gistfile1.java

import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMultiMap<K, V> {
  private final ConcurrentHashMap<K, ConcurrentHashMap<V, Boolean>> map = new ConcurrentHashMap<>();
	
	public void put(K key, V value) {
		map.putIfAbsent(key, new ConcurrentHashMap<V, Boolean>());
		map.get(key).put(value, true);
	}
}