this.columnMap = new ColumnMap();
  }

  /** An immutable map implementation backed by an indexed nullable array. */
  private abstract static class ImmutableArrayMap<K, V> extends IteratorBasedImmutableMap<K, V> {
    private final int size;

    ImmutableArrayMap(int size) {
      this.size = size;
    }

    abstract ImmutableMap<K, Integer> keyToIndex();

    // True if getValue never returns null.