How i could make a jess rule in java

Question

I am a begginer at jess rules so i can't understand how i could use it. I had read a lot of tutorials but i am confused.

So i have this code :

Date choosendate = "2013-05-05";
Date date1 = "2013-05-10";
Date date2 = "2013-05-25";
Date date3 = "2013-05-05";
int var = 0;

    if (choosendate.compareTo(date1)==0)
                {
                    var = 1;
                }
                else if (choosendate.compareTo(date2)==0)
                {
                    var = 2;
                }
                else if (choosendate.compareTo(date3)==0)
                {
                    var = 3;
                }

How i could do it with jess rules? I would like to make a jess rules who takes the dates , compare them and give me back in java the variable var. Could you make me a simple example to understand it?

Answer 1

This problem isn't a good fit for Jess as written (the Java code is short and efficient as-is) but I can show you a solution that could be adapted to other more complex situations. First, you would need to define a template to hold Date , int pairs:

(deftemplate pair (slot date) (slot score))

Then you could create some facts using the template. These are somewhat equivalent to your date1 , date2 , etc, except they associate each date with the corresponding var value:

(import java.util.Date)
(assert (pair (date (new Date 113 4 10)) (score 1)))
(assert (pair (date (new Date 113 4 25)) (score 2)))
(assert (pair (date (new Date 113 4 5))  (score 3)))

We can define a global variable to hold the final, computed score (makes it easier to get from Java.) This is the equivalent of your var variable:

(defglobal ?*var* = 0)

Assuming that the "chosen date" is going to be in an ordered fact chosendate , we could write a rule like the following. It replaces your chain of if statements, and will compare your chosen date to all the dates in working memory until it finds a match, then stop:

(defrule score-date
    (chosendate ?d)
    (pair (date ?d) (score ?s))
    =>
    (bind ?*var* ?s)
    (halt))

OK, now, all the code above goes in a file called dates.clp . The following Java code will make use of it (the call to Rete.watchAll() is included so you can see some interesting trace output; you'd leave that out in a real program):

    import jess.*;
    // ...

    // Get Jess ready
    Rete engine = new Rete();
    engine.batch("dates.clp");
    engine.watchAll();

    // Plug in the "chosen date"
    Date chosenDate = new Date(113, 4, 5);
    Fact fact = new Fact("chosendate", engine);
    fact.setSlotValue("__data", new Value(new ValueVector().add(chosenDate), RU.LIST));
    engine.assertFact(fact);

    // Run the rule and report the result
    int count = engine.run();
    if (count > 0) {
        int score = engine.getGlobalContext().getVariable("*var*").intValue(null);
        System.out.println("Score = " + score);
    } else {
        System.out.println("No matching date found.");
    }

As I said, this isn't a great fit, because the resulting code is larger and more complex than your original. Where using a rule engine makes sense is if you've got multiple rules that interact; such a Jess program has no more overhead than this, and so fairly quickly starts to look like a simplification compared to equivalent Java code. Good luck with Jess!