code: public List<Issue> getIssuesBySummary(String projectKey, String summaryField) throws RedmineException {
          if ((projectKey != null) && (projectKey.length() > 0)) {
              return transport.getObjectsList(Issue.class,
                      new BasicNameValuePair("subject", summaryField),
                      new BasicNameValuePair("project_id", projectKey));
          } else {
              return transport.getObjectsList(Issue.class,
                      new BasicNameValuePair("subject", summaryField));
          }
      }
docs: There could be several issues with the same summary, so the method returns List.
  
  @return empty list if not issues with this summary field exist, never NULL
  @throws RedmineAuthenticationException invalid or no API access key is used with the server, which
  requires authorization. Check the constructor arguments.
  @throws NotFoundException
  @throws RedmineException