Java Applet

A Simple Applet
1 RAJESHREE KHANDE

Applets
2
 Applet are small program use in Internet Computing
 Applet are run using Applet Viewer or Web Browser that support Java.
 Applet can perform arithmetic operations, display graphics play sound,
accept user input, create animations and play interactive games.
 Applet do not use the main() method for initiating the execution of the
code.
 When loaded , automatically calls certain method of applet to start &
execute the applet code.
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Applets
3
 Applets are run from inside a Web Page using a
special features know as applet tag
 Applet can not read from or write to a file form local
computer
 Applet are restricted from using libraries. From other
language like C & C++ .
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Applet Class
4
 Applet code uses services of two classes namely Applet &
Graphics
 The Applet class provide life and behavior to applet
through it’s method such as
- init()
- start()
- paint()
- stop()
- destroy()
 Applet class extends the AWT class Panel.
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Applet Class
5
 When loaded , automatically calls series of Applet class
methods for starting , running & stopping the applet code.
 Therefore maintains a lifecycle of an applet.
 The paint() method of Applet class when called, actually
display the result of the applet code on the screen.
 The paint() method requires a graphics object as an
argument.
syntax : public void paint(Graphics g)
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Skeleton of Applet
6
import java.awt.*;
import java.applet.*;
……………
……………
public class appletclassName extends Applet
{
public void init()
{
…………….
…………….
}
public void paint(Graphics g)
{
…………….
…………….
}
} RAJESHREE KHANDE

Graphic class
7
 public void paint(Graphics g) { … }
 public says that anyone can use this method
 void says that it does not return a result
 A Graphics (short for “Graphics context”) is an object
that holds information about a painting
 It remembers what color you are using
 It remembers what font you are using
 You can “paint” on it (but it doesn’t remember what
you have painted)
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Graphic class
8
 The java.awt package defines a class named Color
 There are 13 predefined colors—here are their fully-
qualified names:
 For compatibility with older programs (before the naming
conventions were established), Java also allows color
names in lowercase: Color.black, Color.darkGray, etc.
Color.BLACK Color.PINK Color.GREEN
Color.DARK_GRAY Color.RED Color.CYAN
Color.GRAY Color.ORANGE Color.BLUE
Color.LIGHT_GRAY Color.YELLOW
Color.WHITE Color.MAGENTA
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New Color
9
 Every color is a mix of red, green, and blue
 You can make your own colors:
new Color( red , green , blue )
 Amounts range from 0 to 255
 Black is (0, 0, 0), white is (255, 255, 255)
 We are mixing lights, not pigments
 Yellow is red + green, or (255, 255, 0)
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Step to develop an Applet
10
1. Building an applet code ( .java file)
2. Creating an executable applet ( .class file)
3. Designing a Web page using HTML tag
4. Prepare an <APPLET> tag
5. Incorporating an <APPLET> tag into the Web
page
6. Creating HTML file.
7. Testing an applet code.
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Example
11
/* applet code i.e. HelloApplet.java
*/
import java.awt.*;
import java.applet.*;
public void HelloApplet extend applet
{
{
g.drawString(“Hello applet”,10,10);
}
}
/* HTML file */
<HTML>
<HEAD>
<TITLE>
Welcome to Java Applet
</TITLE>
</HEAD>
<BODY>
<CENETR>
<H1> Welcome to 1st applet </H1>
</CENTER>
<BR> <CENETR>
<APPLET CODE =HelloApplet.class
WIDTH =400 HEIGHT =200>
</APPLET>
</CENTER>
</BODY>
</HTML>
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Running the Applet
12
 We must have the following file in the current
directory
1. HelloApplet.java
2. HelloApplet.class
3. HelloApplet.html
 To run an applet we require the one of the
following
1 Java enable Web browser
If we use this we will be able to see the entire Web Page
containing the applet
2 Java appletviewer
if we use appletviewer we will only see the applet
output.
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Applet Life Cycle
13
 Every java applet inherits a set of default behaviors
from Applet Class.
 When an applet is loaded, it undergoes a series of
changes in it’s state.
 The applet state includes
1. Born or Initialization state
2. Running State
3. Idle State
4. Dead or Destroyed state
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Applet Life Cycle
( An Applet State transition Diagram )
14
Begin
Born(Load Applet)
Initialization
Running Idle
DeadDestroy
Exit of Browser
End
destroy( )
start( )
paint( )
Display
stop( )
start( )
Stopped
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Applet Life Cycle
15
1. Initialization state
- Enters in initialization state when it’s first loaded.
- Achieved by calling init( ) method of an Applet class.
- The applet is born (creation of object needed by applet,
setting initial values, set up colors, load images and fonts.
- Initialization occurs only once in the applet life cycle
- Therefore override the init() method
public void init()
{
--------
--------
--------
} Action
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Applet Life Cycle
16
2. Running state
- Enter in the running state when system call start( ) method
of an Applet class.
- This occurs automatically when applet is initialized
- start() method may be called more than once.
- We may override the start( ) method to create a thread
to control the applet
public void start()
{
--------
--------
--------
}
Action
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Applet Life Cycle
17
3. Idle or Stopped state
- When applet stopped running it becomes idle
- Stopping occurs when we leave the page containing the
currently
running applet.
- We can also do so by calling stop() method explicitly.
- If we use thread to run an applet then we must use stop()
method
to terminate the thread.
- We can achieve this by overriding the stop() method.
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Applet Life Cycle
18
public void stop()
{
--------
--------
--------
}
Action
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Applet Life Cycle
19
3. Dead state
- An applet is said to be dead when it is removed from memory.
- This occurs automatically by invoking destroy() method when we
quit the browser.
- It occurs only once in applet’s life cycle.
- If applet has created any resource like thread, we may override
destroy() method to clean up the resource.
public void destroy()
{
--------
--------
--------
}
Action
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Applet Life Cycle
20
Display state
- Applet moves to display state whenever some output
operation has to perform on the screen.
- It occurs after the applet enters into running state.
- The paint( ) method is called to do this task.
- Every applet will have a paint( ) method.
- Override paint( ) method whenever we want to display
anything on the screen.
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Applet Life Cycle
21
{
--------
--------
--------
}
Display Statement
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Difference between Applet & Application
Applet Aplication Program
An applet can be embedded in HTML Web
page & easily downloaded over Internet
An application program can not be
embedded in HTML Web page.
An applet runs inside java compatible
container such as internet explorer or
netscape navigator.
Runs on command prompt using java
interpreter , java.exe
An applet executes under strict security
limitation that reject some opreation such as
accessing a file or downloading a file
Has no security restriction.
It is java program that is written for
distribution over a network. i.e. applet is not
standalone program
It is standalone program
It does not require main() method for it’s
execution.
Need main() method for it’s execution.
22

