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Review of Applets

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. Applications and Applets.

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Review of Applets

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  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 Unit 07

  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 Appletor theJAppletclass. Unit 07

  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. Unit 07

  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.BOLD+Font.ITALIC, 24)); • g.drawString("This is Introduction to Computer Science", 20, 150); • } • } Unit 07

  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. Unit 07

  6. Graphics and Graphics2D (cont’d) • 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. Unit 07

  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) ; • } • } Unit 07

  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. Unit 07

  9. How Java Displays Images (cont’d) • 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 Componentclass implements. • An image observer is informed about many aspects of the image. • The image observer usually calls repaint to update the applet. Unit 07

  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); } } Unit 07

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