Multithreading: AtomicIntegerArray updates one slot at a time

AtomicIntegerArray gives every index its own lock-free counter, so two threads bumping different (or the same) slots never lose an update. The final totals only depend on how many times each slot was incremented, not on thread scheduling.

Code

AtomicIntegerArray counts = new AtomicIntegerArray(3);
Runnable bump = () -> {
    for (int i = 0; i < 1000; i++) counts.incrementAndGet(i % 3);
};
Thread t1 = new Thread(bump);
Thread t2 = new Thread(bump);
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println("Slot 0: " + counts.get(0));
System.out.println("Slot 1: " + counts.get(1));
System.out.println("Slot 2: " + counts.get(2));

Output

Slot 0: 668
Slot 1: 666
Slot 2: 666