table.put("one", 'a', 1);
    table.put("two", 'a', 2);
    table.put("three", 'a', 3);
    table.put("four", 'b', 4);
    return Tables.unmodifiableTable(table).column('a');
  }

    return delegate().cellSet();
  }

  @Override
  public void clear() {
    delegate().clear();
  }

  @Override
  public Map<R, V> column(@ParametricNullness C columnKey) {
    return delegate().column(columnKey);
  }

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

  @Override
  public Map<C, Map<R, V>> columnMap() {

 *
 * <p>The allowed row and column keys must be supplied when the table is created. The table always
 * contains a mapping for every row key / column pair. The value corresponding to a given row and
 * column is null unless another value is provided.
 *
 * <p>The table's size is constant: the product of the number of supplied row keys and the number of

 * 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.
 *
 * <p>Lookups by row key are often faster than lookups by column key, because the data is stored in

   * @return the corresponding map from column keys to values
   */
  Map<C, V> row(@ParametricNullness R rowKey);

  /**
   * Returns a view of all mappings that have the given column key. For each row key / column key /
   * value mapping in the table with that column key, the returned map associates the row key with
   * the value. If no mappings in the table have the provided column key, an empty map is returned.
   *

    return delegate().cellSet();
  }

  @Override
  public void clear() {
    delegate().clear();
  }

  @Override
  public Map<R, V> column(@ParametricNullness C columnKey) {
    return delegate().column(columnKey);
  }

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

  @Override
  public Map<C, Map<R, V>> columnMap() {

    assertEquals(ImmutableMap.of("foo", 'a', "bar", 'b'), table.column(1));
  }

  // This test assumes that the implementation does not support null keys.
  public void testColumnNull() {
    table = create("foo", 1, 'a', "bar", 1, 'b', "foo", 3, 'c');
    try {
      table.column(null);
      fail();
    } catch (NullPointerException expected) {
    }
  }

    assertEquals(ImmutableMap.of("foo", 'a', "bar", 'b'), table.column(1));
  }

  // This test assumes that the implementation does not support null keys.
  public void testColumnNull() {
    table = create("foo", 1, 'a', "bar", 1, 'b', "foo", 3, 'c');
    try {
      table.column(null);
      fail();
    } catch (NullPointerException expected) {
    }
  }

 *
 * <p>The allowed row and column keys must be supplied when the table is created. The table always
 * contains a mapping for every row key / column pair. The value corresponding to a given row and
 * column is null unless another value is provided.
 *
 * <p>The table's size is constant: the product of the number of supplied row keys and the number of

    public Set<Cell<R, C, V>> cellSet() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.cellSet();
    }

    @Override
    public Map<R, V> column(C columnKey) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.column(columnKey);
    }

    @Override
    public Set<C> columnKeySet() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.columnKeySet();
    }