1 / 47

Chapter 4

Chapter 4. Introduction to Applets and Graphics. Topics. Applet Structure Executing an Applet Drawing Shapes with Graphics Methods Using Colors. Applets. A Java application is a stand-alone program with a main method (like the ones we've seen so far)

alfonso-rivers
Télécharger la présentation

Chapter 4

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 4 Introduction to Applets and Graphics

  2. Topics • Applet Structure • Executing an Applet • Drawing Shapes with Graphics Methods • Using Colors

  3. Applets • A Java application is a stand-alone program with a main method (like the ones we've seen so far) • A Java applet is a program that is intended to transported over the Web and executed using a web browser • An applet also can be executed using the appletviewer tool of the Java Software Development Kit • An applet doesn't have a main method • Instead, there are several special methods that serve specific purposes

  4. Applets • Executed by a browser or applet viewer • Opens a window, as well as the Java console • Applet viewer comes with Java Software Development Kit (SDK)

  5. Java Translation & Execution Java source code Across the Internet using HTML Java bytecode Java compiler Web browser Java interpreter Java interpreter Bytecode compiler Remote computer Local computer Machine code

  6. Applet Structure • Do not use main method • Two methods called automatically: • init method • Browser calls init when applet starts • Use to initialize variables and objects • paint method • Browser calls after init and whenever window needs to be redrawn • Use to draw to screen

  7. Example 4.01 ShellApplet.java import javax.swing.JApplet; import java.awt.Graphics; public class ShellApplet extends JApplet { // declare variables here public void init( ) { // initialize data here } public void paint( Graphics g ) { super.paint( g ); // include graphics code here } }

  8. Executing an Applet • A Web page tells the browser to run the applet • HTML tags come in pairs, data goes between start and end tags <HTML> </HTML> start and end of HTML code <HEAD> </HEAD> start and end of header <TITLE> </TITLE> text to display in title bar <BODY> </BODY> start and end of page content

  9. <Applet> Tag <APPLET> CODE = Classname.class CODEBASE = . directory of class file WIDTH = nnn width of window in pixels HEIGHT = nnn height of window in pixels </APPLET>

  10. Minimal HTML File <HTML> <HEAD> <TITLE>TitleName</TITLE> </HEAD> <BODY> <APPLET CODE="ClassName.class" CODEBASE=. WIDTH=nnn HEIGHT=nnn> </APPLET> </BODY> </HTML>

  11. HTML File for FirstApplet <HTML> <HEAD> <TITLE>My First Applet</TITLE> </HEAD> <BODY> <APPLET CODE="FirstApplet.class" CODEBASE=. WIDTH=400 HEIGHT=300> </APPLET> </BODY> </HTML>

  12. Executing an Applet • If HTML file is named FirstApplet.html, you can execute the applet using this command: appletviewer FirstApplet.html • Many IDEs automatically create and launch the HTML file for running an applet

  13. Applets in jGrasp • To run an applet in jGrasp, just click on the “apple” button • An applet viewer will automatically be opened

  14. The Graphics Class • Browser or appletviewer sends a Graphics object to the paint method • The Graphics object represents the applet window, current font, and current color • Provides methods to draw shapes and text on the window

  15. The Graphics Coordinate System

  16. Drawing Shapes • Let's explore some of the methods of the Graphics class that draw shapes in more detail • A shape can be filled or unfilled, depending on which method is invoked • The method parameters specify coordinates and sizes • Recall that the Java coordinate system has the origin in the top left corner • Shapes with curves, like an oval, are usually drawn by specifying the shape’s bounding rectangle • An arc can be thought of as a section of an oval

  17. Graphics Class Methods • Methods are available for drawing lines, rectangles, ovals, and other shapes, and for setting the current color • All methods have a void return type, so method calls are standalone statements • draw… methods draw an outlined share • fill… methods draw a solid shape

