Race Condition vs Visibility problem.

Occurs when one thread updates a shared variable in its CPU cache, but another thread doesn't see the update because it's reading from its own cache.
Solution: Use volatile or synchronized.

Occurs when multiple threads access and modify shared data concurrently, and the final result depends on the timing of their execution (e.g., count++).
count++ is actually 3 operations (read, increment, write).
Solution: Use synchronized or AtomicInteger. volatile does not solve race conditions.