### Question:
HttpHeadersImpl implements HttpHeaders { public String getHeaderString(String headerName) { return HttpUtils.getHeaderString(headers.get(headerName)); } 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 testGetHeaderString() throws Exception { Message m = createMessage(createHeaders()); HttpHeaders h = new HttpHeadersImpl(m); String date = h.getHeaderString("Date"); assertEquals("Tue, 21 Oct 2008 17:00:00 GMT", date); }
@Test public void testGetHeaderString2() throws Exception { Message m = createMessage(createHeaders()); HttpHeaders h = new HttpHeadersImpl(m); String date = h.getHeaderString("a"); assertEquals("1,2", date); }