Chapter 3: Introduction to Applets

1
Xavier University Computer Center Sun
Microsystems, Philippines
Java sa Eskwela (JsE) Program
2
Chapter 3
Introduction to Applets
3
Chapter 3

• Outline
• A Simple Java Applet Drawing a String
• Two More Simple Applets Drawing Strings and
  Lines
• Another Java Applet Adding Integers

4
Objectives

• To understand the difference between an applet
  and an application
• To be able to write simple Java applets
• To be able to write simple HyperText Markup
  Language (HTML) files to load an applet into the
  appletviewer or a web browser

5
Applets

• Program that runs in
• appletviewer (test utility for applets)
• Web browser (IE, Communicator)
• Executes when HTML (Hypertext Markup Language)
  document containing applet is opened
• Applications run in command windows

6
Applets

• Sample Applets

7
Applets

• General Process
• an instance of the applet's controlling class (an
  Applet subclass) is created
• the applet initializes itself
• the applet starts running
• the applet stops running
• the applet does final clean-up then unloads

8
Applets

• General Structure
• public class Simple extends JApplet
• public void init() ...
• public void start() ...
• public void stop() ...
• public void destroy() ...

9
Applets

• init()
• to initialize the applet each time it's loaded
  (or reloaded)
• start()
• to start the applet's execution, such as when the
  applet's loaded or when the user revisits a page
  that contains the applet

10
Applets

• stop()
• to stop the applet's execution, such as when the
  user leaves the applet's page or quits the
  browser
• destroy()
• to perform a final cleanup in preparation for
  unloading

11
Applets

• paint()
• basic display method
• many applets implement the paint method to draw
  the applet's representation within a browser page
• update()
• used along with paint to improve drawing
  performance

12
Applets

• Program Analysis
• Create an applet to display
• "Welcome to Java Programming!"
• Show applet and HTML file, then discuss them line
  by line

13
// Fig. 3.6 WelcomeApplet.java // A first applet
in Java import javax.swing.JApplet // import
class JApplet import java.awt.Graphics //
import class Graphics public class WelcomeApplet
extends JApplet public void paint(
Graphics g ) g.drawString( "Welcome to
Java Programming!", 25, 25 )
14
//Open WelcomeApplet.class in a browser through
an //HTML file lthtmlgt ltapplet
code"WelcomeApplet.class" width300
height30gt lt/appletgt lt/htmlgt
15
Applets

• Methods paint, init, and start
• guaranteed to be called automatically for us
• our applet gets a "free" version of these by
  inheriting from JApplet
• free versions have an empty body (do nothing)
• every applet does not need all three - override
  only the ones you need

16
Applets

• Method paint
• draws graphics on screen
• void means paint returns nothing when it finishes
  its task
• parenthesis define parameter list - where methods
  receive data to perform tasks
• normally, data passed by programmer, as in
  JOptionPane.showMessageDialog
• paint gets parameters automatically
• Graphics object used by paint
• mimic paint's first line

17
Applets

• Body of paint
• method drawString (of class Graphics)
• called using Graphics object g and dot operator
  (.)
• method name followed by parenthesis containing
  argument list
• First argument String to draw
• Second x coordinate of location to draw at (in
  pixels)
• Third y coordinate of location to draw at (in
  pixels)

18
Applets

• Java coordinate system
• measured in pixels (picture elements)
• upper left is (0,0)

19
Applets

• Running the applet
• Compile
• javac WelcomeApplet.java
• if no errors, bytecodes stored in
  WelcomeApplet.class
• we must create an HTML file
• loads the applet into appletviewer or a browser
• ends in .htm or .html
• to execute an applet
• create an HTML file indicating which applet the
  browser (or appletviewer) should load and execute

20
Applets

• Program Analysis
• mimics application for adding two integers
• this time, use floating point numbers (numbers
  with a decimal point)
• show program, then we will discuss it

21
// Fig. 3.12 AdditionApplet.java // Adding two
floating-point numbers import java.awt.Graphics
// import class Graphics import javax.swing.
// import package javax.swing public class
AdditionApplet extends JApplet double sum
// sum of the values entered by the user
public void init() String firstNumber,
// first string entered by user
secondNumber // second string entered by user
double number1, // first number to add
number2 // second number to
add // read in first number from user
firstNumber JOptionPane.showInputDialo
g( "Enter first floating-point value"
)
22
// read in second number from user
secondNumber JOptionPane.showInputDialo
g( "Enter second floating-point
value" ) // convert numbers from type
String to type double number1
Double.parseDouble( firstNumber ) number2
Double.parseDouble( secondNumber ) //
add the numbers sum number1 number2
public void paint( Graphics g )
// draw the results with g.drawString
g.drawRect( 15, 10, 270, 20 )
g.drawString( "The sum is " sum, 25, 25 )

23
(No Transcript)
24
Applets

• Method init
• Normally initializes instance variables
• Guaranteed to be first method called
• First line must always appear as above
• Returns nothing (void), takes no arguments

25
Applets

• static method Double.parseDouble
• converts String argument to a double
• returns the double value
• Method drawRect( x1, y1, width, height )
• draws a rectangle with an upper left corner (x1,
  y1), with specified width and height