HttpResponse { @NonNull public Result<String> getHeaderFieldValue(@NonNull final String field) { return absentIfNull(header.get(field.toLowerCase(US))); } private  HttpResponse(final int responseCode, @NonNull final String responseMessage,
      @NonNull final Map<String, String> header, @NonNull final byte[] body); @NonNull static HttpResponse httpResponse(final int responseCode,
      @NonNull final String responseMessage,
      @NonNull final Map<String, String> header, @NonNull final byte[] body); @NonNull byte[] getBody(); @NonNull Result<String> getBodyString(); int getResponseCode(); @NonNull String getResponseMessage(); @NonNull Result<String> getHeaderFieldValue(@NonNull final String field); @Override boolean equals(final Object o); @Override int hashCode(); @Override String toString();  }
@Test public void shouldGetAbsentForAbsentHeaderField() { assertThat(successfulHttpResponse.getHeaderFieldValue("absentfield").isAbsent(), is(true)); }