android/guava/src/com/google/common/collect/AbstractMapBasedMultimap.java
@Override
  public Collection<V> removeAll(@NullableDecl Object key) {
    Collection<V> collection = map.remove(key);

    if (collection == null) {
      return createUnmodifiableEmptyCollection();
    }

    Collection<V> output = createCollection();
    output.addAll(collection);
    totalSize -= collection.size();
    collection.clear();

    return unmodifiableCollectionSubclass(output);
  }
@Override
  public Collection<V> removeAll(@NullableDecl Object key) {
    Collection<V> collection = map.remove(key);

    if (collection == null) {
      return createUnmodifiableEmptyCollection();
    }

    Collection<V> output = createCollection();
    output.addAll(collection);
    totalSize -= collection.size();
    collection.clear();

    return unmodifiableCollectionSubclass(output);
  }
{@inheritDoc}

<p>The returned collection is immutable.