Swing Graphics Not Working In My Line Algorithm?

Swing Graphics Not Working In My Line Algorithm?

I'm trying to utilize swing graphics in order to gauge Bresenham's
algorithm against a less polished solution (I haven't implemented the
timers yet). As things stand, there are no errors when compiling, and it
throws a NullPointer exception at basic, drawthoselines, and main. The
idea is that the lines will appear in the JFrame, but they don't. It's
just a blank frame. I know I have everything set to static, but I get a
lot of errors otherwise.
I'm a novice and I would be grateful to anyone who could provide a
solution and an explanation.
import java.awt.*;
import javax.swing.*;
public class lines extends JPanel {
static int deltaX;
static int deltaY;
static int DY2;
static int DX2;
static int Di;
public static void main (String[] args) {
JFrame f = new JFrame("Line vs Line");
f.pack();
f.setVisible(true);
f.setSize(300,300);
f.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
JPanel p = new JPanel();
f.getContentPane().add(p);
Graphics g = null;
drawthoselines(g);
}
public static void basic(int x1, int y1, int x2, int y2, Graphics g){
int deltaX = x2-x1;
int deltaY = y2-y1;
float m = (float)deltaY/(float)deltaX;
float c = y1 - (m*x1);
for (int x=x1; x<x2; x++){
float floatY = (m*x) + c;
int y = Math.round(floatY);
g.drawLine(x,y,x,y);
}
}
public static void brz(int x1, int y1, int x2, int y2, Graphics g){
deltaX = x2-x1;
deltaY = y2-y1;
DY2 = 2* deltaY;
DX2 = 2* deltaX;
Di = DY2 - deltaX;
int x = x1;
int y = y1;
int prevy;
while (x<x2) {
x++;
prevy = y;
if (Di > 0){
y++;
}
g.drawLine(x,y,x,y);
Di = Di + DY2 - (DX2 * (y - prevy));
}
}
public static void drawthoselines(Graphics g){
basic(10,10,40,30,g);
basic(10,10,40,90,g);
brz(50,50,150,60,g);
brz(50,50,150,120,g);
brz(50,50,150,140,g);
}
}