code: public User findByEmail(String email) throws FlickrException {
          Map<String, Object> parameters = new HashMap<String, Object>();
          parameters.put("method", METHOD_FIND_BY_EMAIL);
  
          parameters.put("find_email", email);
  
          Response response = transportAPI.get(transportAPI.getPath(), parameters, apiKey, sharedSecret);
          if (response.isError()) {
              throw new FlickrException(response.getErrorCode(), response.getErrorMessage());
          }
          Element userElement = response.getPayload();
          User user = new User();
          user.setId(userElement.getAttribute("nsid"));
          user.setUsername(XMLUtilities.getChildValue(userElement, "username"));
          return user;
      }
docs: Find the user by their email address.
  
  This method does not require authentication.
  
  @param email
  The email address
  @return The User
  @throws FlickrException