checkCanReserializeSingleton(Functions.identity());
  }

  public void testToStringFunction_apply() {
    assertEquals("3", Functions.toStringFunction().apply(3));
    assertEquals("hiya", Functions.toStringFunction().apply("hiya"));
    assertEquals(
        "I'm a string",
        Functions.toStringFunction()
            .apply(
                new Object() {
                  @Override
                  public String toString() {

  }

  @Override
  protected Map<String, String> makePopulatedMap() {
    Map<String, Integer> underlying = new HashMap<>();
    underlying.put("a", 1);
    underlying.put("b", 2);
    underlying.put("c", 3);
    return transformValues(
        new UnmodifiableIteratorMap<String, Integer>(underlying), Functions.toStringFunction());
  }

  @Override

    checkCanReserializeSingleton(Functions.identity());
  }

  public void testToStringFunction_apply() {
    assertEquals("3", Functions.toStringFunction().apply(3));
    assertEquals("hiya", Functions.toStringFunction().apply("hiya"));
    assertEquals(
        "I'm a string",
        Functions.toStringFunction()
            .apply(
                new Object() {
                  @Override
                  public String toString() {

   * the JDK {@link java.util.function.Function} class, see {@link Function}.
   */
  public static Function<Object, String> toStringFunction() {
    return ToStringFunction.INSTANCE;
  }

  // enum singleton pattern
  private enum ToStringFunction implements Function<Object, String> {
    INSTANCE;

    @Override
    public String apply(Object o) {
      checkNotNull(o); // eager for GWT.

                    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());

   * 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
   */

   * 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}.
   *

   * 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}.
   *