从番石榴（谷歌）Multimap 中删除永远不会删除密钥本身。 为什么？ 怎么做？

Question

I'm using the google collections library from guava, I believe the most recent version.

I find that once I remove the final (K, V) pair from the map for a given value of K, the map still contains an entry for K, where V is an empty collection.

I would rather have the map not contain this entry. Why can't I remove it? Or, if I can, how?

It's probably something simple that I have missed. Here is a code example. Thanks.

    // A plain ordinary map.
    Map<Integer, Integer> hm = new HashMap<Integer, Integer>();
    hm.put(1, 2);
    hm.remove(1);
    // Value of key 1 in HashMap: null
    System.out.println("Value of key 1 in HashMap: " + hm.get(1));

    // A list multimap.
    ListMultimap<Integer, Integer> lmm = ArrayListMultimap.<Integer, Integer> create();
    lmm.put(1, 2);
    lmm.remove(1, 2);
    // Value of key 1 in ArrayListMultiMap: []
    System.out.println("Value of key 1 in ArrayListMultiMap: " + lmm.get(1));

    // A set multimap.
    SetMultimap<Integer, Integer> smm = HashMultimap.<Integer, Integer> create();
    smm.put(1, 2);
    smm.remove(1, 2);
    // Value of key 1 in HashMultimap: []
    System.out.println("Value of key 1 in HashMultimap: " + smm.get(1));

Answer 1

Actually when you remove the last value for a key in the multimap, the key is removed from the map. See for instance the behaviour of 'containsKey'

System.out.println("ListMultimap contains key 1? " + lmm.containsKey(1));

However when you get the values from the multimap, if there is no collection associated with the key, it will return an empty collection, see the implementation of get in AbstractMultimap:

/**
 * {@inheritDoc}
 *
 * <p>The returned collection is not serializable.
 */
@Override
public Collection<V> get(@Nullable K key) {
  Collection<V> collection = map.get(key);
  if (collection == null) {
    collection = createCollection(key);
  }
  return wrapCollection(key, collection);
}

Answer 2

To totally remove the underlying entry from the Multimap , you need to use the Map view:

multimap.asMap().remove(key);