使用canvas实现数据过多时候的下拉

package ui; import java.util.Vector; import javax.microedition.lcdui.Canvas; import javax.microedition.lcdui.Font; import javax.microedition.lcdui.Graphics; public class MenuCanvas extends Canvas { private int width; //屏幕宽度 private int height; //屏幕高度 private int fontHeight; //文本高度 private static int paddingHeight = 5; //文本间隔高度 private String words = "经贸学院东"; private Vector vector = new Vector(); private int lineNumber = 0; //行号 private Font font = Font.getFont(Font.FACE_MONOSPACE, Font.STYLE_BOLD, Font.SIZE_LARGE); private int xxx; public MenuCanvas() { width = this.getWidth(); height = this.getHeight(); fontHeight = font.getHeight(); for (int i=0; i<20; i++) { vector.addElement(words + i); } xxx = width / (fontHeight + paddingHeight); } protected void paint(Graphics g) { g.setColor(255, 255, 255); g.fillRect(0, 0, width, height); int x = lineNumber; for (int i=0; i<20 && x < vector.size() ; i++,x++) { g.setColor(00, 00, 00); g.setFont(font); g.drawString((String)vector.elementAt(x), 0, (fontHeight + paddingHeight) * i , Graphics.TOP|Graphics.LEFT); } } protected void keyPressed(int keyCode) { if (getGameAction(keyCode) == Canvas.DOWN) { System.out.println(width +"/" + fontHeight + paddingHeight + "=" +xxx); if (lineNumber < vector.size() && vector.size() - lineNumber > xxx) { lineNumber += 1; System.out.println("i pressed it"); this.repaint(); } } else if (getGameAction(keyCode) == Canvas.UP) { if (lineNumber > 0) { lineNumber -= 1; System.out.println("i pressed it"); this.repaint(); } } } }  

package ui; import javax.microedition.lcdui.Canvas; import javax.microedition.lcdui.Display; import javax.microedition.midlet.MIDlet; import javax.microedition.midlet.MIDletStateChangeException; public class Screen extends MIDlet { private Display display; private Canvas canvas; public Screen() { display = Display.getDisplay(this); canvas = new MenuCanvas(); } protected void destroyApp(boolean arg0) throws MIDletStateChangeException { } protected void pauseApp() { } protected void startApp() throws MIDletStateChangeException { display.setCurrent(canvas); } }