CounterConcurrentHashMap.java

public class CounterConcurrentHashMap<K>  {

    private final Map<K, MutableInteger> counters = new ConcurrentHashMap<>();

    public int up(K key) {
        MutableInteger initValue = new MutableInteger(1);
        MutableInteger oldValue = counters.put(key, initValue);

        if (oldValue != null) {
            initValue.set(oldValue.get() + 1);
        }
        return initValue.get();
    }

    public int getCount(K key) {
        return counters.getOrDefault(key, new MutableInteger(0)).get();
    }
}