1、1Features of JavaCS 3331Fall 20092OutlineAbstract classInterfaceApplication - animation applets3Motivation - Drawing Board4Class Shapepublic class Shape private int x, y; private Color c; public Shape(int x, int y, Color c) this.x = x; this.y = y; this.c = c; public void draw(Graphics g) /* */ publi

2、c int getX() return x; public int getY() return y; public Color getColor return c; Q: What is wrong with this definition?5At Least Two Problems6Solution: Abstract Classpublic abstract class Shape private int x, y; private Color c; protected Shape(int x, int y, Color c) this.x = x; this.y = y; this.c

3、 = c; public abstract void draw(Graphics g); / no body here! public int getX() return x; public int getY() return y; public Color getColor return c; 7Abstract Classes?Classes that cant be instantiatedUsed to define common properties that are to be inherited by subclassesOften provide partial impleme

4、ntationsMay include abstract methods, methods that have no body8How Abstract Classes Solve the Problems?9In Sum, Abstract Classes Provide partial implementations to be inherited by subclassesMay include abstract methodsAre good for factoring out common properties among classes10OutlineAbstract class

5、esInterfacesApplication - animation applets11InterfacesDeclare features to be supported by classesProvide no implementationOnly allow public abstract methods and constants (public static final fields)public interface Runnable public void run();12Why Interfaces?To draw automobiles DrawingBoardCircleR

6、ectangleTriangleVehicleAutomobile0.*Shapeabstract13How to Draw Automobiles?By programming to the interface.14In Sum, Interfaces Good for establishing a well-defined boundary between modules (subsystems)Thus, make programs more reusable and maintainable15Abstract Classes vs. InterfacesPartial code vs

7、. no code at allClass vs. interface16ExerciseSeparate the display of DigitalClock to support various ways of displaying time, e.g., digital, analog, customized background, etc. Explain your design by drawing a UML class diagram.DigitalClock# timer: Timer# font: Font# color: Color+ DigitalClock(): vo

8、id+ start(): void+ stop(): void+ paint(g: Graphics): void17Applications - Animation AppletsEnhanced digital clock appletScrolling banner appletlInitial versionlDouble-buffered version18Enhanced Digital Clock AppletSetting applet parameters in the Web page 19Getting Applet Parametersimport java.awt.C

9、olor;public class DigitalClock2 extends DigitalClock public void init() String param = getParameter(“color”); if (“red”.equals(param) color = Color.RED; else if (“blue”.equals(param) color = Color.BLUE; else if (“yellow”.equals(param) color = Color.YELLOW; /* */ else color = Color.GREEN; 20Animation

10、 AppletsEnhanced digital clock appletScrolling banner appletlInitial versionlDouble-buffered version21The java.awt.Graphics ClassClass GraphicslRepresents graphics context, an abstraction of various drawing surfaces, e.g., screen, printer, off-screen image (an image stored in memory).lProvide a rich

11、 set of graphics methods. drawString() drawLine() drawArc() fillArc() drawOval() fillOval()drawPolygon() fillPolygon()drawRect() fillRect() drawRoundRect() fillRoundRect()22Graphics Class (Cont.)Other methodssetColor(color) set the current colorsetFont(font) set the current font setPaintMode() set t

12、he paint, or overwrite mode setXORMode(color) set the XOR mode getColor() get the current color getFont() get the current font getFontMetrics() get the font metrics of the current font getFontMetrics(font) get the font metrics for the specified font 23The java.awt.FontMetrics ClassgetAscent() getDes

13、cent() getHeight() getLeading() stringWidth(s) Upwidthheightleadingascentdescentbaseline24Scrolling Banner Appletimport java.awt.*;import java.awt.event.*;import javax.swing.*;public class ScrollingBanner extends java.applet.Applet public void init() . public void paint(Graphics g) . public void sta

14、rt() . public void stop() . 25Field Declarationsprotected String text; protected Font font = new java.awt.Font(Sans-serif, Font.BOLD, 24); protected Dimension dim; protected int x, y; protected int delay = 100; protected int offset = 1;protected Timer timer; / animation timer26Initializationpublic v

15、oid init() / get parameters delay and text String att = getParameter(delay); if (att != null) delay = Integer.parseInt(att); att = getParameter(text); if (att != null) text = att; else text = “Go Miners!”; / set initial position of the text dim = getSize(); x = dim.width; y = font.getSize();/ initia

16、lize animation timertimer = new Timer(delay, new ActionListener() public void actionPerformed() repaint(); );27Painting the Current FrameGo Miners!Go Miners!Go Miners!lengthviewing area(dim.width, y)(dim.width-1, y)(-length, y)(x, y)(0, 0)leftmost positionrightmost positioncurrent position28Painting

17、 the Current Frame (Cont.)public void paint(Graphics g) / get the font metrics to determine the length of the text g.setFont(font); FontMetrics fm = g.getFontMetrics(); int length = fm.stringWidth(text); / adjust the position of text from the previous frame x = x - offset; / if the text is completel

18、y off to the left end / move the position back to the right end if (x -length) x = dim.width; / set the pen color and draw the background g.setColor(Color.BLACK); g.fillRect(0, 0, dim.width, dim.height); / set the pen color, then draw the text g.setColor(Color.GREEN); g.drawString(text, x, y); 29The

19、 start() and stop() Methodspublic void start() timer.start(); public void stop() timer.stop(); 30ExerciseDefine a subclass of ScrollingBanner, called ScrollingBanner3, that scrolls the banner vertically. Reuse code as much as possible and minimize code duplication.31How to Avoid Flickering?Flickerin

20、g is caused by repaint() lrepaint() calls the update() method. lThe default update() method does the following: paint the whole area with the background color; set the foreground color; call the paint() method. lThe update() method is also called by the system to update windows. Solution: loverride

21、the update() method luse an off-screen image 32Using Off-Screen ImageDouble bufferingimport java.awt.*; public class ScrollingBanner2 extends ScrollingBanner protected Image image; / off-screen image protected Graphics offscreen; / off-screen graphics public update(Graphics g) . public paint(Graphic

22、s g) . 33Using Off-Screen Image (Cont.)public void update(Graphics g) / create the offscreen image if it is the first time if (image = null) image = createImage(dim.width, dim.height); offscreen = image.getGraphics(); / draw the current frame into the off-screen image / using the paint method of the superclass super.paint(offscreen); / copy the off-screen image to the screen g.drawImage(image, 0, 0, t