*
   * @param valueClass class of values stored in the returned array
   */
  @GwtIncompatible // reflection
  public @Nullable V[][] toArray(Class<V> valueClass) {
    @SuppressWarnings("unchecked") // TODO: safe?
    @Nullable
    V[][] copy = (@Nullable V[][]) Array.newInstance(valueClass, rowList.size(), columnList.size());
    for (int i = 0; i < rowList.size(); i++) {