APPLETS

1
APPLETS

• CSC 171 FALL 2004
• LECTURE 6

2
APPLETS

• Graphical Java programs
• Run inside web browser
• Platform-neutral
• Easy deployment--loads when needed
• Secure

3
Applets

• Applets are Java prgrams that can be embedded in
  Hypertext Markup Language (HTML) documents
• The browser that executes an applet is
  generically knows as the applet container
• So, often we use appletviewer

4
How Applets Work
5
HTML

• Text and tagsJava is an ltigtobject-orientedlt/igt
  programming language
• Browser renders the tagsJava is an
  object-oriented programming language
• Bulleted list (like this one) is defined by
  tagsltulgtltligt. . .lt/ligtltligt. . .lt/ligtltligt. .
  .lt/ligtlt/ulgt
• Use ltgt for ltgt symbols

6
Simple HTML

• lthtmlgt
• ltheadgt
• lttitlegt Ted Pawlicki's CSC 171 HTMLlt/titlegt
• lt/headgt
• ltbodygt
• Hello Ted!
• lt/bodygt
• lt/htmlgt

7
Applet Methods

• Applets have no main()
• Rather, it has key methods that deal with the
  unique situation of applets

8
Key Applet Methods

• init
• start
• paint
• stop
• destroy

9
Key Applet Methods

• public void init()
• public void start()
• public void paint(Graphics g)
• public void stop()
• public void destroy()

10
public void init()

• Called once by appletviewer or browser
• Called when an applet is loaded
• Performs initialization
• Instance variables
• GUI components
• Loading sounds images (multimedia)
• Creation of threads (animation)

11
public void start()

• Called after the init method completes
• Called every time the user of the browser returns
  to the HTML page
• Performs tasks that must be repeated when the
  applet is revisited
• Restarting an animation

12
public void paint(Graphics g)

• Called after init() method completes and start()
  has started
• Draws stuff on the applet
• The system hands the applet a Graphics object to
  draw stuff on
• Automatically recalled whenever the applet needs
  to be repainted
• uncovering a covered window

13
public void stop()

• Called when the applet should stop executing
• When the user leaves the HTML page
• Perform tasks necessary to suspend applet
• pausing animations

14
public void destroy()

• Called when the applet is removed from memory
• Performs tasks to de-allocate resources

15
repaint()

• Paint is normally called by the applet container
• We sometimes change the applets appearance based
  on user input
• We might like to call paint() directly
• However, in order to call paint, we have to pass
  it a Graphics object
• But the container owns the Graphics object
• repaint() invokes update() which invokes paint
  with the appropriate graphics object

16
Some HTML

• lthtmlgt
• ltheadgt
• lttitlegt Ted Pawlicki's CSC 171 First
  Appletlt/titlegt
• lt/headgt
• ltbodygt
• ltapplet codeFirstApplet.class" width 256
  height 550 gt
• lt/appletgt
• lt/htmlgt

17
Some Java

• import java.awt.
• import java.applet.Applet
• public class FirstApplet extends Applet
• public void paint(Graphics g)
• g.drawString(Hello Ted!", 20, 40)

18
Cooler Graphics

• class MyApplet extends Applet public void
  paint(Graphics g) Graphics2D g2
  (Graphics2D)g // add drawing operations. .
  .

19
Graphical Shapes

• Shape classes Ellipse2D.Double, Line2D.Double,
  etc. 
• import java.awt.geom.Ellipse2D // no .Double
• Must construct and draw the shape
  Ellipse2D.Double easterEgg new
  Ellipse2D.Double(5, 10, 15, 20)g2.draw(easterEgg
  )

20
Ellipse
21
Lines

• Line2D.Double segment new Line2D.Double(x1, x2,
  y1, y2)
• More object-oriented to use Point2D.Double for
  the end pointsPoint2D.Double from new
  Point2D.Double(x1, y1)Point2D.Double to new
  Point2D.Double(x2, y2)Line2D.Double segment
  new Line2D.Double(from, to)
• Draw thick linesg2.setStroke(new
  BasicStroke(4.0F)) // 4 pixels

22
Colors

• Specify red, green, blue between 0.0F and
  1.0FColor magenta new Color(1.0F, 0.0F, 1.0F)
• Standard colorsColor.blackColor.yellowColor.pin
  k. . .
• Set color in graphics contextg2.setColor(Color.p
  ink)
• Then draw or fill shapesg2.fill(easterEgg)

23
FONTS

• Specify text and base pointg2.drawString("Applet
  ", 50, 100)
• Font object has
• face name (Serif, SansSerif, Monospaced, ...)
• style (Font.PLAIN, Font.BOLD, Font.ITALIC )
• point size (12 point normal size)
• g2.setFont(new Font("Serif", Font.BOLD, 36))

24
Baseline
25
Local or Relative Coordinate System
26
The whole object has a location

• public Car(double x, double y)
•       xLeft  x
•       yTop  y
•  

27
The object is drawn relative to the whole
objects location

• public void draw(Graphics2D g2)   
• Rectangle2D.Double body     
• new Rectangle2D.Double(xLeft, yTop10, 60, 10) 
    
• Ellipse2D.Double rearTire          
• new Ellipse2D.Double(xLeft  40, yTop  20, 10, 1
  0)   
• ..