Re: Newbie question: How to write a set?

From:

Tom Anderson <twic@urchin.earth.li>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 20 May 2008 19:22:57 +0100

Message-ID:

<Pine.LNX.4.64.0805201921500.15168@urchin.earth.li>

On Tue, 20 May 2008, Eric Sosman wrote:

mrstephengross wrote:

I have an object (Foo), and would like to create a set of Foo's.

(1) How should I implement Foo's 'equals()' function so that I can
ensure only unique instances of Foo are placed in the set?

class Foo {
        public boolean equals(Object obj) {
            // return true iff obj is a Foo
            // that you consider "equal" to this one
        }

        public int hashCode() {
            // return an integer whose value depends
            // only on those aspects of a Foo that
            // equals() takes into consideration
        }
    }

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

tom

--
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