An simple example of an *incorrect* use of Java synchronization. Synchronizing the incrementCount() method doesn't actually fix the race condition because synchronization happens on a per *object* basis and we have multiple separate objects, each running in a separate thread, so the synchronization really accomplishes nothing here.

IncorrectSynchronizedIncrement.java

public class IncorrectSynchronizedIncrement 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 IncorrectSynchronizedIncrement());
            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) {
            incrementCount();
        }
    }

    /**
     * This doesn't actually fix the problem because synchronization happens on a per *object*
     * basis and we have multiple separate objects, each running in a separate thread, so the
     * synchronization really accomplishes nothing here.
     */
    private synchronized void incrementCount() {
        ++count;
    }

}