Object identity is a slippery concept. Consider that two Strings
containing the same text are "equal," but they are two different
objects. Now, consider that two arrays containing the same values, and
of the same length, are *not* equal according to the "equals()"
method. Finally, consider that some classes have equals() methods that
depend on mutable data, so that two objects may be equal now, but not
equal a second from now (all the Collectioins classes are like this.)
Anyway, here's an experiment to try. The following little program
shows what happens if you use objects whose hashValue()s can change as
the keys in a HashMap. The HashMap gets quite confused, as you'll
see. The same kind of thing can happen to Jess if the the data in a
slot changes in an uncontrolled way. As long as 1) all changes are
announced by PropertyChangeEvents, and 2) the actual definstanced
object's hashCode doesn't change, however, things work fine.
----------------------------------------------------------------------
import java.util.HashMap;
public class Test {
private int m_i;
public Test(int i) {
m_i = i;
}
public void setHashCode(int i) {
m_i = i;
}
public int hashCode() {
return m_i;
}
public boolean equals(Object o) {
if (o instanceof Test) {
return m_i == ((Test) o).m_i;
} else
return false;
}
public static void main(String[] argv) {
HashMap map = new HashMap();
Test t1 = new Test(1);
Test t2 = new Test(2);
map.put(t1, "tee-one");
map.put(t2, "tee-two");
System.out.println(map.get(t1));
System.out.println(map.get(t2));
t1.setHashCode(9);
t2.setHashCode(23);
System.out.println(map.get(t1));
System.out.println(map.get(t2));
}
}
----------------------------------------------------------------------
I think Alexander Lamb wrote:
[Charset iso-8859-1 unsupported, filtering to ASCII...]
> I am wondering if I might be affected by this problem as well since all my
> facts will be definstances and some of them will be dynamic.
> I thougt Jess was using hascode to handle a unique instance of the Java
> object as a fact and by default hascode was returning a code different for
> each object but immutable (for example the address of the object).
> But somehow from this thread it looks like Jess is using properties to
> identify unicity? Or is it my misunderstanding? Obviously, in a dynamic
> object, properties will change! And I really don't want several copies of
> facts of the same object in memory (representing in a way the history of the
> object), because this would have side effects with:
> - memory consumption
> - how to make sure an undefinstance will remove them all
>
> Alexander
>
---------------------------------------------------------
Ernest Friedman-Hill
Distributed Systems Research Phone: (925) 294-2154
Sandia National Labs FAX: (925) 294-2234
Org. 8920, MS 9012 [EMAIL PROTECTED]
PO Box 969 http://herzberg.ca.sandia.gov
Livermore, CA 94550
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