Can you see the error ?

Question

Here is the code (a few lines removed) :
public class Alone{
    static final int PACKET_SIZE = 8192;
    public static int nbPackets = 0;
    Vector[] queue;
    Thread[] thread;
    boolean run;
    int id;
    public Alone(int port) throws Exception{
     queue = new Vector[NB_OF_QUEUES];
     for(int i=0;i<NB_OF_QUEUES;i++){
         queue[i] = new Vector();
     thread = new Thread[NB_OF_THREADS];
     thread[T_RECEIVE] = new Receive(this);
    public DatagramPacket getPacket(int q){
     DatagramPacket dp;
     if(q==FREE && queue[FREE].isEmpty()){
         nbPackets++;
         dp = new DatagramPacket(new byte[PACKET_SIZE],PACKET_SIZE);
     }else{
         synchronized(queue[q]){
          dp = (DatagramPacket)queue[q].firstElement();
          queue[q].remove(0);
     return dp;
    }I'm calling the "getPacket" method from a thread which receives DatagramPacket through a DatagramSocket this way :
public class Receive extends Thread {
    private Alone alone;
    public Receive(Alone c){
     alone = c;
     setName("Receive");
    public void run(){
     DatagramPacket dp = alone.getPacket(Alone.FREE);
     try{
         while(alone.getRun()){
          System.out.println(getName() +":sleeping");
          alone.socket.receive(dp);
          System.out.println(getName() +":wake up");
          System.out.println(getName() +" >>> packet received");
          alone.addPacket(Alone.RECEIVED, dp, Alone.T_REDIRECT);
     }catch(Exception e){
         System.out.println(e);
}But when it gets to the "alone.socket.receive(dp);" it raises a "java.lang.NullPointerException". So i guess theres is a problem in the passing of the DatagramPacket's reference, but I just can't point out where the error is. Anyone see the problem ?

Answer 1

Okay thanks for the advice ! i'll follow it from now on.
//in class Receive
    public void run(){
     DatagramPacket dp = alone.getPacket(Alone.FREE);
     while(alone.getRun()){
         System.out.println(getName() +":sleeping");
         try{
          alone.socket.receive(dp);
         }catch(IOException e){e.printStackTrace();}
         System.out.println(getName() +":wake up");
         System.out.println(getName() +" >>> packet received");
         alone.addPacket(Alone.RECEIVED, dp, Alone.T_REDIRECT);
//in class Alone
    public DatagramPacket getPacket(int q){
     DatagramPacket dp;
     if(q==FREE && queue[FREE].isEmpty()){
         System.out.println("instanciating a new DatagramPacket");
         nbPackets++;
         dp = new DatagramPacket(new byte[PACKET_SIZE],PACKET_SIZE);
     }else{
         synchronized(queue[q]){
          dp = (DatagramPacket)queue[q].firstElement();
          queue[q].remove(0);
     return dp;
    }running this will prompt
instanciating a new DatagramPacket
Receive:sleeping
java.lang.NullPointerException
        at alone.Receive.run(Receive.java:19)