ex.java

    <K, V> void addToMultimap(ConcurrentHashMap<K, Set<V>> map, K key, V value) {
        map.compute(key, (k, v) -> {
            if (v == null) {
                v = ConcurrentHashMap.newKeySet();
            }
            v.add(value);
            return v;
        });
    }

    <K, V> void removeFromMultimap(ConcurrentHashMap<K, Set<V>> map, K key, V value) {
        map.computeIfPresent(key, (k, v) -> {
            v.remove(value);
            if (v.size() == 0) {
                return null;
            } else {
                return v;
            }
        });
    }

Yeah, something like this:

    <K, V> void addToMultimap(ConcurrentHashMap<K, Set<V>> map, K key, V value) {
        map.compute(key, (k, v) -> {
            if (v == null) {
                v = ConcurrentHashMap.newKeySet();
            }
            v.add(value);
            return v;
        });
    }

    <K, V> void removeFromMultimap(ConcurrentHashMap<K, Set<V>> map, K key, V value) {
        map.computeIfPresent(key, (k, v) -> {
            return (v.remove(value) && !v.isEmpty) ? v : null;
        });
    }