Re: hashCode
On 8/10/2012 6:22 PM, bob smith wrote:
Better in the sense that you would never HAVE to override hashCode.
Now, there are cases where you HAVE to override it, or your code is very broken.
Returning a constant hash code is correct in the same sense that
answering "yes" to the question "Can you tell me the correct way to do
this?" would be--syntactically and semantically correct, but completely
contrary to the actual intent of the question.
The point of the hash code is to provide a cheap way to quickly
distinguish inputs (in the sense that Pr(a.hashCode() == b.hashCode()
and !a.equals(b)) should be as small as possible [1]). A constant-value
hash completely negates the purpose of the hash code, and this renders
the hashCode again completely unusable for anything that actually wants
to use it.
In the default case, a.hashCode() == b.hashCode() only when a == b (this
definitely holds true with 32-bit machines and I'm pretty sure it still
holds true with 64-bit machines, but I'd have to reverify the JVM source
code to be certain). It is thus correct so long as identity equals is
correct. It is also potentially correct in a limited set of cases where
a.equals(b) and a != b. In all of these cases, it would not only be
correct but also extremely useful, having pretty strong guarantees about
the distribution of hash values.
[1] Actually, for good performance, hash codes should go one step
further and make slightly stronger guarantees about independence with
respect to the size of the hash table. But I digress.
--
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth