What guarantees does volatile provide? How does it differ from Atomic?

The volatile keyword in Java guarantees that reading and writing a variable will occur directly from main memory, not from the processor cache, ensuring visibility of changes to the variable between threads. This means that if one thread changes the value of a volatile variable, other threads will immediately see this change.

However, volatile does not guarantee atomicity of operations. For example, incrementing a volatile int is not atomic, as it involves reading, modifying, and writing.

The Atomic class (e.g., AtomicInteger) provides atomic operations on variables, such as incrementAndGet(), which guarantee that operations are performed entirely without interruption and conflicts between threads. Additionally, Atomic uses low-level mechanisms like CAS (Compare-And-Swap) for efficient synchronization.

In summary:

• volatile guarantees visibility of changes between threads but not atomicity.
• Atomic provides atomicity of operations and visibility.

Example:

volatile int counter;

// Non-atomic:
counter++;

AtomicInteger atomicCounter = new AtomicInteger(0);

// Atomic:
atomicCounter.incrementAndGet();