Finding the most recent date in from all dates in Drools object

How can I find the most recent date among all joining dates in a Drools object?

My rule is something like this:

declare RecentDate
      dates : java.util.Date
end

rule "insert_dates"
      when
       //reading dates from back end
       Employee($date : dateOfJoining, $name : name, salary > 10000)
       then
       insert(new RecentDate($date))
end

Now I would like to find the most recent employee based on dateOfJoining. Any help is appreciated. Thank you.

One way to do it is to, given an employee, make sure that there is no other with a higher date:

declare RecentDate
      dates : java.util.Date
end

rule "insert_dates"
when
  //reading dates from back end
  Employee( $date : dateOfJoining, $name : name, salary > 10000)
  not Employee (dateOfJoining > $date)
then
  insert(new RecentDate($date))
end

The rule above only works on the case you have all your Employee facts in your session before you execute fireAllRules(). If this is not the case, you can try to initially insert a RecentDate with a null date and then compare each Employee against it and update it accordingly:

rule "Init RecentDate"
when
  not RecentDate()
then
  insert new RecentDate(null);
end

rule "update_dates"
when
  Employee( $date : dateOfJoining, $name : name, salary > 10000)
  $d: RecentDate ( date == null || date < $date)
then
  modify ($d){
    setDate($date)
  }
end

Hope it helps,