Non Blocking Stack - AtomicReference

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;

class Demonstration {
  public static void main(String[] args) throws Exception {
    Stack<Integer> stack = new Stack<Integer>();
    ExecutorService executor = Executors.newFixedThreadPool(20);
    int numThreads = 2;
    CyclicBarrier barrier = new CyclicBarrier(numThreads);
    try {
      for (int i = 0; i < numThreads; i++) {
        executor.submit(
            new Runnable() {
              @Override
              public void run() {
                for (int j = 0; j < 1000; j++) {
                  stack.push(j);
                }
                try {
                  barrier.await();
                } catch (InterruptedException e) {

                } catch (BrokenBarrierException e) {

                }

                for (int j = 0; j < 1000; j++) {
                  stack.pop();
                }
              }
            });
      }
    } finally {
      executor.shutdown();
      executor.awaitTermination(10, TimeUnit.SECONDS);
    }
  }
}

class Stack<T> {
  private AtomicReference<StackNode<T>> topNode = new AtomicReference<StackNode<T>>(null);

  public void push(T item) {
    StackNode<T> node, currentTop;

    do {
      node = new StackNode<T>(item);
      currentTop = topNode.get();
      if (currentTop != null) {
        node.setNext(currentTop);
      }
    } while (!topNode.compareAndSet(currentTop, node));
  }

  public T pop() {
    if (topNode.get() == null) {
      return null;
    }
    StackNode<T> currentTop;
    StackNode<T> newTop;
    do {
      currentTop = topNode.get();
      if (currentTop == null) {
        return null;
      }
      newTop = currentTop.getNext();
    } while (!topNode.compareAndSet(currentTop, newTop));

    return currentTop.getValue();
  }
}

class StackNode<T> {
  private T value;
  StackNode<T> next;

  public StackNode(T value) {
    this.value = value;
  }

  public void setNext(StackNode<T> next) {
    this.next = next;
  }

  public T getValue() {
    return this.value;
  }

  public StackNode<T> getNext() {
    return this.next;
  }
}