Multi-threading a merge sorting algorithm

I have a class that does some recursive merge sorting on a generic List, as long as the element implements Comparable. I'm trying to make the code multi-threaded to improve performance, and to do this, I have a static variable maxThreads which keeps the number of threads I create from not exploding, and I have a static variable currentThreads that keeps track of the number of threads I currently have running. There seems to be a race condition on my currentThreads variable, but I haven't been able to come up with a solution to fix it.

import java.util.ArrayList;
import java.util.List;

public class ThreadedMergeSorter<E extends Comparable<? super E>> implements, Runnable  
{
  private List<E> list;
  private List<E> left, right;
  private Thread t1, t2;
  private static final int maxThreads = 4;
  private static AtomicInteger currentThreads = new AtomicInteger(0);

  private ThreadedMergeSorter(List<E> list)
  {
    this.list = list;
  }

  public ThreadedMergeSorter(){}


  /**
   * Sorts a List<E> using the merge sorting algorithm
   * @param list the list to be merge sorted
   * @return 
   * @throws InterruptedException 
   */
  public void sort(List<E> list) 
  {
    if(list.size() > 1)
    {                  
      left = new ArrayList<E>(list.subList(0, list.size()/2));
      right = new ArrayList<E>(list.subList(list.size()/2, list.size()));

      list.clear();

      if(currentThreads.get() < maxThreads)
      {
        t1 = new Thread(new ThreadedMergeSorter<E>(left));
        t1.start();
        currentThreads.incrementAndGet();
      }
      else sort(left);

      if(currentThreads.get() < maxThreads)
      {
        t2 = new Thread(new ThreadedMergeSorter<E>(right));
        t2.start();
        currentThreads.incrementAndGet();
      }
      else sort(right);

      try{
        if(t1 != null)
        {
          t1.join();
          currentThreads.decrementAndGet();
        }
        if(t2 != null)
        {
          t2.join();
          currentThreads.decrementAndGet();
        }
      }catch(InterruptedException e){}

      list.addAll(mergeSortedLists(left, right)); 
    } 
  }

  /**
   * Merges two previously sorted List<E extends Comparable<E>> into a single List
   * @param leftArray a List of previously sorted elements
   * @param rightArray a List of previously sorted elements
   * @return an new sorted List
   */
  private List<E> mergeSortedLists(List<E> leftList, List<E> rightList)
  {
    ArrayList<E> list = new ArrayList<E>();

    while(!leftList.isEmpty() && !rightList.isEmpty())
    {
      if((leftList.get(0)).compareTo(rightList.get(0)) <= 0)
        list.add(leftList.remove(0));        
      else
        list.add(rightList.remove(0));
    }

    if(!leftList.isEmpty())
      list.addAll(leftList);
    if(!rightList.isEmpty())
      list.addAll(rightList);

    return list;
  }


  @Override
  public void run() 
  {
    sort(this.list);
  }
}

The problem is in the sort(List<E> list) method by the if statements and the try catch block.