Session4 applets

Applets / 2 of 27
Objectives
 Differentiate between Java applications and Java
applets
 Explain how to create a program to work as both
applet and an application
 Identify how parameters are passed to applets
 Discuss Event handling with applets
 Describe the graphics class
 Discuss the Font class
 Examine the FontMetrics class
 Discuss the Color class
 Define an applet

Applets / 3 of 27
Introduction
 There are a number of ways to create interactive
programs on the Web but one of the most remarkable
ways is using a Java applet
 An applet is a Java program designed to work on the
Internet through a web browser
 An applet can be used for a variety of purposes right
from web-based communications to designing
graphical user interfaces for backend applications
 HTML pages by themselves are very passive

Applets / 4 of 27
Applets
 Created by subclassing the ‘java.applet.Applet’
class
 Examples of Java enabled web browsers are
Internet Explorer
 An ‘Applet’ is a Java program that can be
embedded in an HTML page and executed on
a Java enabled browser
and Netscape Communicator

Applets / 5 of 27
Difference between
Applets and Applications (1)
 Applets are created by extending the
java.applet.Applet class.
There is no such constraint for an application
 Applets run on any browser while
applications run using Java interpreter
 An applet is basically designed for deploying on
the web whereas an
application is designed to work as a standalone
program

Applets / 6 of 27
Difference between
Applets and Applications (2)
 Applet must contain at least one public class
failing which the compiler reports an error. There
it is not mandatory to declare main( ) for an
applet where as
In case of application, ‘main( )’ has to be included
in a public class
 Execution of applets begin with the init() method
while Execution of applications begins with
main() method

Applets / 7 of 27
Life cycle of an Applet (1)
 An applet defines its structure from four
events that take place during execution
 For each event, a method is automatically
called
 Life cycle of an object specifies stages the
object has to pass right from its creation until
it is destroyed

Applets / 8 of 27
 init() – called during initialization
 start() – starts the applet once it is initialized
 stop() – used to pause the execution of an applet
 destroy() – used to destroy the applet
 The method paint() is used to display a line, text
or an image on the screen
 Whenever an applet has to be painted again after
it has been drawn once, the repaint() method is
used.
 The methods are as follows:

Applets / 9 of 27
Redraw
Applet
stop( )
Start
state
start( ) paint( )
Applet
Working
Applet
Born
Applet
Displayed
Idle
State
Applet
Destroyed
Initialization
state
Ifstart()
calledagain
destroy( )
Destroy
Appletinit( )

Applets / 10 of 27
A simple applet
Output

Applets / 11 of 27
 Create a HTML page to display the applet
<html>
<appletcode=Firstapplet width=200 height=200>
</applet>
</html>
 Then type the following at command prompt:
 appletviewer abc.html //‘abc.html’- name of
the html file
Compiling and running
an applet
 An applet is compiled using the Java compiler:
javac
 javac Firstapplet.java

Applets / 12 of 27
Displaying images
using applets (1)
Output

Applets / 13 of 27
Displaying images
using applets (2)
 getCodeBase() method gets the base URL
 getImage() method returns an Image object
which can be drawn on the screen
 drawImage() takes four parameters – Image
object, location in terms of x and y
coordinates and an object of type
ImageObserver
 To display images, we need to make use of
the Image and Graphics classes

Applets / 14 of 27
Passing parameters
 Parameters are passed to the applet using
the <param> tag in the HTML file
 Parameter value is retrieved in the applet
using the getParameter() method which
returns a string
 By the help of parameters we can allow the
user to control certain factors

Applets / 16 of 27
Applets and GUI
 Default layout of an applet is FlowLayout
 The figure below depicts the various controls
that can be created
 Graphical User Interface is used to create a
pictorial interface that is easy to work with

Applets / 17 of 27
Handling events with applets
 While designing applets we need to trap these
events and provide suitable actions to be
performed in response to each of those events
 Mouse events in an applet can be handled by
overriding mouseDown(), mouseUp(),
mouseDrag() methods
 Besides the methods, we can also use Listener
interfaces
 Clicking or pressing the Enter key on GUI
components generates event

Applets / 18 of 27
Graphics class (1)
 Graphics class is a part of the java.awt
package
 It has to be imported into the program
 Apart from text, it is possible to draw images,
rectangles, lines, polygons and various other
graphical representations

Applets / 19 of 27
Graphics class (2)
Method Purpose
abstract void drawLine(int
x1, int y1, int x2, int y2)
Draws a line from starting point
specified by x1,y1 to ending point
x2,y2 in the graphic context’s
coordinate system
void drawRect( int topx, int
topy, int width, int height)
Draws a rectangle whose top left
coordinates, width and height are
given
abstract void fillRect (int
topx, int topy, int width, int
height)
Draws a solid rectangle whose top
left coordinates, width and height
are given

Applets / 20 of 27
Graphics class (3)
Method Purpose
abstract void drawOval(int
topx, int topy, int width,
int height)
Draws an oval whose top left coordinates,
width and height are given
abstract void fillOval(int
topx, int topy, int width, int
height)
Draws a solid oval whose top left
coordinates, width and height are given
abstract void drawArc(int
x, int y, int width, int
height, int startangle, int
degrees)
Draws an arc whose x, y coordinates are
given with given height and width and whose
start angle and degrees travelled are given

Applets / 21 of 27
Example (1)

Applets / 22 of 27
Example (2)
Output

Applets / 23 of 27
Font class
 One constructor of the Font is:
 public Font(String name, int style, int pointsize)

name can be “Times New Roman”, “Arial”, etc.

style can be Font.PLAIN, Font.BOLD, Font.ITALIC

pointsize for fonts can be 11,12,14,16,etc.
 java.awt.Font class is used to set or retrieve
fonts

Applets / 24 of 27
Example
Output

Applets / 25 of 27
FontMetrics class
 In such a case, the FontMetrics class proves
useful
 Commonly used methods of FontMetrics class
 int stringWidth(String s) – returns full width of string
 int charWidth(char c) – returns width of that
character
 int getHeight() – returns total height of the font
 At times, it is necessary to know the attributes
of fonts used within a program

Applets / 26 of 27
Example
Output

Applets / 27 of 27
Color class
 Colors can be constructed as shown :
 Color a=new Color(255,255,0);
 Color b=new Color(0.907F,2F,0F);
 To change or set colors for a component :
 void setColor(Color) of Graphics class
 void setForeground(Color) of Component class ,inherited
by various components
 void setBackground(Color) of Component class ,inherited
by various components
 java.awt.Color class is used to add color to
applications and applets

Session4 applets

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Session4 applets