Review of Applets

1
Review of Applets

• Learning Outcomes
• Distinguish between Java Applications and Java
  Applets.
• Write applet programs that can load images and
  play sound.
• Explain the limitations of constructors in applet
  programs.
• Applications and Applets
• Example 1 Passing Parameters to Applets
• The Graphics and the Graphics2D Classes
• Example 2 Loading Images and Playing Sounds
• How Java Loads and Displays Images
• Example 3 Applet and Application Hybrid

2
Applications and Applets

• A Java program can be an application, applet or
  both.
• A Java application is a stand-alone, unrestricted
  program.

• A Java applet is a restricted program that relies
  on another program to execute.
• A Java applet executes under a Web browser or
  applet viewer.
• An applet is defined by extending the Applet or
  the JApplet class.

3
Life Cycle of an Applet

• An applet may call the following methods in its
  life cycle
• init()
• start()
• stop()
• paint()
• destroy()
• By default, these methods have empty
  implementations.

4
Example 1 Passing Parameters to Applets
1 import javax.swing. import java.awt. 2
public class AppletParameters extends JApplet 3
private int size 4 private String font
5 private String message 6 public void
init() 7 size Integer.parseInt(getParam
eter("size")) 8 font getParameter("font"
) 9 message getParameter("message") 10
11 public void paint(Graphics g) 12
g.setColor(Color.green) 13 g.setFont(new
Font(font, Font.PLAIN, size)) 14
g.drawString(message,20, 50) 15 Font
myFont new Font("Dialog", Font.BOLD, 36) 16
g.setFont(myFont) g.setColor(Color.red) 17
g.drawString("You are Welcome to CCSE", 20,
100) 18 g.setColor(Color.blue) 19
g.setFont(new Font("Courier", Font.BOLDFont.ITALI
C, 24)) 20 g.drawString("This is
Introduction to Computer Science", 20, 150) 21
22
5
The Graphics and the Graphics2D Classes

• Drawing graphics is done differently on different
  computer platforms.
• Java provides an abstract Graphics class that
  covers all platforms.
• The Graphics class was included with the first
  version of Java.
• A more sophisticated graphics class, Graphics2D,
  was introduced later.
• The Graphics2D class extends rather than replace
  Graphics.

6
Graphics and Graphics2D (contd)

• In many earlier programs, the public void
  paint(Graphics g) method includes the statement
• Graphics2D g2 (Graphics2D)g
• Each time paint is called it is passed a
  Graphics2D object for drawing in the display area
• An instance of Graphics or Graphics2D is called a
  graphics context.
• A graphics context represents a drawing surface.
• The Graphics2D object passed to paint is up-cast
  to Graphics.
• Additional functionality in Graphics2D is
  available after the down-cast.

7
Example 2 Loading Images, Playing Sounds
1 import javax.swing.import java.awt.import
java.applet. 2 public class ImageAndSound
extends Applet 3 Image image AudioClip
sound 4 public void init( ) 5 image
getImage(getDocumentBase() , "myImage.gif" )
6 sound getAudioClip(getDocumentBase()
, "mySound.au" ) 7 setBackground(Color.r
ed) 8 9 public void start( ) 10
repaint() 11 if (sound ! null) 12
sound.loop() 13 14 public void stop(
) 15 sound.stop() 16 17
public void paint(Graphics g) 18
Graphics2D g2 (Graphics2D)g 19
g2.drawImage(image , 5 , 5 , this) 20 21

8
How Java Loads and Displays Images

• The preceding example reminds us of loading
  images and playing sounds
• To display an image you need to
• 1. retrieve the image from a file or from an
  Internet source
• 2. draw the image.
• The getImage method starts the loading process
  and returns immediately.
• Thus, Java loads images in an asynchronous
  manner.
• The benefit is that the program can do something
  if loading the image takes time.

9
How Java Displays Images (contd)

• Java will start loading a new image only when we
  attempt to display or manipulate the image
• Images are drawn using the overloaded drawImage
  method
• drawImage(Image img, int x, int y, Color bgC,
  ImageObserver obs)
• The drawImage method starts the download and
  returns immediately.
• ImageObserver is an interface that the Component
  class implements.
• An image observer is informed about many aspects
  of the image.
• The image observer usually calls repaint to
  update the applet.

10
Example 3 Applet and Application Hybrid

• import java.awt.
• import javax.swing.
• public class AppletApplication extends JApplet
• public void paint(Graphics g)
• Graphics2D g2 (Graphics2D)g
• g2.drawString("Salaam Shabab!", 30,30)
• public static void main(String args )
• JFrame f new JFrame("Applet and
  Application")
• AppletApplication applet new
  AppletApplication()
• Container cp f.getContentPane()
• cp.add(applet)
• f.setSize(150,150)
• f.setVisible(true)