Re: hashCode
On 08/10/2012 04:30 PM, Arne Vajh??j wrote:
On 8/10/2012 6:32 PM, Lew wrote:
bob smith wrote:
Now, there are cases where you HAVE to override it, or your code is very
broken.
No.
As long as 'hashCode()' fulfills the contract, your code will work -
functionally. But a bad
'hashCode()' could and likely will noticeably affect performance. There is
more to correctness
than mere functional conformance.
If the code per specs is guaranteed to work then it is correct.
Good (or just decent) performance is not necessary for code to
be correct.
At least not in the traditional programming terminology.
In plain English maybe.
I see your point, but that is not to say that the specs exclude performance
considerations.
In the case of 'hashCode()', the Javadocs do say, "This method is supported
for the benefit of hash tables such as those provided by HashMap."
<http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#hashCode()>
The key question here is how you define "benefit". I argue that a hash code
that is constant does not benefit, say, a 'HashMap' because one of our desired
uses is constant-order retrieval.
"This implementation provides constant-time performance for the basic
operations (get and put), assuming the hash function disperses the elements
properly among the buckets."
<http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html>
Each specification refers to the other. Ergo they are meant to be considered
together. Taken together, the documentation clearly specifies that "correct"
or "proper" includes performance considerations. Therefore, by what you say,
the simple "return 1;" is not correct.
It certainly would not be correct for the 'Object' implementation.
"As much as is reasonably practical, the hashCode method defined by class
Object does return distinct integers for distinct objects." [op. cit.]
As you say, Arne, "correct" means it follows the spec. The OP's suggested
implementation violates the spec on two fronts.
--
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg