how to create a Graphics object?

I am trying to create a small animation applet which requires to generate a Graphics object dynamically. I am using getGraphics() method on a Canvas object and then painting on that but when I run my program it says NullPointerException.

I don't know why the method getGraphics() is not returning any Graphics object. A part of my code is below.

class Fan extends Thread
   {
        private int r,x,y,deg;
        private Canvas myfan;
        public boolean direction;

        Fan(int new_r, int new_x, int new_y, int new_deg, Canvas C)
        {
                this.r=new_r;
                this.x=new_x;
                this.y=new_y;
                this.deg=new_deg;
                this.myfan=C;
                this.direction = true;
            
        }// constructor

        public void draw()
        {
                int rx, ry;
                Graphics g = myfan.getGraphics();
            if (myfan.getGraphics()==null)
                 System.out.println("error");
                rx=(int) (r*Math.cos(deg));
                ry=(int) (r*Math.sin(deg));
                g.drawLine(x-rx,y-ry,x+rx,y+ry);
                g.fillOval(x-rx-3,y-ry-3,6,6);
                g.fillOval(x-10,y-10,20,20);
        }


I appreciate your help.

Thank you,
Kunala