code: public void setVisitorId(String visitorId){
          if (visitorId != null){
              if (visitorId.length() != ID_LENGTH){            
                  throw new IllegalArgumentException(visitorId+" is not "+ID_LENGTH+" characters long.");
              }
              // Verify visitorID is a 16 character hexadecimal string
              else if (!visitorId.matches("[0-9A-Fa-f]+")){
                  throw new IllegalArgumentException(visitorId+" is not a hexadecimal string.");
              }
          }
          setParameter(VISITOR_ID, visitorId);
      }
docs: Set the unique visitor ID, must be a {@value #ID_LENGTH} characters hexadecimal string.
  Every unique visitor must be assigned a different ID and this ID must not
  change after it is assigned. If this value is not set Piwik will still
  track visits, but the unique visitors metric might be less accurate.
  @param visitorId the visitor id to set.  A null value will remove this parameter