Designing a Web Page
23
<HTML>
</HTML>
<!
………
>
<HEAD>
</HEAD>
Title Tag
<BODY>
</BODY>
<APPLET> Applet Tag
</APPLET>
Comment Section
Head Section
Body Section
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Applet Tag
24
<APPLET
[ CODEBASE = codebase_URL]
CODE =AppletFileName.class
[ALT = alternate text]
[NAME = applet_instance_name]
WIDTH = pixel
HEIGHT =pixel
[ALIGNMENT = alignment]
[VSPACE = pixel ]
[HSPACE = pixel ]
>
[<PARAM NAME =name1 VALUE =value1 >]
[<PARAM NAME =name2 VALUE =value2 >]
………………………………………….
…………………………………………
</APPLET>
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Attribute of Applet Tag
25
1. CODE : =AppletFileName.class
Specifies a name of the applet class to be loaded
i.e the name of already compiled file
2. CODEBASE=codebase_URL(optional)
Specifies the URL of the directory in which the applet
resides. If applet resides in the same directory as HTML
file, then this attribute is ommitted.
3. WIDTH=pixel HEIGHT=pixel
This specifies width & height of the space on HTML
page.
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26
4. NAME =applet_instance_name (optional)
A name for the applet , so that other applet on the
page may refer to this applet. Used for inter applet
communication.
5. ALIGN =alignment (optional)
This specifies where on the page the applet will
appear. Values are : TOP,BOTTOM,LEFT,RIGHT
MIDDLE,ABSMIDDLE, ABSBOTTOM, TEXTTOP and
BASELINE
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27
6. HSPACE=pixel (optional)
Used only when align is set to LEFT or RIGHT.
Specifies horizontal blank spaces the browser should
leave surrounding the applet.
7. VSPACE=pixel (optional)
Used only when align is set to TOP or BOTTOM.
Specifies vertical blank spaces the browser should
leave surrounding the applet.
8. ALT=alternate_text (optinal)
Non-Java browser will display this text where the
applet would normally go.RAJESHREE KHANDE

