code: public BoxRequestsFile.DownloadFile getDownloadRequest(File target, String fileId) throws IOException{
              if (!target.exists()){
                  throw new FileNotFoundException();
              }
              BoxRequestsFile.DownloadFile request = new BoxRequestsFile.DownloadFile(fileId, target, getFileDownloadUrl(fileId),mSession);
              return request;
      }
docs: Gets a request that downloads a given file to a target file
  
  @param target    target file to download to, target can be either a directory or a file
  @param fileId    id of the file to download
  @return  request to download a file to a target file
  @throws IOException throws FileNotFoundException if target file does not exist.