assertEquals(Optional.absent(), Optional.absent().transform(Functions.toStringFunction()));
  }

  public void testTransform_presentIdentity() {
    assertEquals(Optional.of("a"), Optional.of("a").transform(Functions.identity()));
  }

  public void testTransform_presentToString() {
    assertEquals(Optional.of("42"), Optional.of(42).transform(Functions.toStringFunction()));
  }

            .put("1", "1")
            .put("2", 2)
            .put("2", 2L)
            .build();

    ImmutableMultimap<String, Object> outputMap =
        Multimaps.index(stringToObject.values(), Functions.toStringFunction());
    assertEquals(stringToObject, outputMap);
  }

  public void testIndexIterator() {
    Multimap<String, Object> stringToObject =
        new ImmutableMultimap.Builder<String, Object>()

   * relation. That is, invoking {@link Function#apply} multiple times for a given value must return
   * equivalent results. For example, {@code
   * Equivalence.identity().onResultOf(Functions.toStringFunction())} is broken because it's not
   * guaranteed that {@link Object#toString}) always returns the same string instance.
   *
   * @since 10.0
   */

                    Table<String, Integer, Character> table = HashBasedTable.create();
                    populateForRowKeySet(table, elements);
                    return transformValues(table, Functions.toStringFunction()).column(1).keySet();
                  }
                })
            .named("TransformValues.column.keySet")
            .withFeatures(COLLECTION_FEATURES_REMOVE)
            .createTestSuite());

  }

  public void testToMultiset_empty() {
    assertThat(fluent().toMultiset()).isEmpty();
  }

  public void testToMap() {
    assertThat(fluent(1, 2, 3).toMap(Functions.toStringFunction()).entrySet())
        .containsExactly(immutableEntry(1, "1"), immutableEntry(2, "2"), immutableEntry(3, "3"))
        .inOrder();
  }

  public void testToMap_nullKey() {
    assertThrows(

   * Color red = new Color("red", 255, 0, 0);
   * ...
   * FluentIterable<Color> allColors = FluentIterable.from(ImmutableSet.of(red, green, blue));
   *
   * Map<String, Color> colorForName = allColors.uniqueIndex(toStringFunction());
   * assertThat(colorForName).containsEntry("red", red);
   * }
   *
   * <p>If your index may associate multiple values with each key, use {@link #index(Function)
   * index}.
   *

   * string forms, in a case-insensitive manner, use:
   *
   * {@snippet :
   * Ordering.from(String.CASE_INSENSITIVE_ORDER)
   *     .onResultOf(Functions.toStringFunction())
   * }
   *
   * <p><b>Java 8+ users:</b> Use {@code Comparator.comparing(function, thisComparator)} instead
   * (you can omit the comparator if it is the natural order).
   */

   * Color red = new Color("red", 255, 0, 0);
   * ...
   * Iterator<Color> allColors = ImmutableSet.of(red, green, blue).iterator();
   *
   * Map<String, Color> colorForName =
   *     uniqueIndex(allColors, toStringFunction());
   * assertThat(colorForName).containsEntry("red", red);
   * }
   *
   * <p>If your index may associate multiple values with each key, use {@link
   * Multimaps#index(Iterator, Function) Multimaps.index}.

   * Color red = new Color("red", 255, 0, 0);
   * ...
   * Iterator<Color> allColors = ImmutableSet.of(red, green, blue).iterator();
   *
   * Map<String, Color> colorForName =
   *     uniqueIndex(allColors, toStringFunction());
   * assertThat(colorForName).containsEntry("red", red);
   * }
   *
   * <p>If your index may associate multiple values with each key, use {@link
   * Multimaps#index(Iterator, Function) Multimaps.index}.