1. APPLETS

2. What are the differences between Applets and Application?
What is Applets?
An applet is a program written in the Java programming language that can be included in an HTML page, much in the same way an image is included in a page.
When you use a Java technology-enabled browser to view a page that contains an applet, the applet's code is transferred to your system and executed by the browser's Java Virtual Machine (JVM).
What are the differences between Applets and Application?
Applets do not use the main() method for initiating the execution of the code. Applets, when loaded, automatically call certain methods of Applet class to start and execute the applet code.
Unlike stand – alone applications, applets cannot be run independently. They are run from inside a Web page using a special feature known as HTML tag.
Applets cannot read from or write to the files in the local computer.
Applets cannot communicate with other servers on the network.
Applets cannot run any program from the local computer.
Applets are restricted from using libraries from other languages such as C or C++. (Java language supports this feature through native methods.)

3. Applet Architecture An applet is a window – based program.
First, applets are event driven.
Event – driven architecture impacts the design of an applet.
An applet resembles a set of interrupt service routines.
An applet waits until an event occurs.
The AWT notifies the applet about an event by calling an event handler appropriate action and then quickly return control to the AWT.
Second, the user initiates interaction with an applet - not the other way around.
As you know, in a non - windowed program, when the program needs input, it will prompt the user and then call some input method, such as readLine().
This not the way it works in an applet. Instead, the user interacts with the applet as he or she wants, when must respond.
For example, when the user clicks a mouse inside the applet’s window, a mouse – clicked event is generated. If the user presses a key while the applet’s window has input focus, a keypress event is generated.
Applets can contain various controls, such as push buttons and check boxes.

4. Applet Architecture
12. When the user interacts with one of these controls, an event is generated.
13. While the architecture of an applet is not as easy to understand as that of a console – based program, Java’s AWT makes it as simple as possible.
14. If you have written programs for windows, you know how intimidating that environment can be.
15. Fortunately, Java’s AWT provides a much cleaner approach that is more quickly mastered.

5. An Applet Skeleton or Applet Life Cycle
Born
Begin
(Load Applet)
Initialization
start( )
Running
stop( )
Idle
Stopped
start( )
Display
paint( )
destroy( )
Dead
End
Destroyed
Exit of Browser

6. An Applet Skeleton or Applet Life Cycle
All but the the most trivial applets override a set of methods that provides the basic mechanism by which the browser or applet viewer interfaces to the applet and controls its execution.
Four of these methods – init( ), start( ), stop( ), and destroy( ) – are defined by Applet. Another, paint(), is defined by the AWT Component class.
Default implementations for all of these methods are provided.
Applets do no need to override those methods they do not use.
However, only very simple applets will not need to define all of them.
The applet states is:
1. Born or initialization state
2. Running state
3. Idle state
4. Dead or destroyed state
5. Display state

7. Initialization State
Applet enters the initialisation state when it is first loaded. This is achieved by calling the init() method of Applet Class. The applet is born. At this stage, we may do the following, if required,
a. Create objects needed by the applet.
b. Set up initial values
c. Load images or fonts
d. set up colors
public void init( ) { }
Action

8. 2. Running State
Applets enters the running state when the system call the start () method of Applet Class. This occurs automatically after the applet is initailized. Starting can also occur if the applet is already in “stopped” state. For example, we may leave the web page containing the applet temporarily to another page and return back to the page. This again starts the applet running. Note that, unlike init() method, the start() method may be called more than once. We may override the start() method to create a threa to control the applet.
public void start( ) { }
Action

9. 3. Idle or Stopped State
An applet becomes idle when it is stopped from running. Stopping occurs automatically when we leave the page containing the currently running applet. We can also do so by calling the stop() method explicitly. If we use a thread to run the applet, then we must use stop() method to terminate the thread. We can achieve by overriding the stop( ) method:
public void stop( ) { }
Action

10. 4. Dead State
An applet is said to be dead when it is removed from memory. This occurs automatically by invoking the destroy( ) method when we quit the browser. Like initialization, destroying stage occurs only once in the applet’s life cycle. If the applet has created any resources. Like threads we may override the destroy( ) method to clean up these resouces.
public void destroy( ) { }
Action

11. 5. Display State
Applet moves to the display state whenever it has to perform some output operations on the screen. This happens immediately after the applet enters into the running state. The paint() method is called to accomplish this task. Almost every applet will have a paint() method. Like other methods in the life cycle, the default version of paint() method does absolutely nothing. We must therefore override this method if we want anything to be displayed on the screen.
public void paint (Graphics g) { }
Display Statements

