### Question:
StringUtils { public static String toLowerCase(String s) { return Optional.ofNullable(s) .map(String::toLowerCase) .orElse(null); }  static String toLowerCase(String s); static String join(CharSequence delimiter, Iterable<?> elements);  }### Answer:
@Test public void testToLowerCase() { assertThat(StringUtils.toLowerCase("foO")).isEqualTo("foo"); assertThat(StringUtils.toLowerCase(null)).isNull(); }