Java图形绘制

public void paintComponent(Graphics g)
            {
          Graphics2D g2=(Graphics2D) g;  //这行代码是什么意思，为什么在一些代码中可有可无？？？



/**
public class Geom extends JFrame{
    public Geom(){
        super("draw");
    }
    public void paint(Graphics g){
        g.setColor(Color.red);
        g.drawOval(50, 50, 10, 10);
    }
    public static void main(String[] args){
        Geom kj=new Geom();
        kj.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        kj.setSize(500, 400);
        kj.setVisible(true);
    }
}

*/


/**
class GeomFrame extends JComponent
        {
            private static final int DEFAULT_WIDTH = 400;
            private static final int DEFAULT_HEIGHT = 400;
            
            public void paintComponent(Graphics g)
            {
            Graphics2D g2=(Graphics2D) g;
            
            double leftX = 100;
            double topY = 100;
            double width = 200;
            double height = 150;
            
            Rectangle2D rect = new Rectangle2D.Double(leftX, topY, width, height);
            g2.draw(rect);
            Ellipse2D ellipse =new Ellipse2D.Double();
            ellipse.setFrame(rect);
            g2.draw(ellipse);
            
            g2.draw(new Line2D.Double(leftX, topY, width, height));
            
            double centerX = rect.getCenterX();
            double centerY = rect.getCenterY();
            double radius = 150;
            
            Ellipse2D circle = new Ellipse2D.Double();
            circle.setFrameFromCenter(centerX, centerY, centerX + radius, centerY + radius);
            g2.draw(circle);
            }
            
            public Dimension getPreferredSize(){
                return new Dimension(DEFAULT_WIDTH, DEFAULT_HEIGHT);
            }
        }

*/