### Question:
ResponseImpl extends Response { @Override public String getHeaderString(String name) { List<Object> headers = getMetadata().get(name); if (headers == null) { return null; } List<String> headerStrings = headers.stream().map(HeaderHelper::getHeaderAsString).collect(toList()); return HeaderHelper.convertToString(headerStrings); } protected  ResponseImpl(int status, Object entity, Annotation[] entityAnnotations, MultivaluedMap<String, Object> headers); @Override Object getEntity(); Annotation[] getEntityAnnotations(); @Override T readEntity(Class<T> entityType); @SuppressWarnings("unchecked") @Override T readEntity(GenericType<T> entityType); @Override T readEntity(Class<T> entityType, Annotation[] annotations); @SuppressWarnings("unchecked") @Override T readEntity(GenericType<T> entityType, Annotation[] annotations); @Override boolean hasEntity(); @Override boolean bufferEntity(); @Override void close(); boolean isClosed(); @Override MediaType getMediaType(); @Override Locale getLanguage(); @Override int getLength(); @Override Set<String> getAllowedMethods(); @Override Map<String, NewCookie> getCookies(); @Override EntityTag getEntityTag(); @Override Date getDate(); @Override Date getLastModified(); @Override URI getLocation(); @Override Set<Link> getLinks(); @Override boolean hasLink(String relation); @Override Link getLink(String relation); @Override Link.Builder getLinkBuilder(String relation); @Override MultivaluedMap<String, Object> getMetadata(); @Override MultivaluedMap<String, String> getStringHeaders(); @Override String getHeaderString(String name); @Override int getStatus(); @Override StatusType getStatusInfo(); @Override String toString();  }### Answer:
@Test public void getsNullIfHeaderNotExist() throws Exception { MultivaluedMap<String, Object> headers = new MultivaluedHashMap<>(); ResponseImpl response = new ResponseImpl(200, "foo", null, headers); assertNull(response.getHeaderString("foo")); }