Re: equals and hascode

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 18 Dec 2010 06:05:10 -0800

Message-ID:

<gtudnZW6k_82IpHQnZ2dnUVZ_sGdnZ2d@earthlink.com>

mark jason wrote:

On Dec 18, 6:12 pm, Lew <no...@lewscanon.com> wrote:

Others have pointed out that the "colt" library itself has the bug. This
means that its matrix classes will behave strangely in hashed collections such
as 'HashSet'.

thanks everyone for the replies,so if I still want to subclass the
class from colt package,should I implement the method as given below?
regards,
mark
@Override
public int hashCode() {
throw new UnsupportedOperationException();
}

As indicated in the findbugs documentation, that is a good fix if you
will *never* use an instance of your class as a key in a HashMap, an
element in a HashList, or in any other way that requires its hashCode.
It is definitely better than leaving hashCode inconsistent with equals.

I would perhaps add a message to the exception constructor to indicate
the nature of the problem, because hashCode is not documented as an
optional method.

You can use instances of your class as the keys in an IdentityHashMap,
which is probably all that makes sense for matrix classes.

Patricia