### Question:
Strings { public static String toLowerCase(final String s) { if (s == null) { return null; } else { return s.toLowerCase(Locale.ENGLISH); } } private  Strings(); static boolean isNull(final String s); static boolean isEmpty(final String s); static boolean isNotEmpty(final String s); static boolean isBlank(final String s); static String trim(final String s); static String trimToNull(final String s); static String trimToEmpty(final String s); static String toLowerCase(final String s);  }### Answer:
@Test public void testToLowerCase() { final String nullString = null; final String emptyString = ""; final String blankString = " "; final String string = "A"; assertThat(Strings.toLowerCase(nullString), is(nullValue())); assertThat(Strings.toLowerCase(emptyString), is("")); assertThat(Strings.toLowerCase(blankString), is(" ")); assertThat(Strings.toLowerCase(string), is("a")); }