Stupid null pointer exception

From: Tony McGrath (fatmanmcgrath_at_hotmail.com)
Date: 10/30/04 

Date: 30 Oct 2004 05:50:41 -0700

This here is some code I have been writing as part of my final year
project in Comp Sci. It is a traffic simulator and this is a test
class to try and run a text based version of a simle straiht road.
However when i run it i only seem to get a null pointer exception the
whole time. Can anyone help. The arrray should contain objects of type
Bollox, which in this case contains informationon each cell. ie does
it contain a car or not and if so whats its speed.

Much appreciated cheers.

public class Bollox {

  Cells road1[] = new Cells[200];
  Cells road2[] = new Cells[200];

  private class Cells{

    public void Cells(){
      Cells Cell = new Cells();
    }
    boolean isEmpty;
    boolean isCar;
    int velocity = 0;
    int maxVelocity = 0;

    public void setVelocity(int vel){
      velocity = vel;
    }

    public void setEmpty(){
      isEmpty = true;
    }

    public void setNotEmpty(){
      isEmpty = false;
    }

    public void setIsCar(){
      isCar = true;
    }

    public void setIsNotCar(){
      isCar = false;
    }

    public boolean isEmpty(){
      return isEmpty = true;
    }

    public boolean isCar(){
      return isCar = true;
    }

    public int getVelocity(){
      return velocity;
    }

    public void velocity(int i){
      int x = 1;
      int temp = 0;

      if(isCar()){
        maxVelocity = 12;
      }else{
        maxVelocity = 10;
      }

       do{
        if(road1[i+x].isEmpty() && temp < maxVelocity){
          temp++;
          road2[i+x].setEmpty();
          x++;
        }else{
          if(temp < velocity){
            velocity = temp;
            break;
          }else{
            int ran = (int)(Math.random()*1);
            if(ran == 0 && velocity + 2 <= maxVelocity){
              velocity = velocity + 2;
              isEmpty = false;
              road2[i+x] = road1[i+x];
              break;
            }else if(velocity + 1 <= maxVelocity){
              velocity++;
              isEmpty = false;
              road2[i+x] = road1[i+x];
              break;
            }else{
              velocity = velocity;
              road2[i+x] = road1[i+x];
              isEmpty = false;
              break;
            }
          }
        }
      }
      while (road1[i+x].isEmpty());
    }

    public void addVehicle(){
      road1[0].setNotEmpty();
      int ran = (int)(Math.random()*8);
      if(ran == 3){
        road1[0].setIsNotCar();
        road1[0].setVelocity(7);

      }else{
        road1[0].setIsCar();
        road1[0].setVelocity(9);
      }
    }

    public void printOut(){
      System.out.println("");
      for(int i = 0; i < 200; i++){
        if(road1[i].isEmpty()){
          System.out.print(" ");
        }else{
          if(road1[i].isCar()){
            System.out.print(".");
          }else{
            System.out.print("#");
          }
        }
      }
    }

    public void swap(int i){
        road1[i] = road2[i];
    }
  }

  void run(){
    for(int i = 0; i < 200; i++){
      Cells cell1 = road1[i];
      Cells cell2 = road2[i];
      cell1.swap(i);
    }
  }

  //void swap(){
    //swap();
  //}
}

class MainC extends Thread{
  public static void main(String[] args){
   Bollox b = new Bollox();
   boolean go = true;
   while(go){
     b.run();
     try{Thread.sleep(100);}
     catch(InterruptedException e){}
   }
 }
}