A simple example that illustrates the existence of race conditions when using Java Threads. If you run this code on a multi-core box you're likely to get a lower count (potentially much lower) than you might naively expect.

SimpleIncrement.java

public class SimpleIncrement implements Runnable {

    private static final int NUM_THREADS = 4;
    private static final int NUM_INCREMENTS = 10000;
    private static int count = 0;

    public static void main(String[] args) throws InterruptedException {
        Thread[] threads = new Thread[NUM_THREADS];
        
        for (int i=0; i<NUM_THREADS; ++i) {
            threads[i] = new Thread(new SimpleIncrement());
            threads[i].start();
        }
        
        for (int i=0; i<NUM_THREADS; ++i) {
            threads[i].join();
        }
        
        System.out.println("total count = " + count + " vs. expected = " + (NUM_THREADS * NUM_INCREMENTS));
    }

    @Override
    public void run() {
        for (int i=0; i<NUM_INCREMENTS; ++i) {
            ++count;
        }
    }

}