Applet Class
28
 Applet extends the awt class Panel .
 Panel extends Container which
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The java.awt package
29
 The java.awt package includes classes for:
 Drawing lines and shapes
 Drawing letters
 Setting colors
 Choosing fonts
 If it’s drawn on the screen, then java.awt is probably
involved!
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The applet so far
30
import java.applet.Applet;
import java.awt.*;
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Your applet class
31
public class Drawing extends Applet {
… }
 Drawing is the name of your class
 Class names should always be capitalized
 extends Applet says that our Drawing is a kind of
Applet, but with added capabilities
 Java’s Applet just makes an empty window
 We are going to draw in that window
 The only way to make an applet is to extend Applet
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The applet so far
32
import java.awt.*;
// CIT 591 example
…we still need to put some code in here...
}
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The paint method
33
 Our applet is going to have a method to paint some
colored rectangles on the screen
 This method must be named paint
 paint needs to be told where on the screen it can draw
 This will be the only parameter it needs
 paint doesn’t return any result
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The applet so far
34
import java.awt.*;
// CIT 591 example
public void paint(Graphics g) {
…we still need to put some code in here…
}
}
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New colors
35
 Every color is a mix of red, green, and blue
 You can make your own colors:
new Color( red , green , blue )
 Amounts range from 0 to 255
 Black is (0, 0, 0), white is (255, 255, 255)
 We are mixing lights, not pigments
 Yellow is red + green, or (255, 255, 0)
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Setting a color
36
 To use a color, we tell our Graphics g what color we want:
g.setColor(Color.RED);
 g will remember this color and use it for everything until
we tell it some different color
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The paint method so far
37
g.setColor(Color.BLUE);
…draw a rectangle…
…draw another rectangle…
}
}
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Pixels
38
 A pixel is a picture (pix) element
 one pixel is one dot on your screen
 there are typically 72 to 90 pixels per inch
 java.awt measures everything in pixels
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Java’s coordinate system
39
 Java uses an (x, y) coordinate system
 (0, 0) is the top left corner
 (50, 0) is 50 pixels to the right of (0, 0)
 (0, 20) is 20 pixels down from (0, 0)
 (w - 1, h - 1) is just inside the bottom right corner, where
w is the width of the window and h is its height
(0, 0)
(0, 20)
(50, 0)
(50, 20)
(w-1, h-1)
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Drawing rectangles
40
 There are two ways to draw rectangles:
 g.drawRect( left , top , width , height );
 g.fillRect(left , top , width , height );
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The complete applet
41
import java.awt.*;
// CIT 591 example
g.setColor(Color.BLUE);
g.fillRect(20, 20, 50, 30);
g.fillRect(50, 30, 50, 30);
}
} RAJESHREE KHANDE

