Review of Applets

1
Review of Applets

• 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()
• These methods have empty implementations in the
  Applet class.

4
Example 1 Passing Parameters to Applets

• import javax.swing. import java.awt.
• public class AppletParameters extends JApplet
• private int size
• private String font
• private String message
• public void init()
• size Integer.parseInt(getParameter("size"))
  
• font getParameter("font")
• message getParameter("message")
• public void paint(Graphics g)
• g.setColor(Color.green)
• g.setFont(new Font(font, Font.PLAIN, size))
• g.drawString(message,20, 50)
• Font myFont new Font("Dialog", Font.BOLD,
  36)
• g.setFont(myFont) g.setColor(Color.red)
• g.drawString("You are Welcome to CCSE", 20,
  100)
• g.setColor(Color.blue)
• g.setFont(new Font("Courier",
  Font.BOLDFont.ITALIC, 24))

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 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

• import javax.swing.import java.awt.import
  java.applet.
• public class ImageAndSound extends Applet
• Image image AudioClip sound
• public void init( )
• image getImage(getDocumentBase() ,
  "myImage.gif" )
• sound getAudioClip(getDocumentBase() ,
  "mySound.au" )
• setBackground(Color.red)
• public void start( )
• repaint()
• if (sound ! null)
• sound.loop()
• public void stop( )
• sound.stop()
• public void paint(Graphics g)
• Graphics2D g2 (Graphics2D)g
• g2.drawImage(image , 5 , 5 , this)

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)

• 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)