'X');
    assertEquals("[20, 15, 10, 5, 3, 2, 1]", table.columnKeySet().toString());
  }

  public void testColumnKeySet_empty() {
    table = create();
    assertEquals("[]", table.columnKeySet().toString());
  }

  public void testColumnKeySet_oneRow() {
    table = create("a", 2, 'X', "a", 1, 'X');
    assertEquals("[1, 2]", table.columnKeySet().toString());
  }

                    populateForColumnKeySet(table, elements);
                    return table.columnKeySet();
                  }
                })
            .named("ArrayTable.columnKeySet")
            .withFeatures(
                CollectionSize.ONE,
                CollectionSize.SEVERAL,
                CollectionFeature.KNOWN_ORDER,

    }

    @Override
    public Set<R> rowKeySet() {
      return fromTable.rowKeySet();
    }

    @Override
    public Set<C> columnKeySet() {
      return fromTable.columnKeySet();
    }

    @Override
    Collection<V2> createValues() {
      return Collections2.transform(fromTable.values(), function);
    }

    @Override

   * table.rowKeySet()} and the column keys in {@code table.columnKeySet()}. Calling {@link #put}
   * with other keys leads to an {@code IllegalArgumentException}.
   *
   * <p>The ordering of {@code table.rowKeySet()} and {@code table.columnKeySet()} determines the
   * row and column iteration ordering of the returned table.
   *

  }

  @WeakOuter
  private final class ColumnKeySet extends TableSet<C> {
    @Override
    public Iterator<C> iterator() {
      return createColumnKeyIterator();
    }

    @Override
    public int size() {
      return Iterators.size(iterator());
    }

    @Override
    public boolean remove(@Nullable Object obj) {

    checkNotNull(columnKey, "columnKey");
    return MoreObjects.firstNonNull(
        (ImmutableMap<R, V>) columnMap().get(columnKey), ImmutableMap.<R, V>of());
  }

  @Override
  public ImmutableSet<C> columnKeySet() {
    return columnMap().keySet();
  }

  /**
   * {@inheritDoc}
   *
   * <p>The value {@code Map<R, V>} instances in the returned map are {@link ImmutableMap} instances
   * as well.
   */

    checkNotNull(columnKey, "columnKey");
    return MoreObjects.firstNonNull(
        (ImmutableMap<R, V>) columnMap().get(columnKey), ImmutableMap.<R, V>of());
  }

  @Override
  public ImmutableSet<C> columnKeySet() {
    return columnMap().keySet();
  }

  /**
   * {@inheritDoc}
   *
   * <p>The value {@code Map<R, V>} instances in the returned map are {@link ImmutableMap} instances
   * as well.
   */

 * returns a {@link SortedMap}, instead of the {@link Set} and {@link Map} specified by the {@link
 * Table} interface.
 *
 * <p>The views returned by {@link #column}, {@link #columnKeySet()}, and {@link #columnMap()} have
 * iterators that don't support {@code remove()}. Otherwise, all optional operations are supported.
 * Null row keys, columns keys, and values are not supported.
 *

 * returns a {@link SortedMap}, instead of the {@link Set} and {@link Map} specified by the {@link
 * Table} interface.
 *
 * <p>The views returned by {@link #column}, {@link #columnKeySet()}, and {@link #columnMap()} have
 * iterators that don't support {@code remove()}. Otherwise, all optional operations are supported.
 * Null row keys, columns keys, and values are not supported.
 *

  /**
   * Returns a set of column keys that have one or more values in the table. Changes to the set will
   * update the underlying table, and vice versa.
   *
   * @return set of column keys
   */
  Set<C> columnKeySet();

  /**
   * Returns a collection of all values, which may contain duplicates. Changes to the returned
   * collection will update the underlying table, and vice versa.
   *
   * @return collection of values