public String validate(){
  StringBuilder sb=new StringBuilder();
  if (group == null)   sb.append("group is required. ");
  if (host == null)   sb.append("host is required. ");
  if (sql == null)   sb.append("sql is required. ");
  return sb.toString();
}

public String validate ( ) { StringBuilder sb = new StringBuilder ( ) ; if ( group == null ) sb . append ( "group is required. " ) ; if ( host == null ) sb . append ( "host is required. " ) ; if ( sql == null ) sb . append ( "sql is required. " ) ; return sb . toString ( ) ; }
validate the parameters . return null or empty if validated . otherwise , error messages todo
