HeadersImpl implements Headers { @Override public Set<String> headerNames() { return Collections.unmodifiableSet(headers.keySet()); } private  HeadersImpl(boolean readOnly);  HeadersImpl();  HeadersImpl(Headers headers, boolean readOnly); @Override List<String> get(String headerName); @Override void add(String headerName, String value); @Override void put(String headerName, String value); @Override String getFirst(String headerName); @Override Set<String> headerNames(); @Override long getContentLength(); @Override void setContentLength(long contentLength); @Override ContentType getContentType(); @Override void setContentType(ContentType contentType);  }
@Test public void shouldReturnEmptySetOfHeadernames() { final HeadersImpl headers = new HeadersImpl(); assertEquals(emptySet(), headers.headerNames()); }