Re: Newbie question: How to write a set?

From:
Tom Anderson <twic@urchin.earth.li>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 21 May 2008 01:51:31 +0100
Message-ID:
<Pine.LNX.4.64.0805210150590.22052@urchin.earth.li>
On Tue, 20 May 2008, Eric Sosman wrote:

Tom Anderson wrote:

On Tue, 20 May 2008, Eric Sosman wrote:

[...]
The important thing is to implement *both* equals() and
hashCode(), or to implement *neither* of them. It is
almost always a misteak to implement just one of the pair.


If the OP wants to use a TreeSet, he needs to implement
Comparable.compareTo,


   Right. (Alternatively, he could specify a Comparator when building
the TreeSet.) Good catch.


Ditto on the Comparator!

and hashCode doesn't matter. Although it would still be prudent to
implement it.


   I'd still go beyond "prudent." If he implements Comparable it's a
very good idea to implement equals(); it's legal to have a natural order
that is inconsistent with equals(), but it's asking for trouble. And if
he implements equals() without implementing hashCode() (or vice versa),
he's not asking for trouble but begging for it.


Absolutely.

tom

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