Incrementing AtomicInteger in Java in 1000 threads does not generate value 1000 [duplicate]

Question

I am executing a java code where I have an AtomicInteger on which 1000 threads are trying to perform an incrementAndGet(). I was expecting the final value to be 1000. But each run is generating all sorts of different values. The code is as follows :

class task implements Runnable {
    AtomicInteger i ;
    task(AtomicInteger ai) {i =ai ;}
    public void run() { i.incrementAndGet() ; }
}

class jlt {
    public static void main(String[] args) throws Exception {
        AtomicInteger atomicInt = new AtomicInteger(0);

        ExecutorService executor = Executors.newFixedThreadPool(2);
        for(int i=1; i<=1000; i++)
            executor.submit(new task(atomicInt)) ;

        executor.shutdown() ;

        System.out.println(atomicInt.get()); // 1000 is expected 
    }
}

I am not able to figure out the mistake that I am making here or in my understanding

Answer 1

You're not waiting for the threads to finish before printing out the number.

Add a executor.awaitTermination(10, TimeUnit.SECONDS) after shutdown() and you might see something that you expected.