code: public void setVisitorCustomId(String visitorCustomId){
          if (visitorCustomId != null){
              if (visitorCustomId.length() != ID_LENGTH){            
                  throw new IllegalArgumentException(visitorCustomId+" is not "+ID_LENGTH+" characters long.");
              }
              // Verify visitorID is a 16 character hexadecimal string
              else if (!visitorCustomId.matches("[0-9A-Fa-f]+")){
                  throw new IllegalArgumentException(visitorCustomId+" is not a hexadecimal string.");
              }
          }
          setParameter(VISITOR_CUSTOM_ID, visitorCustomId);
      }
docs: Set a custom visitor ID for this request. You must set this value to exactly
  a {@value #ID_LENGTH} character hexadecimal string (containing only characters 01234567890abcdefABCDEF).
  We recommended to set the UserId rather than the VisitorCustomId.
  @param visitorCustomId the visitor's custom id to set.  A null value will remove this parameter