  18. Displaying Text • Example: g.drawString( "Hello", x, y ); • See Example 4.4 DrawingTextApplet.java

  19. Drawing a Line g.drawLine( xStart, yStart, xEnd, yEnd ); • See Example 4.5 LineDrawingApplet.java

  20. X Y page.drawLine (10, 20, 150, 45); or page.drawLine (150, 45, 10, 20); Drawing a Line 10 150 20 45

  21. Drawing A Rectangle g.drawRect( x, y, width, height );

  22. X 40 100 Y Drawing a Rectangle 50 20 page.drawRect (50, 20, 100, 40);

  23. Drawing A Solid Rectangle g.fillRect( x, y, width, height );

  24. Drawing An Oval g.drawOval( x, y, width, height );

  25. X 80 50 Y Drawing an Oval 175 20 bounding rectangle page.drawOval (175, 20, 50, 80);

  26. Drawing A Solid Oval g.fillOval( x, y, width, height );

  27. Drawing Squares and Circles • To draw a square, use drawRect or fillRect with equal values for width and height. • To draw a circle, use drawOval or fillOval with equal values for width and height

  28. Example 4.6 ShapeDrawingApplet.java public void paint( Graphics g ) { super.paint( g ); g.drawRect( 100, 50, 40, 100 ); // rectangle g.fillRect( 200, 70, 80, 80 ); // solid square g.fillOval( 100, 50, 40, 100 ); // oval inside the rectangle g.drawOval( 100, 200, 100, 40 ); // same-size oval rotated 90 degrees int centerX = 250, centerY = 225; int radius = 25; g.drawOval( centerX - radius, centerY - radius, radius * 2, radius * 2 ); // circle using radius and center } }

  29. When drawing a figure using Graphics methods, specify coordinate values as offsets from a starting (x,y) coordinate. • This will make your figure easier to move or resize.

  30. Example 4.7 Astronaut.java import javax.swing.JApplet; import java.awt.Graphics; public class Astronaut extends JApplet { int sX, sY; // starting x and y coordinates public void init( ) { sX = 95; sY = 20; }

  31. public void paint( Graphics g ) { super.paint( g ); // helmet g.drawOval( sX + 60, sY, 75, 75 ); g.drawOval( sX + 70, sY + 10, 55, 55 ); // face g.drawOval( sX + 83, sY + 27, 8, 8 ); g.drawOval( sX + 103, sY + 27, 8, 8 ); g.drawLine( sX + 97, sY + 35, sX + 99, sY + 43 ); g.drawLine( sX + 97, sY + 43, sX + 99, sY + 43 ); g.drawOval( sX + 90, sY + 48, 15, 6 ); // neck g.drawRect( sX + 88, sY + 70, 20, 10 ); // torso g.drawRect( sX + 65, sY + 80, 65, 85 );

  32. // arms g.drawRect( sX, sY + 80, 65, 20 ); g.drawRect( sX + 130, sY + 80, 65, 20 ); // legs g.drawRect( sX + 70, sY + 165, 20, 80 ); g.drawRect( sX + 105, sY + 165, 20, 80 ); // flag g.drawLine( sX + 195, sY + 80, sX + 195 , sY ); g.drawRect( sX + 195, sY, 75, 45 ); g.drawRect( sX + 195, sY, 30, 25 ); // caption g.drawString( "One small step for man…", sX + 25, sY + 270 ); } }

  33. Representing Color • A black and white picture can be stored using one bit per pixel (0 = white and 1 = black) • A colored picture requires more information; there are several techniques for representing colors • For example, every color can be represented as a mixture of the three additive primary colors Red, Green, and Blue (RGB system) • In Java, each color is represented by three numbers between 0 and 255 that collectively are called an RGB value (one byte per color stored)

  34. Using Color • Every drawing surface has a background color • The Graphics context has a current foreground color • Both can be set explicitly • All drawing is done in current color; the current color is in effect until changed • The default color is black. • To use color, import the Color class from the java.awt package

