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Applets

Applets. An applet is similar to a Java program, but it runs within a Web-browser on a client machine. For security, you cannot change anything on a client system in a Java applet The browser gives the applet a piece of the browser window, and Java controls that region

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Applets

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  1. Applets • An applet is similar to a Java program, but it runs within a Web-browser on a client machine. • For security, you cannot change anything on a client system in a Java applet • The browser gives the applet a piece of the browser window, and Java controls that region • The structure of an applet is different than an application • There is an init method instead of a main() method • An applet inherits (extends) a Applet class. This means all the methods in the applet class are available to the applet you create • Some methods that are available include init(), paint(), update(), setSize(), and others.

  2. Graphics class object • Methods • g.drawArc(x, y, width, height, startAngle, endAngle); • g.drawLine(x1, y, x2, y2); • g.drawOval(x, y, width, height); • g.drawRect(x, y, width, height); • g.fillArc(x, y, width, height); • g.fillOval(x, y, width, height); • g.setColor(color); • Notes • x, x1, x2 = distance from the left, y, y1, y2 = distance from the top • width = how wide to draw, height = how high to draw • color = the color to use for drawing

  3. The Color Class • Instantiate a color String s = “ 0000ff”; int i = Integer.parseInt( s.trim(), 16); Color c = new Color(i); • Set the current color for drawing graphics.setColor(c); • Fill a rectangle using the current color Graphics.fillRect(0,0,800,800);

  4. A Rectangle applet We override JApplet init and paint methods, this is polymorphism import java.awt.*; import javax.swing.*; public class Picture extends JApplet { public void init() { setSize(new Dimension(800,800)); } public void paint(Graphics g) { g.setColor(new Color(256*255)); g.fillRect(0,0,799,799); g.setColor(new Color(0)); g.fillRect(50, 100, 100, 200); } } Import gets Java features that can be used if we ask for them A Dimension object defines a width and height setSize defines a size for the applet window extends is a reserved word for inheriting from the Applet class

  5. Testing and running an Applet • In Jgrasp, simply click on the picture of the apple • Create a web-page with the applet <html> <head><title>My Applet</title></head> <body> <applet code="MyPicture.class" width="800" height="800"></applet> </body></html>

  6. Applet parameters • HTML: Add the following immediately after your applet tag, but before </applet> <param name=“background” value=“00ff00” /> <param name=“foreground” value= “ff0000” /> • The applet: Retrieve parameters from the HTML String background = getParam(“background”); String foreground = getParam(“foreground”); • Your init() method (Formatting in your applet to use colors) int i = Integer.parseInt( background.trim(), 16); Color c = new Color(i); • Your paint(Graphics g) method: setColor(c); g.fillRect(0, 0, 800, 800);

  7. Review • What is an applet? • What security concerns relate to an applet? • What is inheritance? • What is polymorphism? • How do you test your applet in Jgrasp? • How do you use html parameters in an applet? • What is a parameter name? • What does the init() method do? • What does the paint() method do? • How are colors represented in the computer? • What is the Graphics class? How do you use it in an applet?

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