How to make a counter thread-safe

Short answer

AtomicInteger increments the value atomically — as one indivisible operation, even with many threads:

void main() {
    var counter = new AtomicInteger();
 
    try (var pool = Executors.newFixedThreadPool(4)) {
        for (int i = 0; i < 10_000; i++) {
            pool.submit(counter::incrementAndGet);
        }
    }
    IO.println(counter.get());
}

Output:

synchronized

A synchronized block lets only one thread in at a time. It fits when several values must change together:

class Account {
    private int balance;
    private int operations;
 
    synchronized void deposit(int amount) {
        balance += amount;
        operations++;
    }
 
    synchronized String summary() {
        return balance + " in " + operations + " operations";
    }
}
 
void main() {
    var account = new Account();
    try (var pool = Executors.newFixedThreadPool(4)) {
        for (int i = 0; i < 1000; i++) {
            pool.submit(() -> account.deposit(5));
        }
    }
    IO.println(account.summary());
}

Output:

5000 in 1000 operations

Counters by key

ConcurrentHashMap.merge() safely updates a value by key:

void main() {
    var hits = new ConcurrentHashMap<String, Integer>();
    var pages = List.of("home", "blog", "home");
 
    try (var pool = Executors.newFixedThreadPool(4)) {
        for (int i = 0; i < 1000; i++) {
            String page = pages.get(i % pages.size());
            pool.submit(() -> hits.merge(page, 1, Integer::sum));
        }
    }
    IO.println(new TreeMap<>(hits));
}

Output:

{blog=333, home=667}

Common mistake

Incrementing a plain field with count++ from several threads. The operation consists of a read, an add and a write, and threads overwrite each other's results:

int count = 0;
 
void main() {
    try (var pool = Executors.newFixedThreadPool(4)) {
        for (int i = 0; i < 100_000; i++) {
            pool.submit(() -> count++);
        }
    }
    IO.println(count);
}

Output:

The result is almost always less than 100,000 and changes from run to run.

Thanks for reading!

Short answer

synchronized

Counters by key

Common mistake

Functions used in this recipe

Thanks for reading!