A good technical question!
The method listActivations() is not used internally by Jess; it's
just provided for use by tools of various kinds. It's a fairly
expensive method because it makes a copy of the whole agenda, and
iterating over this copy does a full sort. The copy is necessary; the
process of iterating over all the activations is destructive. They're
not kept in sorted order -- they're pulled out "just in time." Anyway
concern over using == vs equals() may be misplaced -- this is a small
difference compared to the overall expense of copying the whole
agenda. This much has always been true.
What's changed in 7.0 is that an Activation object can't be part of
more than one agenda at a time, because the Activation "knows" where
in the agenda it lies. In older versions, the same Activation objects
would live in the copied agenda as in the original; now copies are
made of all the Activations, too. This new property of Activation
objects is an optimization; it makes the process of removing
activations from the agenda before firing them (due to a retraction,
for example) much more efficient.
I thought of this as an internal implementation detail and so didn't
document it, but the change is permanent.
On Dec 2, 2006, at 8:07 PM, Jonathan Sewall wrote:
I've found that the method below behaves differently in Jess 7.0b7
and the formal v7.0 release. In the formal release, the
Activations provided by the Iterator in 2 successive calls to
Rete.listActivations() are different objects, even if nothing has
happened between the calls. The change has ramifications for those
with custom Strategy implementations. In our case, we scan the
agenda for a particular Activation to favor before calling run(),
and then our Strategy's compare() method tests its arguments for
that object with the == operator.
We can, with some impact on performance (presumably negligible on
short agendas), use the Activation.equals() method instead of the
== operator. But I wanted to ask about the intent of the change,
and whether it's likely to be permanent. I hadn't seen (but could
have simply missed) mention of it in the notes since beta 7.
Thanks very much,
Jonathan
________
public void dumpAgenda(String label) {
Map actMap = new HashMap();
Iterator it = listActivations(); // get the agenda and
store Activations
for (int i = 0; it.hasNext(); ++i) {
Activation a = (Activation) it.next();
actMap.put(new Integer(a.hashCode()), a);
}
it = listActivations(); // get the agenda again; no
intervening activity
for (int i = 0; it.hasNext(); ++i) {
Activation a = (Activation) it.next();
Activation aa = (Activation) actMap.get(new Integer
(a.hashCode()));
System.out.println("agenda a["+i+"]"+
(aa == null ? " not" : "")+" in map,"+ // aa !
= null in both v7.0b7 and 7.0
(a == aa ? "" : " not")+ " same obj"); // aa
== a only in v7.0b7
}
}
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Ernest Friedman-Hill
Advanced Software Research Phone: (925) 294-2154
Sandia National Labs FAX: (925) 294-2234
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