COM214J2: Introduction to Programming Applets

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COM214J2 Introduction to ProgrammingApplets

• Sarabjot Singh Anand
• 16E17
• ss.anand_at_ulster.ac.uk
• http//ijsr32.infj.ulst.ac.uk/e95346/COM214J2/ind
  ex.html

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Applets Vs Applications

• Java applications are run by the Java Interpreter
  that
• loads the applications main class
• loads other classes as required
• Java applets runs within an Internet browser that
  supports Java
• it is a graphics window which can be controlled
  by a Java program
• makes Internet content interactive
• must be included in an HTML page using HTML tags
  just like an image or text that needs to appear
  in the web page
• when the page is loaded, the applet is run by the
  Java interpreter built into the browser

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(No Transcript)
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lthtmlgt ltheadgt lttitlegtWatch Appletlt/titlegt lt/headgt
ltbody bgcolor"996633"gt ltpgtThe current
timeltbrgt ltobject height"50" width350"
classid"clsid8AD9C840-044E-11D1-B3E9-00805F499D9
3" codebase"http//java.sun.com/products/plugin/a
utodl/jinstall-1_4_0-win.cab"gt ltparam name"Code"
valueNewWatch.classgt This program requires a
Java-enabled browser. lt/objectgt lt/bodygt lt/htmlgt
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The Applets Environment

• As applets run within a browser
• There is an existing window for the applet to run
  in
• A place to display graphics and receive
  information
• A predefined interface with the Browser
• Can react to events such as the page reloading
• An applet is a subclass of JApplet (javax.swing
  package) or Applet (which is the superclass of
  JApplet in the java.applet package)
• public class myApplet extends javax.swing.JApplet
  //java code

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Anatomy of an Applet

• Does not contain a main method
• The browser creates an instance of the applet
  class and calls different methods whenever
  specific events occur
• Specific methods in an applet
• init()
• start()
• stop()
• destroy()
• paint()
• These methods are defined in the Applet class or
  its superclasses but can be overridden in your
  applet

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Method OverriddingHow does Java know which paint
method to use?
paint()
init() start() stop() destroy()
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Paint

• This is a key method that displays the applet on
  the screen
• Painting occurs whenever the browser window is
• initialised
• brought out from behind a page
• Resized
• ..
• Explicitly requested by the repaint() method call
• The method to override is defined as public void
  paint(Graphics g)

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The paint method

• This method must be overridden as it defines what
  must be shown in the java applet you are writing
• Called by the browser whenever the applet is to
  be redrawn
• The parameter to the method is an instance of the
  class Graphics
• The object is created by the browser and passed
  to the paint method
• Hence the applet must import the
  java.awt.Graphics class

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The Graphics object

• Represents the area being drawn to
• The Applet Window
• It is the equivalent of a blank canvas upon which
  the paint method draws graphics like lines,
  rectangle,.
• Object used to draw text and other graphical
  components in the applet window

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Co-ordinates on the Applet Window
(35,0)
(0,0)
(0,30)
(35,30)
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Methods provided by the Graphics Object

• drawString(String text, int xvalue, int yvalue)
• g.drawString(My Applet, 30,40 )
• drawLine(int xStart, int yStart,int xFinish, int
  yFinish)
• g.drawLine(30,40,60,40)
• drawRect(int xStart, int yStart, int width, int
  height)
• g.drawRect(25,35,40,10)
• setColor(Color myColor)
• g.setColor(Color.black)
• setFont(Font myFont)
• g.setFont(new Font(Arial, Font.BOLD,10))

Upper Left corner of String on Canvas
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import java.applet. import java.awt. public
class ColorandFontDemo extends Applet
public void paint(Graphics g) // paint
method Font f new Font3 // an
array to hold 3 fonts f0 new
Font("TimesRoman",Font.BOLD,12) f1
new Font("Courier", Font.ITALIC,16) f2
new Font("Arial",Font.PLAIN,20)  
g.setColor(Color.green) // set the colour to
green g.setFont(f0) // choose the
first font g.drawString(First Green
Applet Message, 10 ,20) // display
message at (x10,y20)
g.setColor(Color.red) // change colour to
red g.drawLine(20,30, 200, 250) // draw
a straight red line from (20,30) to (200,250)
g.setColor(Color.yellow) // change colour
to yellow g.drawRect(30, 40, 200, 150)
// draw yellow rectangle 200 wide, 150 high with
top left corner at (30,40)
g.setColor(Color.blue) // change colour to
blue g.setFont(f1) // choose second
font g.drawString(Second Blue Message,
200 ,250) // display message starting at
(10,350)
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Viewing the Applet
ltHTMLgt ltAPPLET code"ColorandFontDemo.class"
width400 height300gt lt/APPLETgt lt/HTMLgt