How would you ensure atomicity without using the synchronized keyword?

Question

Accepted Answer

Atomicity ensures that an operation is completed as a single, indivisible unit of work without interference from other threads. When synchronization is not desired, Java provides lock-free mechanisms through atomic classes that perform thread-safe operations using low-level CPU instructions such as Compare-And-Swap (CAS). These classes help achieve high concurrency with better performance than traditional locking in many scenarios.

Key Points:
• Atomic classes such as AtomicInteger, AtomicLong, and AtomicReference provide thread-safe operations without using synchronized.
• They use CAS (Compare-And-Swap) internally to update values atomically and avoid race conditions.
• Lock-free operations generally offer better scalability and lower contention in highly concurrent applications.

Example:
Consider a website visitor counter being updated by thousands of concurrent requests. Using AtomicInteger allows multiple threads to increment the counter safely without acquiring explicit locks.

Code Example:
```
import java.util.concurrent.atomic.AtomicInteger;

public class VisitorCounter {

private static final AtomicInteger counter =
            new AtomicInteger(0);

public static void main(String[] args) {

counter.incrementAndGet();

System.out.println(
                "Visitors: "
                + counter.get());
    }
}
```
Interview Tip:
A concise interview answer is: To achieve atomicity without synchronized, I use atomic classes from java.util.concurrent.atomic such as AtomicInteger or AtomicReference. These classes rely on CAS operations to perform thread-safe updates without locking, providing better performance and scalability in concurrent applications.