Publishing this [Was: Re: Discrepancy in current hour]

Eric Sosman <esosman@ieee-dot-org.invalid>
Sun, 21 Mar 2010 16:35:18 -0400
On 3/21/2010 3:54 PM, Lew wrote:

Regardless of "manner of speaking", it is dangerous to use an instance
before it's fully constructed, super, sub and barrel.

     Slight topic branch: One of the unpleasant consequences is
that it's unsafe for a constructor to publish a `this' reference.
When I was a Javanewbie I sometimes wrote classes that maintained
collections of all their instances, thus (genericized for current
audiences, although generics weren't around in my newbie days):

    class Wotsit {
        private static final Set<Wotsit> universe
            = new HashSet<Wotsit>();
        Wotsit(...) {

I later learned that this is a Bad Thing, because if another
thread traverses the contents of `universe' it could come across
a Wotsit (or Wotsit subclass) that wasn't yet fully operational.

     The only way I know of to maintain such a collection safely
is to hide the constructor and make folks use a factory method:

    class Dingbat {
        private static final Set<Dingbat> universe
            = new HashSet<Dingbat>();
        private Dingbat(...) {
        static Dingbat newInstance(...) {
            Dingbat edith = new Dingbat(...);
            return edith;

This has the unfortunate side-effect that Dingbat cannot be
subclassed, because of the private constructor. Yet if you
make the constructor anything other than private, there's a
risk that someone will do `new Dingbat()' and `universe' will
be incomplete.

     How do others deal with this? Give up on "all instances"
collections? Live with effectively-final classes? Document
the living daylights out of it and hope for the best? Or is
there something niftier that can be done?

Eric Sosman

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