What is the difference?

Question

Can you explain me what is the difference between

i.compareAndSet(i.get(), i.get() + 1)

and

int s = i.get();
int nextS = s + 1;
i.compareAndSet(s, nextS);

where i is an AtomicInteger. Am I right that first way is wrong if I want to get increment value of i? But I can't explain why.

Answer 1

The first way calls i.get() twice. Since there's no locking here, the two calls may return different values, which is probably not what you expected.

Answer 2

I want to get next value of int

Then you probably don't want compareAndSet at all, you want updateAndGet:

updated = i.updateAndGet(value -> value + 1);

Or getAndUpdate if you want the previous value before it was updated:

previous = i.getAndUpdate(value -> value + 1);