### Question:
HttpHeadersImpl implements HttpHeaders { public Locale getLanguage() { List<String> values = getListValues(HttpHeaders.CONTENT_LANGUAGE); return values.isEmpty() ? null : HttpUtils.getLocale(values.get(0).trim()); } HttpHeadersImpl(Message message); List<MediaType> getAcceptableMediaTypes(); Map<String, Cookie> getCookies(); Locale getLanguage(); MediaType getMediaType(); MultivaluedMap<String, String> getRequestHeaders(); List<Locale> getAcceptableLanguages(); List<String> getRequestHeader(String name); Date getDate(); String getHeaderString(String headerName); int getLength();  }### Answer:
@Test public void testGetNullLanguage() throws Exception { Message m = createMessage(createHeaders()); HttpHeaders h = new HttpHeadersImpl(m); assertNull(h.getLanguage()); }
@Test public void testGetLanguage() throws Exception { Message m = createMessage(createHeader(HttpHeaders.CONTENT_LANGUAGE, "en-US")); HttpHeaders h = new HttpHeadersImpl(m); assertEquals("en_US", h.getLanguage().toString()); }