Problems on draw method

Question

Hi everyone,
I made a rectangle class and it extends abstract class(Shape) and implements Drawable.
However when I instantiate it and call draww method in a Frame it always creates a NullPointerException.
Why ??
import s.*;
import java.awt.*;
class Rectangle extends Shape implements Drawable {
     // properties
     double height, width;
     static int num = 0;
     // constructor
     public Rectangle (double h, double w){
          height = h;
          width = w;
          num++;     
     // methods
      public double getArea(){
          return height * width;
     public String toString(){
          return height + ", " + width;
     public void draw(Graphics g){
          setLocation(num,num + 10);     
          g.drawRect(super.x1,super.y1,10,10);     
}

Answer 1

And the code snippet that I get the exception from .
import s.*;
import java.awt.*;
class Demo {
     static Graphics gr;
     public static void main(String args[]){
          Frame f = new Frame();
          f.setBounds( 50, 50, 400, 400);
          f.setVisible( true);
          f.getGraphics();
          Shape shapes;
          shapes = new Rectangle(10,20);
          shapes.setLocation( 12,23);
          ( (Rectangle)shapes ).draw(gr);
}