF.setBounds(120, 50, 100, 20);  
        TextField lastNameTF = new TextField();  
        lastNameTF.setBounds(120, 80, 100, 20);  
        TextField dobTF = new TextField();  
        dobTF.setBounds(120, 110, 100, 20);  
        Button sbmt = new Button("Submit");  
        sbmt.setBounds(20, 160, 100, 30);  
        Button reset = new Button("Reset");  
        reset.setBounds(120,160,100,30);  
        add(firstName);  
        add(lastName);  
        add(dob);  
        add(firstNameTF);  
        add(lastNameTF);  
        add(dobTF);  
        add(sbmt);  
        add(reset);  
        setSize(300,300);  
        setLayout(null);  
        setVisible(true);  
    }  
    public static void main(String[] args) {  
    // TODO Auto-generated method stub  
        AwtDemo awt = new AwtDemo ();    
    }      
}

OUTPUT:

eclipse output done by author

SWING Applets:

• Swing is a set of classes that provide more flexible and powerful components and it contains some extra features like Scroll panes, trees, tables and tabbed panes.
• It is a part of Java Foundation classes (JFC), which is used to create window based applications.
• It is built on the top of AWT API and written in java.
• Swing class can be accessed from javax.swing.package.
• Some Swing Classes are:
• JButton
• JTextField
• JTextArea
• JRadioButton
• JCheckbox
• JMenu
• JColorChooser

Example Program:

import javax.swing.*; 
public class SwingDemo{ 
    public static void main(String[] args) { 
        JFrame f=new JFrame(); 
        JButton b=new JButton("clickable button");
        b.setBounds(100,100,150, 40);
        f.add(b);
        f.setSize(400,500);
        f.setLayout(null);
        f.setVisible(true);
    } 
}

OUTPUT:

eclipse output done by author

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