guava/src/com/google/common/collect/AbstractMapBasedMultimap.java
@Override
  public Collection<V> get(@Nullable K key) {
    Collection<V> collection = map.get(key);
    if (collection == null) {
      collection = createCollection(key);
    }
    return wrapCollection(key, collection);
  }
@Override
  public Collection<V> get(@Nullable K key) {
    Collection<V> collection = map.get(key);
    if (collection == null) {
      collection = createCollection(key);
    }
    return wrapCollection(key, collection);
  }
{@inheritDoc}

<p>The returned collection is not serializable.