public <T extends @Nullable Object> T[] toArray(T[] a) {
      if (needsAllocArrays()) {
        if (a.length > 0) {
          @Nullable Object[] unsoundlyCovariantArray = a;
          unsoundlyCovariantArray[0] = null;
        }
        return a;
      }
      Map<K, V> delegate = delegateOrNull();
      return (delegate != null)
          ? delegate.keySet().toArray(a)