public class CounterAman {
    private static class Counter {
        private long nilai = 0;

        // synchronized menjamin hanya satu thread yang menjalankan
        // read-modify-write ini pada satu waktu.
        synchronized void naik() {
            nilai++;
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        Runnable tugas = () -> {
            for (int i = 0; i < 100_000; i++) {
                counter.naik();
            }
        };
        Thread a = new Thread(tugas, "teller-A");
        Thread b = new Thread(tugas, "teller-B");
        a.start();
        b.start();
        a.join();
        b.join();
        System.out.println("nilai akhir: " + counter.nilai);
    }
}