### Question:
ResourceUtils { public static String formatTag(String eTag) { if (eTag == null) { return null; } if (!eTag.matches(QUOTED_STRING_REGEX)) { throw new IllegalArgumentException("tag must matches to regexp '" + QUOTED_STRING_REGEX + '\''); } return '\"' + eTag + '\"'; } private  ResourceUtils(); static String formatWeakTag(String eTag); static String formatTag(String eTag); static boolean matchTag(String eTag, String eTagHeaderValue); static long millisToSecond(long millisecond); static long secondToMillis(long second);  }### Answer:
@Test public void testFormatTag() throws Exception { assertNull(ResourceUtils.formatTag(null)); assertEquals("\"123-456789\"", ResourceUtils.formatTag("123-456789")); try { ResourceUtils.formatTag("123\"456789"); fail(); } catch (IllegalArgumentException e) { } }