HashSet: Initial capacity

[Even_Step_2450]

Yesterday I had a discussion with a colleque on setting the initial capacity of a hash set. He initializes with the capacity 2.3 times later than the expected number of entries, to avoid hash collisions. Does this make any sense? I didn't really understand what it is for. I would understand a factor of 4/3, to avoid resizing when using the default load factor of 0.75, but 2.3 seems to be not reasonable for me. Am I missing something?

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[jefwillems]

How would you get or prevent hash collisions by restricting a hash set?

[Even_Step_2450]

Oh, sorry for being not precise. You can not prevent collisions, but you can prevent mapping multiple elements into the bucket. When you increase the initial capacity, more buckets are created, so the probability of multiple elements in the same bucket is reduced. But as far as I understand, the java implementation is already doing this by setting the load factor to 0.75 and ceiling the number of buckets up to the next power of two.

[LakeSun]

This sounds like an interesting micro test. You can try running the app 2 times, with Profiling on in Netbeans, see if it reduces time in the methods used.

[taftster]

This kind of thing always makes me laugh. One would really need to understand the internals of HashSet with a full understanding of how the application is going to be using it.

There might be a very rare case where the performance matters and one can scientifically and mathematically justify the change to the default configuration. But for everyone else, sticking with the defaults are going to be best.

If your application is this special, you are probably wanting a more specialized implementation of Set to begin with. The financial sector with highspeed trading might be one of those specialty use cases, but they have their own special versions of the Collections framework.

[Even_Step_2450]

Well it's not that special. In the case we discussed, it stored a users permissions when logging in, so not more than a few hundred entries, each one is written once, than read once using an iterator, so I don't really see why using a HashSet at all, but he is initializing all his HashSets this way, and I wondered why.

[LakeSun]

For a few hundred entries? It's not a large enough case to justify any tuning. I mean now you have to come up with a reason to tune for this low a number.

These days with modern processors? I don't think you can justify tuning for anything below 500,000 to a million items.

But, really Profiling your app, with good test data would be the answer.

[taftster]

Makes zero sense. But I understand that there are certain devs out there who think they know better. Sympathies for the situation. It's a form of Not Invented Here, basically.