android/guava/src/com/google/common/collect/AbstractMapBasedMultimap.java
final void setMap(Map<K, Collection<V>> map) {
    this.map = map;
    totalSize = 0;
    for (Collection<V> values : map.values()) {
      checkArgument(!values.isEmpty());
      totalSize += values.size();
    }
  }
final void setMap(Map<K, Collection<V>> map) {
    this.map = map;
    totalSize = 0;
    for (Collection<V> values : map.values()) {
      checkArgument(!values.isEmpty());
      totalSize += values.size();
    }
  }
Used during deserialization only.