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APPLETS

APPLETS CSC 171 FALL 2004 LECTURE 6 APPLETS Graphical Java programs Run inside web browser Platform-neutral Easy deployment--loads when needed Secure Applets Applets are Java prgrams that can be embedded in Hypertext Markup Language (HTML) documents

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APPLETS

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  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 tags:Java is an <i>object-oriented</i> programming language • Browser renders the tags:Java is an object-oriented programming language • Bulleted list (like this one) is defined by tags<ul><li>. . .</li><li>. . .</li><li>. . .</li></ul> • Use &lt;&gt; for <> symbols

  6. Simple HTML <html> <head> <title> Ted Pawlicki's CSC 171 HTML</title> </head> <body> Hello Ted! </body> </html>

  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 applet’s 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 <html> <head> <title> Ted Pawlicki's CSC 171 First Applet</title> </head> <body> <applet code=“FirstApplet.class" width = 256 height = 550 > </applet> </html>

  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 points:Point2D.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 lines:g2.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.pink. . . • Set color in graphics context:g2.setColor(Color.pink); • Then draw or fill shapesg2.fill(easterEgg);

  23. FONTS • Specify text and base point:g2.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 object’s location public void draw(Graphics2D g2)   { Rectangle2D.Double body    =  new Rectangle2D.Double(xLeft, yTop+10, 60, 10);    … Ellipse2D.Double rearTire         =  new Ellipse2D.Double(xLeft + 40, yTop + 20, 10, 10);    …..

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