Re: hashCode

"Eric Sosman" <eric.sosman@1:261/38.remove-pjg-this>
Sat, 11 Aug 2012 18:17:54 GMT
  To: bob smith
From: Eric Sosman <esosman@ieee-dot-org.invalid>

On 8/10/2012 6:22 PM, bob smith wrote: [... many blank lines removed for
legibility's sake ...]

On Friday, August 10, 2012 11:34:28 AM UTC-5, Eric Sosman wrote:

On 8/10/2012 11:47 AM, bob smith wrote:

Is it always technically correct to override the hashCode function like so:

    public int hashCode() {
            return 1;

Would it be potentially better if that was Object's implementation?

      Define "better."

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


     I cannot think of a case where you HAVE to override hashCode(),
except as a consequence of other choices that you didn't HAVE to make. You
don't HAVE to invent classes where distinct instances are considered equal, and
even if you do you don't HAVE to put those instances in HashMaps or HashSets or

     But that's a bit specious: All it says is that you don't HAVE
to override hashCode() because you don't HAVE to use things that call it. It's
like "You don't HAVE to pay taxes, because you don't HAVE to live outside
prison." So, let's take it as a given that you will often need to write
classes that override equals() and hashCode() -- I imagine you understand that
they go together.

     Okay: Then returning a constant 1 (or 42 or 0 or whatever)
would in fact satisfy the letter of the law regarding hashCode(): Whenever
x.equals(y) is true, x.hashCode() == y.hashCode(). In your example this would
be trivially true because x,y,z,... all have the same hashCode() value, whether
they're equal or not -- You have lived up to the letter of the law.

     Of course, such a hashCode() would make all those hash-based
containers pretty much useless: They would work in the sense that they would
get the Right Answer, but they'd be abominably slow, with expected performance
of O(N) instead of O(1). See
for a survey of some denial-of-service attacks that work by driving hash tables
from O(1) to O(N), resulting in catastrophic failure of the attacked system.

     In other words, the letter of the law on hashCode() is a bare
minimum that guarantees correct functioning, but it is not enough to guarantee
usability. Why isn't the law more specific? Because nobody knows how to write
"hashCode() must be correct *and* usable" in terms that would cover all the
classes all the Java programmers have dreamed up and will dream up. Your
hashCode() meets the bare minimum requirement, but is not "usable." The actual
hashCode() provided by Object also meets the bare minimum requirement, and *is*
usable as it stands, until (and unless; you don't HAVE to) you choose to
implement other equals() semantics, and a hashCode() to match them.

Eric Sosman

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