关于空指针异常····求解决··在线等

--------------------编程问答-------------------- 瞎蒙一下：
在所有组件还没画出时,是不能获得Graphics的,只会返回空.只有在JFrame全部显示后才能get到Graphics. --------------------编程问答-------------------- 用paint(Graphics g){
draw...
} --------------------编程问答-------------------- JPanel不是有void paintComponent(Graphics g)方法么。
这是我以前弄的一个绘图JPanel

class MyPanel extends JPanel

{

	public void paintComponent(Graphics g)

	{

		super.paintComponent(g);

		Graphics2D gps2D = (Graphics2D)g ;

		//图形颜色

		gps2D.setPaint(Color.BLACK);

		//背景色

		gps2D.setBackground(Color.BLUE);   //好像没成功

		//绘制矩形

		Rectangle2D r2d = new Rectangle2D.Double(leftX,topY,width,height);

		gps2D.draw(r2d);

		//绘制对角线

		Line2D l2d = new Line2D.Double(leftX,topY,leftX+width,topY+height);

		gps2D.draw(l2d);

		//绘制椭圆

		Ellipse2D e2d = new Ellipse2D.Double(leftX,topY,width,height);

		gps2D.draw(e2d);

		//绘制圆

		double radius = Point2D.distance(leftX,topY,leftX+width,topY+height)/2;

		double centX = r2d.getCenterX();

		double centY = r2d.getCenterY();

		Ellipse2D cyc = new Ellipse2D.Double();

		cyc.setFrameFromCenter(centX,centY,centX+radius,centY+radius);

		gps2D.setPaint(Color.LIGHT_GRAY);

		gps2D.draw(cyc);

		//图形填充

		gps2D.setPaint(Color.LIGHT_GRAY);

		gps2D.fill(cyc);

		gps2D.setPaint(Color.BLACK);

		gps2D.fill(r2d);

		gps2D.setPaint(Color.GRAY);

		gps2D.fill(e2d);

		//设置字体

		Font tempF = new Font("SansSerif",Font.BOLD,24);

		gps2D.setFont(tempF);

		String message = "Shape World";

		FontRenderContext cont = gps2D.getFontRenderContext();

		Rectangle2D rect = tempF.getStringBounds(message,cont);

		//高度、宽度和上坡度

		double sHeight = rect.getHeight();

		double sWidth = rect.getWidth();

		double ascent = -rect.getY();

		LineMetrics lms = tempF.getLineMetrics(message,cont);

		//下坡度和行间距

		float descent = lms.getDescent();

		float lead = lms.getLeading();

		gps2D.drawString("Shape,World",225,50);

		Rectangle2D rS2d = new Rectangle2D.Double(225,50-ascent,sWidth,sHeight);

		gps2D.draw(rS2d);

		Line2D lS2d = new Line2D.Double(225,50,225+sWidth,50);

		gps2D.draw(lS2d);

		Line2D lS2dd = new Line2D.Double(225,50+descent,225+sWidth,50+descent);

		gps2D.draw(lS2dd);

	}

	public static final	double leftX = 200;

	public static final double topY = 150;

	public static final double width =200;

	public static final double height = 100;

}

补充：Java ,  Java SE