* 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.

   * 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.

  }

  @SuppressWarnings("nullness") // test of a bogus call
  public void testToStringFunction_apply() {
    assertThat(Functions.toStringFunction().apply(3)).isEqualTo("3");
    assertThat(Functions.toStringFunction().apply("hiya")).isEqualTo("hiya");
    assertThat(
            Functions.toStringFunction()
                .apply(
                    new Object() {
                      @Override

  }

  @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

  }

  @SuppressWarnings("nullness") // test of a bogus call
  public void testToStringFunction_apply() {
    assertThat(Functions.toStringFunction().apply(3)).isEqualTo("3");
    assertThat(Functions.toStringFunction().apply("hiya")).isEqualTo("hiya");
    assertThat(
            Functions.toStringFunction()
                .apply(
                    new Object() {
                      @Override

  }

  @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

   * 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}.
   *

    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>()