12. Specified Colors Specified Colors Color.black 10. Color.pink
import java.awt.*;
import java.applet.*; /*
<applet code = "AppletDemo" width = 300 height = 300>
</applet> */
public class AppletDemo extends Applet {
String msg;
public void init()
setBackground(Color.cyan);
setBackground(Color.red);
setForeground(Color.blue); }
public void start()
msg = "This is Start method";
public void stop()
msg = "This is Stop method";
public void destroy()
msg = "This is Destroy method";
public void paint(Graphics g)
g.drawString(msg,100,100);
Specified Colors
Color.black
Color.blue
Color.cyan
Color.darkGray
Color.gray
Color.green
Color.lightGray
Color.magenta
Color.orange
Specified Colors
10. Color.pink
11. Color.red
12. Color.white
13. Color.yellow

13. How the applets to be displayed in the Browser
<HTML>
<HEAD>
<TITLE>WELCOME TO cHETTINAD</TITLE>
</HEAD>
<BODY>
<APPLET
CODE = AppletDemo.class
WIDTH = 300
HEIGHT = 200
</APPLET>
</BODY>
</HTML>

14. The HTML APPLET Tag < APPLET [CODEBASE = codebaseURL]
CODE = appletFile
[ALT = alternateText]
[NAME = appletInstanceName]
WIDTH = pixels
HEIGHT = pixels
[ALIGN = alignment]
[VSPACE = pixels]
[HSPACE = pixels]
>
[<PARAM NAME = AttributeName VALUE = AttributeValue>]
[HTML Displayed in the absence of Java]
</APPLET>

15. ATTRIBUTE MEANING CODE = AppletFileName.class
Specifies the name of the applet class to be loaded. That is, the name of the already – compiled .class file in which the executable. Java bytecode for the applet is stored. This attribute must be specified.
CODEBASE = codebase_URL
(Optional)
Specifies the URL of the directory in which the applet resides. If the applet resides in the same directory as the HTML file, then the CODEBASE attirbute may be omitted entirely.
WIDTH = pixels
HEIGHT = pixels
These attributes specify the width and height of the space on the HTML page that will be reserved for the applet.
NAME=applet_instance_name
A name for the applet may optionally be specified so that other applets on the page may refer to this applet. This facilitates inter applet communication.
ALIGN = alignment
This optional attribute specifies where on the page the applet will appear. Possible values for alignment are TOP, BOTTOM, LEFT, RIGHT, MIDDLE, ABSMIDDLE, ABSBOTTOM, TEXTTOP, AND BASELINE.
HSPACE = pixels
Used only when ALIGN is set to LEFT or RIGHT , this attribute specifies the amount of horizontal blank space the browser should leave surrounding the applet.
VSPACE = pixels
Used only when some vertical alignment is specified with the ALIGN attribute (TOP, BOTTOM, etc.,) VSPACE specifes the amount of vertical blank space the browser should leave surrounding the applet.
ALT = alternate_text
Non – java browser will display this text where the applet would normally go. This attribute is optional.
PARAM NAME AND VALUE
The PARAM tag allows you to specify applet – specific arguments in an HTML page. Applets access their attributes with the getParameter() method

16. Passing Parameters to Applets
import java.awt.*;
import java.applet.*; /*
<applet code="ParamDemo" width = 300 height = 300>
<param name = fontName value=Courier>
<param name = fontSize value = 30>
<param name = leading value=2>
<param name = accountEnabled value = true>
</applet> */
public class ParamDemo extends Applet {
String fontName;
int fontSize;
float leading;
boolean active;
public void start()
String param;
fontName = getParameter("fontName");
if(fontName == null)
fontName = "Not Found";

17. param = getParameter("fontSize");
try {
if(param != null) //if not found
fontSize = Integer.parseInt(param);
else
fontSize = 0; }
catch(NumberFormatException e)
fontSize = -1;
param = getParameter("leading");
if(param != null) //if not found
leading = Float.valueOf(param).floatValue();
leading = 0;
leading = -1;

18. param = getParameter("accountEnabled");
if(param != null)
active = Boolean.valueOf(param).booleanValue(); }
public void paint(Graphics g) {
g.drawString("Font name: " + fontName,0,10);
g.drawString("Font Size: " + fontSize,0,20);
g.drawString("Leading: " + leading, 0,42);
g.drawString("Account Active: " + active,0,58);

19. getCodeBase() and getDocumentBase()
import java.awt.*;
import java.applet.*;
import java.net.*; /*
<applet code=ParamDemo_2.class width = 300 height = 300>
</applet> */
public class ParamDemo_2 extends Applet {
public void paint(Graphics g)
String msg;
URL url = getCodeBase();
msg = "Code Base: " + url.toString();
g.drawString(msg,10,20);
url = getDocumentBase();
msg = "Document Base: " + url.toString();
g.drawString(msg,10,40); }