Some more java.awt methods
42
 g.drawLine( x1 , y1 , x2 , y2 );
 g.drawOval( top, left , width , height );
 g.fillOval( top, left , width , height );
 g.drawRoundRect( top, left , width , height,xDiam,yDiam);
 g.fillRoundRect( top, left , width , height, xDiam, yDiam );
 g.drawArc( top, left, width , height ,startAngle, sweepAngle
);
 g.fillArc( top, left, width , height ,startAngle, sweepAngle );
 g.drawString( string , x , y );
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The HTML page
43
 You can only run an applet in an HTML page
 The HTML looks something like this:
 <html>
<body>
<h1>DrawingApplet Applet</h1>
<applet code="DrawingApplet.class"
width="250" height="200">
</applet>
</body>
</html>
 BlueJ will create this HTML for you
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44
 getDocumentBase()
 getCodeBase()
Example
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Working with Font
45
 AWT support multiple type of Fonts
 Font have a
1. Family name : A general name of the font such as
Courier
2. a logical font name : Specifies category of font, such
as Monospaced.
3. a face name : Specifies a specific font such a
Courier Italic
 Font have encapsulated by Font class
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Methods of Font Class
46
 String getFontName()
returns the face name of the invoking font
 String getName():
returns the logical name of the invoking font
 int getSize():
returns the size, in points of the invoking font
 int getStyle():
returns the Style value of the invoking font
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47
 boolean isBold():
returns true if the font includes BOLD style value
o.w. returns false
 boolean isItalic():
returns true if the font includes ITALIC style value
o.w. returns false
 boolean isPlain()
returns true if the font includes PLAIN style value
o.w. returns false
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48
 String toString() :
Returns the string equivalent of the invoking font
 Font class defines following variables
1. String name : name of the font
2. float pointSize : Size of the invoking font
3. int size : Size of the font in points
4. int style : Font style
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Determining available font
49
 To know which fonts are a available on your
machine we can use the
String[] getAvailableFomtFamilyNames()
defined by GraphicsEnvironment class
 To returns an array of Font object
Font[] getAllFont()
 Since these method belong to GraphicsEnvironment
we need GraphicsEnvironment reference to call
them
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Determining available font
50
 We can obtain the referenced them by
getLocalGraphicsEnvironment()
 This is static method
GraphicsEnvironment ge =
GraphicsEnvironment.getLocalGraphicsEnvironment()
 Example

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Creating and Selecting Font
51
 To select new font first construct Font object
Font(String fontName, int fontStyle,
intfontSize)
logical /face Name
 To use the created font
setFont(Font obj)
 Example
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Event Model
52
Java.lang.Object
Java.util.EventObject
Java.awt.AWTEvent
ActionEvent
(Click button
/Menu Option)
AdujustmentEvent
(Moves Scrollbar)
ComponentEvent
(When awt component
are shown ,Hidden
moved /resized)
ItemEvent
(When list item, radio
Button, check box choice
menu option selected)
TextEvent
(when Text changes
for an object
ContainerEvent FocusEvent InputEvent PaintEvent WindowEvent
KeyEvent MouseEvent
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Java Event handling
53
 The delegation event model :
The modern approach to handle the event is based on
delegation event model, which defines standard and
consistent mechanism to generate the event.
1. A source generate an event & sent it to one or more listener,
in this listener wait until it receives an event.
2. Once received the listener process the event & return.
3. The advantages of this design is that the application logic is
separated from the user interface logic that generates an
event
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Java Event handling
54
4. A user element is able to delegates the processing of
an event to a separate piece of code.
5. In the delegation event model , listener must register
with a source in order to receive an event notification.
6. This provide a benefit that notification are sent only
to the listener that want to receive them.
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Event Delegation Model
55
 Event :
In event Delegation model an event is an Object
that describe state change in source
 Event Source:
1. A source is an object that generates an event. Source
may
generate more than one type of event.
2. A source must register listener in order for the
listener to
receive notification about a specific type of event.
3. Each type of Event has its own registration method
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56
format :
public void addTypeListener(TypeListener el)
Here
Type : name of the Event and
el : reference to event listener
 Event Listener :
A listener is an object that is notified when event is occur. It has two
major requirement
1. must have been registered with one or more source to receive
notification about the specific type of event.
2. Must Implement method to receive & process these notification
Event Delegation Model
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Event Classes : EventObject
57
 At the root of Java Event class hierarchy is
EventObject , Which is in java.util.
 EventObject contain two method
1. getSource() :returns the source of event
Object getSource()
2. toString(): returns the string equivalent of the
event
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Event Classes : AWTEvent
58
 Defined in java.awt package & is a Subclass of
EventObject
 It is super class of all AWT-based event used by
event delegation model
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Main Event classes in java.awt.event
Event Class Description
ActionEvent Generates when button is pressed, a list item is double-
Clicked or menu item is selected
AdjustmentEvent Generstes when Scrollbar is manipulated
ComponentEvent When component is hidden, moved,resized,or becomes
visible
ContainerEvent When container is added or removed from container
FocusEvent When component gain or losses focus
ItemEvent Enerates when Checkbox or list item is clicked also occur
when choice selection is made / checkable menu item is
selected or deselected
59

