Re: Distinct ID Number Per Object?
Daniel Dyer wrote:
"As much as is reasonably practical, the hashCode method defined by
class Object does return distinct integers for distinct objects. (This
is typically implemented by converting the internal address of the
object into an integer, but this implementation technique is not
required by the JavaTM programming language.)"
Which seems to be entirely consistent with what you are saying. So I'm
not sure where Lew is coming from when he says:
Because they do not say the algorithm for "converting the internal address".
Which part of the JVM address would they use? The handle, which comprises a
pointer to two other pointers in some Sun implementations? The pointer
values? The heap offset into which one of the pointers points? The pointer
to the class area? All these ingredients are necessary to make up a real
"address" in the JVM, but only an int appears in the hashCode() output.
Also, notice the words, "As much as is reasonably practical". They are
telling you right in the Javadocs that it is not a guarantee.
Once again, the int of a hashCode() and the "address" of an object have
different structures, different interpretations and different semantic spaces.
it is not correct that the "usual implementation of the defaulthash
code for Object is the memory address of that object"
It isn't. One implementation is to derive the int from some part of the
"address" of the object by an unspecified algorithm. The derived int is not
the same thing as the source "address".
Unless he merely means that the value returned is not necessarily the
address itself but is trivially derived from the address.
Maybe trivially, maybe not, but certainly derived from, and not equivalent to.
It can't be. As others pointed out, the theoretical space of addresses is
larger than can be represented in 32 bits.
All of this goes to what others on this thread have also pointed out, that
identityHashCode() and the underlying Object.hashCode() cannot be relied upon
to achieve a guaranteed unique handle for an object.
--
Lew