return fromMap.size();
    }

    @Override
    public boolean containsKey(@CheckForNull Object key) {
      return fromMap.containsKey(key);
    }

    // safe as long as the user followed the <b>Warning</b> in the javadoc
    @SuppressWarnings("unchecked")
    @Override
    @CheckForNull
    public V2 get(@CheckForNull Object key) {
      V1 value = fromMap.get(key);