Re: Distinct ID Number Per Object?
Lew wrote:
Daniel Dyer wrote:
On Sat, 16 Jun 2007 20:17:28 +0100, Lew <lew@lewscanon.nospam> wrote:
Eric Sosman wrote:
Lew wrote:
Hal Vaughan wrote:
So is it only in extreme cases like this where hashcodes would be
duplicated?
Hash codes have even fewer values than Strings. That means there
must be proportionately more collisions. Have you read the
Javadocs on the hashCode() method? You should. Also read the
Javadocs on Map, HashMap and IdentityHashMap.
As Twisted pointed out, the "Identity", i.e., the internal
"address" of an object, is unique for the lifetime of that object.
[...]
Can you find this guarantee in the Javadoc or other
authoritative place? Does this rule out 64-bit JVM's?
You mean the guarantee that an object's "address" is unique during
its lifetime? How else would the JVM find a particular instance? In
other words, how could it possibly not be?
If two objects had the same "address", then a reference using that
"address" would not reference a single object, which contradicts the
very definition of an object reference.
<http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.5>
a reference to the newly created object is returned as the result
[of] the indicated constructor
There is no way for one "address" to point to two objects
simultaneously.
This question has nothing to do with bit width, AFAICS. I'm not
really sure how there could even be a question here.
I think Eric's point is that the number of possible hash codes is
smaller than the number of objects that can be addressed in a 64-bit
JVM. Therefore System.identityHashCode(Object) cannot, in a 64-bit VM
at least, guarantee to return unique values for all objects on the heap.
The fact that he didn't mention that method in his question, but instead
referenced my comments about "address", meant it would've taken quite
the leap for me to understand that context.
Nothing in Eric's post makes mention of
System.identityHashCode(Object). How did you infer it?
The inferential error, if there was one, was mine: When
you wrote about the "Identity" of an Object, I assumed you
meant its System.identityHashCode() value. My assumption was
(I thought) strengthened when you described the "Identity" as
"the internal `address'" of the object, which matches the
highly suggestive (but not 100% prescriptive) Javadoc. Did
you have some other way to find "the internal `address'" of a
Java object? System.hashCode() is the closest thing I can
think of, but there are many things I haven't thought of ...
And yes, the point of my question was that a 64-bit JVM
can (given enough heap) create more distinct objects than
there are hashCode() or identityHashCode() values. I was
attempting what's known as "Socratic questioning;" Socrates
was evidently better at it than I am -- and he got poisoned
for it, so perhaps I ought to quit while I still have the
option ...
--
Eric Sosman
esosman@acm-dot-org.invalid