问java.awt.graphics中图形的自定义方法

EN

    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;
    public class Screen extends JPanel{
        Graphics g = null;
        public void createBox(int x, int y, int width, int height){
            g.fillRect(x, y, width, height);
        }
    }

    import javax.swing.*;
    public class Draw{
        public static void main(String[] args) {

            JFrame f = new JFrame("Shapes");
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            Screen s = new Screen();
            s.createBox(10,10,10,10);
            f.add(s);
            f.setSize(512, 512);
            f.setVisible(true);
        }

    }

Exception in thread "main" java.lang.NullPointerException
at Screen.createBox(Screen:java:7)
at Draw.main(Draw:java:8)