Main Event classes in java.awt.event
KeyEvent Generates when input is received from keyboard
MouseEvent Generates when mouse is dragged, moved clicked
pressed/released. Also when mouse enters or exits a
component.
MouseWheelEvent Generates When mouse wheel is moved
TextEvent Generates When the value of text area or text field is
changed.
WindowEvent Generates When window is activated .closed, deactivated,
deiconified, iconified, opened or quit
60

Event Listener Interfaces
Interface Description
ActionListener Define one method to receive action event
void actionPerformed(ActionEvent ae)
AdjustmentListener Define one method to receive adjustment event
void adjustmentValueChange(AdjustmentEvent ae)
ComponentListener Define four method to recognize when component is hidden
, moved, resized or shown
void componentResized(ComponentEvent ce)
void componentMoved(ComponentEvent ce)
void componentShown(ComponentEvent ce)
void componentHidden(ComponentEvent ce)
61

Event Listener Interfaces
ContainerListener Define two method to recognize when component is
added or removed from container
void componentAdded(ContainerEvent ce)
void componentRemoved(ContainerEvent ce)
FocusListener Define two method to recognize when component gain
or losses keyboard focus
void focusGained(FocusEvent fe)
void focusLost(FocusEvent fe)
ItemListener Define one method to recognize when the state of item is
changed
void itemStateChanged(ItemEvent ie)
62

Eevnt Listener Interfaces
KeyListener Define three method to recognize when key is pressed,
released or typed
void keyPressed(KeyEvent ke)
void keyReleased(KeyEvent ke)
void keyTyped(KeyEvent ke)
MouseListener Define five method to recognize when mouse is clicked,
enters a component, exit a component , is pressed or released.
void mouseClicked(MouseEvent me)
void mouseEntered(MouseEvent me)
void mouseExited(MouseEvent me)
void mousePressed(MouseEvent me)
void mouseReleased(MouseEvent me)
63

MouseMotionListener Define two method to recognize when mouse is
dragged or moved.
void mouseDragged(MouseEvent me)
void mouseMoved(MouseEvent me)
MouseWheelListener Define one method to recognize when mouse wheel is
moved
void mouseWheelMoved(MouseWheelEvent
mwe)
TextListener One method when text value is changed
void textChanged(TextEvent te)
64

Eevnt Listener Interfaces
WindowFocusListener Defines Two method to recognize a window
gain or loses input focus
void windowGainedFocus(WindowEvent we)
void windowLostFocus(WindowEvent we)
WindowListener Seven methods to recognize when window
is activated closed, deactivated, deiconified,
iconified, opened or quit
void windowActivated(WindowEvent we)
void windowClosed(WindowEvent we)
void windowClosing (WindowEvent we)
void windowDeactivated(WindowEvent we)
void windowIconified(WindowEvent we)
void windowDeiconified(WindowEvent we)
void windowOpened(WindowEvent we) 65

Adapter Classes
66
1. Simplifies the creation of event handler.
2. These are useful when you want to receive and process only
some of the event that are handled by a particular event listener
interface
3. You can define a new class to act as an event listener by
extending one of the adapter classes and implementing only
those event in which you are interested.
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Adapter Classes
67
 The Adapter classes
are
1. ComponentAdap
ter
2. ContainerAdapte
r
3. FocusAdapter
4. KeyAdapter
5. MouseAdapter
6. MouseMotionAdapte
r
7. WindowAdapter
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WindowEvent Class
68
 The class has 7 type of
integer constant
1. WINDOW_ACTIVATED
2. WINDOW_CLOSED
3. WINDOW_CLOSING
4. WINDOW_DEACTIVATED
5. WINDOW_DEICONIFIED
6. WINDOW_ICONIFIED
7. WINDOW_OPENED
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WindowEvent Class- Methods
69
1. public void windowActivated(WindowEvent we)
2. public void windowClosed(WindowEvent we)
3. public void windowClosing(WindowEvent we)
4. public void windowDeactivated(WindowEvent we)
5. public void windowDeiconified(WindowEvent we)
6. public void windowOpened(WindowEvent we)
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Java Applet

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