ConcurrentHashMap — Thread-Safe Map

import java.util.concurrent.*;

class Counter {
    private final ConcurrentHashMap<String, Long> counts = new ConcurrentHashMap<>();

    public void increment(String key) {
        counts.merge(key, 1L, Long::sum);      // atomic — no separate get/put race
    }

    public Long get(String key) {
        return counts.getOrDefault(key, 0L);
    }

    // compute — atomic read-modify-write with the supplied function
    public void touchLastSeen(String key, long now) {
        counts.compute(key, (k, v) -> (v == null) ? now : Math.max(v, now));
    }

    public static void main(String[] args) throws Exception {
        var c = new Counter();
        var pool = java.util.concurrent.Executors.newFixedThreadPool(8);
        for (int i = 0; i < 8; i++) {
            pool.submit(() -> {
                for (int j = 0; j < 100_000; j++) c.increment("hits");
            });
        }
        pool.shutdown();
        pool.awaitTermination(1, TimeUnit.MINUTES);
        System.out.println(c.get("hits"));     // 800000
    }
}