Re: Newbie question: How to write a set?

From:

Eric Sosman <Eric.Sosman@sun.com>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 20 May 2008 12:41:40 -0400

Message-ID:

<1211301684.946053@news1nwk>

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.

(2) How should I create the actual set to store the Foo instances?

Set<Foo> set = new HashSet<Foo>();
    // or LinkedHashSet<Foo>();
    // or TreeSet<Foo>();
    // or ...

The type of the `set' reference should usually be Set,
sometimes SortedSet, but very rarely HashSet or TreeSet
or whatever.

--
Eric.Sosman@sun.com