Atomic variables

AtomicInteger is supposed to be thread-safe.

As per my understanding of atomic integers, it works on the principle of Compare and Swap algorithm.

Compare and swap is atomic at the CPU level.

You can implement an increment operation explicitly with compare and swap:

int value;
do { 
  value = var.get();
} while (!var.compareAndSwap(value, value + 1));

The compareAndSwap is guaranteed by the CPU to be atomic (there will be a native implementation of this).

If two threads hit this compareAndSwap at the same time, only one of them will "win", receiving true as the result of the compareAndSwap call, so the loop stops.

The other thread will "lose", and receive false as the result, and so will go round the loop again: it reads a new value, and then tries the CAS again. If this succeeds (because either no other thread was trying to do it at the same time, or it "won" over another thread), the loop stops; otherwise, it just tries again.