有关paintComponent会删除之前绘制的组件

最近做模拟Buffon投针，要在一个panel上模拟多次投针的动作，调用paint绘制“针”时，总是会把上次绘制的擦除掉...注释掉super.paint虽然可以防止擦除，但是整个panel的布局都会毁掉。
解决方法，使用bufferImage，每一次投针动作，都把针画到bufferImage对象中，paint bufferImage对象。

package buffonneedle;import java.awt.Color;public class BuffonPanel extends JPanel{private static final long serialVersionUID = -4803828492748385106L;BuffonSimulate bs = new BuffonSimulate();int x1=0,y1=0,x2=0,y2=0;private Image pic = createImage(1000, 500, true);private Graphics graphics = pic.getGraphics();final static public BufferedImage createImage(int width, int height,boolean flag){if (flag) {return new BufferedImage(width, height, 2);} else {return new BufferedImage(width, height, 1);}}@Overridepublic void update(Graphics g) {graphics.setColor(Color.black);int loop = BuffonSimulate.ScreenHeight/BuffonSimulate.RowHeight;for(int i=0;i<=loop;i++){graphics.drawLine(0, i*BuffonSimulate.RowHeight, BuffonSimulate.ScreenWidth, i*BuffonSimulate.RowHeight);}graphics.drawLine(x1, y1, x2, y2);g.drawImage(pic, 0, 0, null);g.dispose();}@Overridepublic void paint(Graphics g) {super.paint(g);update(g);}public boolean startSimulate(){Point[] p = bs.Simulate();boolean isCross = bs.isIntersection(p);x1 = p[0].x;y1 = p[0].y;x2 = p[1].x;y2 = p[1].y;repaint();return isCross;}}