Atomic operations are performed in a single unit of task without interference from other operations. Atomic operations are necessity in multi-threaded environment to avoid data inconsistency.
int++ is not an atomic operation. So by the time one thread read its value and increment it by one, another thread has read the older value leading to the wrong result.
To solve this issue, we will have to make sure that increment operation on count is atomic, we can do that using Synchronization but Java 5 java.util.concurrent.atomic provides wrapper classes for int and long that can be used to achieve this atomically without the usage of Synchronization. Go to this article to learn more about atomic concurrent classes.
