The Thread-Safe Counter

Implement inside Solution: - static class RacyCounter with void inc() (count++ on a plain field) and long value(). - static class SafeCounter with the same API backed by an AtomicLong. - static long hammer(Object counter, int threads, int perThread): starts threads threads, each calling inc() perThread times via reflection (RacyCounter or SafeCounter), joins them, then returns value() via reflection. With hammer(c, 4, 1000): RacyCounter usually loses updates (< 4000); SafeCounter must ALWAYS return exactly 4000. The safe test runs the hammer five times to make flakiness visible.

Difficulty: intermediate