code: @SuppressWarnings("unchecked")
      public String getUserEmail(String userId) {
          if (userId == null) {
              return null;
          }
          // map userId to email
          // NOTE: this gets convoluted if they use OAuth logins because the email
          // field is in the service!
          Map<String, Object> user = getDocument("users", userId); 
          String email = userId;
          if (user != null) {
              ArrayList<Map<String, String>> emails = (ArrayList<Map<String, String>>) user
                      .get("emails");
              if ((emails != null) && (emails.size() > 0)) {
                  // get first email address
                  Map<String, String> emailFields = emails.get(0);
                  email = emailFields.get("address");
              }
          }
          return email;
      }
docs: Gets email address for user
  @param userId user ID
  @return email address for that user (lookup via users collection)