Why Volatile doesn't work when the value of a field depends on its previous value

"Doesn't work" is a pretty vague claim, but I suspect that the point is that if you have two threads each executing a loop with this code:

field = field + 1

... then even when it's volatile, each iteration is effectively:

1. Read
2. Perform computation with value we've read
3. Write

If one thread performs the read, then another thread performs a write, then by the time the first thread performs its write, it will effectively be ignoring the second thread's write.

So going back to our example, if you have two threads each executing a million iterations of field = field + 1, the total increase in the value of field may not be two million.