1. 1 A Simple Applet

2. 2 Applets and applications An application is an “ordinary” program Examples: Notepad, MS Word, Firefox, Halo, etc. An applet is a Java program that runs “within” another program (usually a browser) Applets can be run within any browser To run Java applets, browsers need an up-to-date Java plugin appletviewer is a program that can run applets When you download the Java SDK, appletviewer comes with it appletviewer is always up-to-date with your Java system Eclipse has an built-in applet viewer

3. 3 Importing some things we need To create an applet, you will import the JApplet class The JApplet class is in the javax.swing package This is the only class we will need from the swing package import javax.swing.JApplet; Since you will want to use many classes from the java.awt package, we will just import them all: import java.awt.*;

4. 4 The applet structure import javax.swing.JApplet; import java.awt.*; // CIT 591 example public class Drawing extends JApplet { …we still need to put some code in here... }

5. 5 The paint method Our applet is going to have a method to paint some colored rectangles on the screen This method must be named paint In applets, this is a special method name paint needs to be told where on the screen it can draw This will be the only parameter it needs paint doesn’t return any result

6. 6 The paint method, part 2 public void paint(Graphics g ) { … } public says that anyone can use this method void says that it does not return a result paint is the name of the method Method names should begin with a lowercase letter The argument, or parameter (there’s only one) is inside parentheses The method’s commands are inside braces

7. 7 The paint method, part 3 public void paint(Graphics g ) { … } A Graphics (short for “Graphics context”) is an object that holds information about a painting It remembers what color you are using It remembers what font you are using You can “paint” on it (but it doesn’t “remember” what you have painted) In this program, The type of the parameter is Graphics The name of the parameter is g

8. 8 The applet so far import javax.swing.JApplet; import java.awt.*; // CIT 591 example public class Drawing extends JApplet { public void paint(Graphics g) { …we still need to put some code in here… } }

9. 9 Colors The java.awt package defines a class named Color There are 13 predefined colors—here are their fully-qualified names: For compatibility with older programs (before the naming conventions were established), Java also allows color names in lowercase: Color.black, Color.darkGray, etc. Color.BLACKColor.PINK Color.GREEN Color.DARK_GRAYColor.REDColor.CYAN Color.GRAYColor.ORANGEColor.BLUE Color.LIGHT_GRAYColor.YELLOW Color.WHITE Color.MAGENTA

10. 10 New colors Every color is a mix of red, green, and blue You can make your own colors: new Color( red, green, blue ) Amounts range from 0 to 255 Black is (0, 0, 0), white is (255, 255, 255) We are mixing lights, not pigments Yellow is red + green, or (255, 255, 0)

11. 11 Setting a color To use a color, we tell our Graphics g what color we want: g.setColor(Color.RED); g will remember this color and use it for everything until we tell it some different color

12. 12 The paint method so far public void paint(Graphics g) { g.setColor(Color.BLUE); …draw a rectangle… g.setColor(Color.RED); …draw another rectangle… } }

13. 13 Pixels A pixel is a picture (pix) element one pixel is one dot on your screen there are typically 72 to 90 pixels per inch java.awt measures everything in pixels

14. 14 Java’s coordinate system Java uses an (x, y) coordinate system (0, 0) is the top left corner (50, 0) is 50 pixels to the right of (0, 0) (0, 20) is 20 pixels down from (0, 0) (w - 1, h - 1) is just inside the bottom right corner, where w is the width of the window and h is its height (0, 0) (0, 20) (50, 0) (50, 20) (w-1, h-1) (50, 0) (0, 0) (0, 20) (50, 20)

15. 15 Drawing rectangles There are two ways to draw rectangles: g.drawRect( left, top, width, height ); g.fillRect( left, top, width, height );

16. 16 Drawing strings A String is a sequence of characters enclosed in double quote marks "Hello, World!" A double quote mark in a String must be preceded by a backslash ( \ ) "He said, \"Please don't go!\" " To draw a string, you need to specify not only what you want to say, but where to say it g.drawString( string, left, top ); For example, g.drawString("Example JApplet", 20, 80);

17. 17 The complete applet import javax.swing.JApplet; import java.awt.*; // CIT 591 example public class Drawing extends JApplet { public void paint(Graphics g) { g.setColor(Color.BLUE); g.fillRect(20, 20, 50, 30); g.setColor(Color.RED); g.fillRect(50, 30, 50, 30); g.setColor(Color.BLACK); g.drawString("Example JApplet", 20, 80); } }

18. 18 More java.awt.Graphics methods g.drawLine( x1, y1, x2, y2 ); g.drawOval( left, top, width, height ); g.fillOval( left, top, width, height ); g.drawRoundRect( left, top, width, height, arcWidth, arcHeight ); arcWidth, arcHeight define the “roundedness” of corners g.fillRoundRect( left, top, width, height, arcWidth, arcHeight ); g.drawArc( left, top, width, height, startAngle, arcAngle ); Angles are in degrees 0 degrees is the 3 o’clock position Positive angles are to the right g.FillArc( left, top, width, height, startAngle, arcAngle );

19. 19 Still more Graphics methods g.drawPolygon( xPoints, yPoints, n ); g.fillPolygon( xPoints, yPoints, n ); xPoints and yPoints are int arrays of size n One way to write an int array is: new int[] { value1, value2,..., valueN } Example: g.drawPolygon(new int[] { 250, 290, 210 }, new int[] { 210, 290, 290 }, 3); draws a triangle using the 3 points (250, 210), (290, 290), and (210, 290). g.drawPolyline( xPoints, yPoints, n ); A “polyline” is like a polygon, except the first and last points are not automatically connected Hence, there is no “fillPolyline” method

20. 20 The HTML page You can only run an applet from an HTML page The HTML looks something like this: Drawing Applet Eclipse will create this HTML for you You don’t even need to think about the HTML just yet

21. 21 The End