builder.put('a', 2, "baz");
    builder.put('c', 3, "cat");
    builder.put('d', 4, "dog");
    Table<Character, Integer, String> table = builder.build();
    assertThat(table).isInstanceOf(SparseImmutableTable.class);
    validateReserialization(table);
  }

  public void testSparseSerialization_rowOrder() {
    ImmutableTable.Builder<Character, Integer, String> builder = ImmutableTable.builder();

    return (cellList.size() > (((long) rowSpace.size() * columnSpace.size()) / 2))
        ? new DenseImmutableTable<R, C, V>(cellList, rowSpace, columnSpace)
        : new SparseImmutableTable<R, C, V>(cellList, rowSpace, columnSpace);
  }

  /** @throws IllegalArgumentException if {@code existingValue} is not null. */
  /*

   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */
  @SuppressWarnings("unchecked")
  public static <R, C, V> ImmutableTable<R, C, V> of() {
    return (ImmutableTable<R, C, V>) SparseImmutableTable.EMPTY;
  }

  /** Returns an immutable table containing a single cell. */
  public static <R, C, V> ImmutableTable<R, C, V> of(R rowKey, C columnKey, V value) {