assertEquals(INT_TO_STRING_MAP, outputMap);
  }

  /** Can't create the map if more than one value maps to the same key. */
  public void testUniqueIndexDuplicates() {
    IllegalArgumentException expected =
        assertThrows(
            IllegalArgumentException.class,
            () -> Maps.uniqueIndex(ImmutableSet.of("one", "uno"), Functions.constant(1)));