  35. Object Color.black Color.blue Color.cyan Color.orange Color.white Color.yellow RGB Value 0, 0, 0 0, 0, 255 0, 255, 255 255, 200, 0 255, 255, 255 255, 255, 0 The Color Class • A color is defined in a Java program using an object created from the Color class • The Color class also contains several static predefined colors, including:

  36. staticColor Constants Color.BLACK Color.GRAY Color.WHITE Color.ORANGE Color.RED Color.YELLOW Color.GREEN Color.PINK Color.BLUE Color.MAGENTA Color.CYAN Color.LIGHT_GRAY Color.DARK_GRAY

  37. Setting the Current Color Example: g.setColor( Color.RED );

  38. Custom Colors • Colors consist of red, green, and blue components (RGB). • Color constructor: • Example: Color green = new Color( 0, 255, 0 );

  39. AstronautWithColor.java import javax.swing.JApplet; import javax.swing.JOptionPane; import java.awt.Graphics; import java.awt.Color; public class AstronautWithColor extends JApplet { int sX, sY; Color spacesuit; public void init( ) { spacesuit = new Color( 195, 175, 150 ); sX = Integer.parseInt( JOptionPane.showInputDialog ( null, "Enter the starting x position") ); sY = Integer.parseInt( JOptionPane.showInputDialog ( null, "Enter the starting y position") ); }

  40. public void paint( Graphics g ) { super.paint( g ); // helmet g.setColor( spacesuit ); g.fillOval( sX + 60, sY, 75, 75 ); g.setColor( Color.LIGHT_GRAY ); g.fillOval( sX + 70, sY + 10, 55, 55 ); // face g.setColor( Color.DARK_GRAY ); g.drawOval( sX + 83, sY + 27, 8, 8 ); g.drawOval( sX + 103, sY + 27, 8, 8 ); g.drawLine( sX + 97, sY + 35, sX + 99, sY + 43 ); g.drawLine( sX + 97, sY + 43, sX + 99, sY + 43 ); g.drawOval( sX + 90, sY + 48, 15, 6 ); // neck g.setColor( spacesuit ); g.fillRect( sX + 88, sY + 70, 20, 10 );

  41. // torso g.fillRect( sX + 65, sY + 80, 65, 85 ); // arms g.fillRect( sX, sY + 80, 65, 20 ); g.fillRect( sX + 130, sY + 80, 65, 20 ); // legs g.fillRect( sX + 70, sY + 165, 20, 80 ); g.fillRect( sX + 105, sY + 165, 20, 80 ); // flag g.setColor( Color.BLACK ); g.drawLine( sX + 195, sY + 80, sX + 195 , sY ); g.setColor( Color.RED ); g.fillRect( sX + 195, sY, 75, 45 ); g.setColor( Color.BLUE ); g.fillRect( sX + 195, sY, 30, 25 ); // caption g.setColor( Color.BLACK ); g.drawString( "One small step for man...", sX + 25, sY + 270 ); } }

  42. Graphics class methods • void drawArc (int x, int y, int width, int height, int startAngle, int arcAngle) • void drawLine (int x1, int y1, int x2, int y2) • void drawOval (int x, int y, int width, int height) • void drawRect (int x, int y, int width, int height) • void drawString (String str, int x, int y) • void fillArc (int x, int y, int width, int height, int startAngle, int arcAngle) • void fillOval (int x, int y, int width, int height) • void fillRect (int x, int y, int width, int height) • Color getColor( ) • void setColor (Color color)

  43. Applet Methods • In previous examples we've used the paint method of the Applet class to draw on an applet • The Applet class has several methods that are invoked automatically at certain points in an applet's life • The init method, for instance, is executed only once when the applet is initially loaded • The start and stop methods are called when the applet becomes active or inactive • The Applet class also contains other methods that generally assist in applet processing

  44. Applet class methods • void init ( ) • void start ( ) • void stop ( ) • void destroy ( ) • URL getCodeBase ( ) • URL getDocumentBase ( ) • AudioClip getAudioClip (URL url, String name) • Image getImage (URL url, String name)

More Related