@SuppressWarnings("Immutable") // We don't modify this after construction.
  private final int[] rowCounts;

  @SuppressWarnings("Immutable") // We don't modify this after construction.
  private final int[] columnCounts;

  @SuppressWarnings("Immutable") // We don't modify this after construction.
  private final @Nullable V[][] values;

  // For each cell in iteration order, the index of that cell's row key in the row key list.