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

Answer 1

From https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html#shutdown()

Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no additional effect if already shut down. This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

That means that when you call atomicInt.get() not all threads have been executed and some had no chance of incrementing the AtomicInteger.

Answer 2

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.