Alternatively... instead of putting the onus on the compiler, the
hashcode could be computed by the JVM at initialization time for the
Descriptor instance, (which would also help performance of dynamically
parsed Descriptor instance hashcode calls).
i.e.
private final int computedHashcode;
public Descriptor() {
//initialization
computedHashcode = do_compute_hashCode();
}
public int hashCode() {
return computedHashcode;
}
punlic int do_compute_hashCode(){
return // compute hashcode
}
This is all talking towards optimum performance implementation... the
real problem is the need for a hashCode implementation for Descriptor
based on the actual Descriptor's content...
On May 11, 4:54 pm, Ben Wright <compuware...@gmail.com> wrote:
> Jay:
>
> Using the class name to generate the hashcode is logically incorrect
> because the class name can be derived by the options java_package_
> name and java_outer_classname.
>
> Additionally (although less likely to matter), separate protocol
> buffer files can define an identical class names with different
> protocol buffers.
>
> Lastly, and most importantly...
>
> If the same Message is being used with generated code and with dynamic
> code, the hash code for the descriptor would still be identical if
> generated from the descriptor instance, whereas the dynamic usage does
> not have a classname from which to derive a hashcode. While in your
> case this should not matter, it does matter for other users of
> protobuf. The hashcode function would be better served by being
> implemented correctly from state data for the descriptor.
> Additionally, in generated code it seems that this hashcode could be
> pre-computed by the compiler and Descriptor.hashcode() could return a
> constant integer - which would be much more efficient than any other
> method.
>
> On May 11, 3:02 pm, Jay Booth <jaybo...@gmail.com> wrote:
>
>
>
>
>
>
>
> > It can be legitimate, especially in the case of Object.hashCode(), but
> > it's supposed to be in sync with equals() by contract. As it stands,
> > two objects which are equal() will produce different hashes, or the
> > same logical object will produce different hashes across JVMs. That
> > breaks the contract.. if the equals() method simply did return (other
> > == this), then it'd be fine, albeit a little useless.
>
> > I created an issue and posted a 1-liner patch that would eliminate the
> > problem by using getClass().getName().hashCode() to incorporate type
> > information into the hashCode without depending on a Descriptor
> > object's memory address.
>
> > On May 11, 12:01 am, Dmitriy Ryaboy <dvrya...@gmail.com> wrote:
>
> > > Hi Jay,
>
> > > I encountered that before. Unfortunately this is a legitimate thing to
> > > do, as documented in Object.hashCode()
>
> > > I have a write-up of the problem and how we wound up solving it (not
> > > elegant.. suggestions welcome)
> > > here:http://squarecog.wordpress.com/2011/02/20/hadoop-requires-stable-hash...
>
> > > D
>
> > > On Mon, May 9, 2011 at 8:25 AM, Jay Booth <jaybo...@gmail.com> wrote:
> > > > I'm testing an on-disk hashtable with Protobufs and noticed that with
> > > > the java generated hashcode function, it seems to return a different
> > > > hashcode across JVM invocations for the same logically equivalent
> > > > object (tested with a single string protobuf, same string for both
> > > > instances).
>
> > > > Is this known behavior? Bit busy right now backporting this to work
> > > > with String keys instead but I could provide a bit of command line
> > > > code that demonstrates the issue when I get a chance.
>
> > > > Glancing at the generated hashcode() function, it looks like the
> > > > difference comes from etiher getDescriptorForType().hashCode() or
> > > > getUnknownFields().hashCode(), both of which are incorporated.
>
> > > > --
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