### Question:
Strings { public static String trimToNull(final String s) { final String trimmed = trim(s); if (trimmed == null || trimmed.isEmpty()) { return null; } else { return trimmed; } } 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 testTrimToNull() { final String nullString = null; final String emptyString = ""; final String blankString = " "; final String string = " a "; assertThat(Strings.trimToNull(nullString), is(nullValue())); assertThat(Strings.trimToNull(emptyString), is(nullValue())); assertThat(Strings.trimToNull(blankString), is(nullValue())); assertThat(Strings.trimToNull(string), is("a")); }