issue-atomicity

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@Slf4j
public class AtomicityIssue {
    static volatile int count = 0;

    public static void main(String[] args) throws InterruptedException {
        Runnable task = () -> {
            for (int i = 0; i < 10_000; i++) {
                count++; // ❌ not atomic
            }
        };

        Thread t1 = new Thread(task, "T1");
        Thread t2 = new Thread(task, "T2");

        t1.start();
        t2.start();
        t1.join();
        t2.join();

        log.info("Final count = {}", count);
    }
}

Diagram Because count++ is three operations under the hood:

  • Read count
  • Add 1
  • Write result back

If two threads do it at the same time, they can both read the same value and overwrite each other → lost updates.

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@Slf4j
public class AtomicityIssue {
    static AtomicInteger count = new AtomicInteger(0);

    public static void main(String[] args) throws InterruptedException {
        Runnable task = () -> {
            for (int i = 0; i < 10_000; i++) {
                count.incrementAndGet(); // ✅ atomic
            }
        };

        Thread t1 = new Thread(task, "T1");
        Thread t2 = new Thread(task, "T2");

        t1.start();
        t2.start();
        t1.join();
        t2.join();

        log.info("Final count = {}", count.get());
    }
}

Diagram Always prints Final count = 20000 because AtomicInteger.incrementAndGet() is thread-safe.