Problem with threads, program always crash

Question

new to threads, may be doing something COMPLETELY wrong. When I run the program I get a couple of NullPointerExceptions in Thread-0 and Thread-1. I'm confused because it screws up when I call size() for arrayList, and in the API size doesn't throw anything.
public class ProducerConsumerRunner
   public static void main(String args[])
      Queue q = new Queue(QUEUE_MAX_SIZE);
      ProducerRunnable producer = new ProducerRunnable(q, ITERATIONS);
      ConsumerRunnable consumer = new ConsumerRunnable(q, ITERATIONS);
      //works good up to here
      Thread t1 = new Thread(producer);
      Thread t2 = new Thread(consumer);
      t1.start();
      t2.start();
   private static int ITERATIONS = 50;
   private static int QUEUE_MAX_SIZE = 25;
import java.util.ArrayList;
import java.util.concurrent.locks.ReentrantLock;
public class Queue {
     public Queue(int maxSize)
          list = new ReentrantLock();
          underLimit = list.newCondition();
          MAX_SIZE = maxSize;
     public void add(String line)
          list.lock();
          try{
                       System.out.println("add method of Queue");
               while(record.size() >= MAX_SIZE){//things get screwed up when this condition is evaluated
                    //size method is crashing the program
                    System.out.println("2");
                    underLimit.await();
                    System.out.println("3");
               record.add(line);
          }catch(java.lang.InterruptedException e){
               System.out.println("await interupted: "+e.getMessage());
          }finally{
               list.unlock();
     public void remove(int line)
          list.lock();
          record.remove(line);
          list.unlock();
     public  ArrayList<String> record;
     private final ReentrantLock list;
     private final java.util.concurrent.locks.Condition underLimit;
     private final int MAX_SIZE;
import java.util.Date;
public class ProducerRunnable implements Runnable{
     public ProducerRunnable(Queue q, int itterations)
          this.q = q;
          ITTERATIONS = itterations;
     public void run()
          String date;
          for(int i = 0; i<ITTERATIONS; i++)
               date = new Date().toString();
               q.add(date);//this is where it screws up
               System.out.println(date+" added");
               try{
               Thread.sleep(100);
               }catch(java.lang.InterruptedException e){
                    System.out.println("ProduccerRunnable's sleep was interupted");
     private final Queue q;
     private final int ITTERATIONS;
public class ConsumerRunnable implements Runnable{
     public ConsumerRunnable(Queue q, int itterations)
          this.q = q;
          ITTERATIONS = itterations;
     public void run()
          //Queue q = new Queue();
          int length;
          for(int i = 0; i<ITTERATIONS; i++)
               length = q.record.size();//things are getting screwed up here
               while(q.record.get(length) == null)
                    length--;
               System.out.println((String)q.record.get(length));
               q.remove(length);
               try{
               Thread.sleep(100);
               }catch(java.lang.InterruptedException e){
                    System.out.println("ConsumerRunnable's sleep was interupted");
     private final Queue q;
     private final int ITTERATIONS;
}

Answer 1

question:
this works right
     public void remove()
          list.lock();
          int line = 0;
          try{
               *while(record.size() < 1)*
                    range.await();
               record.remove(line);
               range.signalAll();
          }catch(java.lang.InterruptedException e){
               System.out.println("await interupted: "+e.getMessage());
          }finally{
               list.unlock();
     }this is deadlock
*int list = 0;*
*while((list = record.size)<1) was screwing things up because of the assignment*record is ArrayList of strings. when better to use Vector instead of ArrayList? i notice no differnce and i use threads
initially I thought record.size was being evaluated and before could be assigned to list, time slice ran out. this could not be case because I use ReentrantLock (list) to lock it.
Edited by: bean-planet on Apr 1, 2009 4:31 PM